tr2010 052 construction of stormwater management...

Construction of Stormwater Devices in the Auckland Region 177

16 Sand Filters

16.1 Introduction

Sand filters are used to attenuate peak stormwater flows, and treat stormw ater to reduce (or

remove) contamination by using different types of filter media. Sand filters are often used in

built up commercial or industrial areas because they can be installed underground and not

take up valuable surface land.

16.2 Device description

Sand filters are usually multi-chamber structures designed to treat stormwater runoff through

sedimentation (settling of heavier particles from the water column) and filtration (runoff

passes through the sand media to filter out pollutants). Stormwater run off from hard surfaces

including roofs, car parks and roads is normally collected by a series of roadside drains (kerb

and channel to catchpits) or spouting (downpipes for roof runoff). The stormwater is then

conveyed by a piped stormwater system to the sand filter for treatment.

Sand filters can be made up of a number of buried chambers, above ground chambers or a

combination of the two. Many sand filters arrive on-site as prefabricated units pre-drilled for

the connection with all of the internal components already installed (except the filter media

which still needs to be placed) and require only a connection to the inlet, outlet and joining of

the chambers (where there is more than one chamber).

The sand filter media is often made up of a mix, or layered with, sand and other materials,

such as peat or aged, mature compost to increase the removal of heavy metals. Care should

be taken when incorporating compost into sand filters, ensuring only mature, contaminant

free compost is used to reduce the risk of contaminant or nutrient leaching to filtered

stormwater. For industrial situations where specific contaminants may enter the stormwater,

the sand can be replaced or partially replaced with specially designed media such as ZPG

(zeolite, perlite and granulated activated carbon) to increase the removal of heavy metals and

so that additional pollutants may be removed. These special media systems are often

associated with cartridge filters. Two generic operational types of sand filters are typically

constructed, “surface” and “depth” filters.

Surface filters operate as a "cake" filter where a “schmutzdecke”, a complex biofilm layer

builds up on the surface of the filter that hydraulically controls the filtration rate. This is

generally very slow in the order of 2–5 cm hr-1

for surface filters. Solids are typically deposited

on the surface of the filter where they accumulate over time. Maintenance typically involves

removal of accumulated sediment and the top few centimetres of sand, which is replaced.

These filters are designed to capture a specified water quality volume and drain out between

24 and 48 hours.


The other style of filter are depth filters, typically constructed from cylindrical cartridges. The

systems (typically manufactured) utilise coarse media with much higher flow rates (and

remove solids or other pollutants throughout the entire bed volume. Maintenance is

performed by complete removal and replacement of the filter media. These filters can be

designed to treat a design hydrograph or a water quality volume using upstream water quality

volume storage (sumps, rain tanks etc.) that captures, pre -treats and delivers a controlled

discharge to the filters. Since there are many variants to these types of filters, only systems

that have been reviewed and approved by the council are allowed. Designs must be

consistent with council design criteria and, in the case of an industrial or trade process, its risk

classification.

Sand filters include five main elements in their design as listed below.

Table 33

Key components of a typical sand filter.

Component Description

Inlet Stormwater runoff

enters the sand

filter from a piped

stormwater

reticulation system

passing through an

inlet manhole

where a diversion

weir is located (so

that in larger

storms the sand

filter is safely

bypassed).

Sedimentation

chamber

The first chamber in the sand filter slows stormwater runoff so that the

coarser, heavier particles contained in the runoff drop out. Sometimes

this is assisted by baffles (small walls or plates) at the bottom of the

chamber. Baffles are also often included in the top of the sedimentation

chamber to trap floatables (such as litter).

They are usually 25% of the area of the filtration chamber.

Filtration

chamber

Runoff enters the filtration chamber via a weir from the sedimentation

chamber. Runoff drains down through the sand filter media to the under

drainage system.

A spreader may be in place between the sedimentation and filtration

chamber to ensure the flows distribute evenly over the filter bed and do

not erode the top of the filter bed.

The filter media is normally 400 mm deep with a ponding area above at a

minimum of 400 mm deep.



Underdrainage

system

Located beneath the sand filter, the underdrainage system collects the

treated water via perforated pipes and conveys this to the outlet.

Outlet Collects flows from the underdrainage system and discharges them to

either a piped system or directly to the receiving environment (i.e.

waterways such as streams, the coast, etc).

For most sand filters, the outlet is piped to an outlet manhole which is

configured to allow for water quality samples of the effluent to be

collected.

Overflow

system

Usually consists of pipes located above the normal operating level of the

sand filter (i.e. above the sand filter bed and ponded water level). Flows in

the larger storms overflow from the sand filter through the pipes to the

stormwater reticulation system.

The ‘Washington’ sand filter is the most common in the Auckland region. Delaware sand

filters are often used around service stations or loading bays. Austin sand filters are usually

open sand filters and are not extensively used in the Auckland region.

Figure 83

Typical Washington sand filter.

Washington sand filters consist of a sedimentation chamber followed by the sand filter media

bed. They are completely enclosed, often pre -cast underground concrete units.

Manufacturers include Hynds and Humes. Pre-fabricated units treat smaller impervious areas

(usually up to 5,000 m2). Larger areas are normally serviced by a number of sand filter units,

or made to unique specifications. They are also known as ‘Underground Sand Filters’ or ‘Vault

Sand Filters’


Figure 84

Typical Delaware sand filter (Source: ARC, TP10).

Delaware sand filters are often used around service stations or loading bays. They are

typically used for direct pavement runoff (from roads, car parks or forecourts). They are

generally not very wide but extend along the main drainage path and collect flows via a series

of grated covers (such as a dish channel). Water then overflows, usually by a series of weirs, or

single long weir to a layer of sand through which stormwater is filtered before entering an

outlet pipe. They are often called ‘Perimeter Sand Filters’ because they are often located at

the edge of paved areas.

Figure 85

Typical Austin sand filter [NZTA].

Austin filters consist of an inlet bay (sometimes with energy dissipaters), sedimentation basin

and a characteristic ‘jagged’ weir to distribute flows evenly on the sand filter bed. The


sedimentation chamber is usually enclosed but the sand filter bed is not. They are usually

constructed on-site. An example of an Austin sand filter is located on the Upper Harbour

Highway in Hobsonville.

16.3 Guideline documents

Table34

Guidelines relat ing part icularly to sand filters include:

Publisher Title Description

ARC TP90 Erosion and Sediment

Control Guidelines for Land

Disturbing Activities in the

Auckland Region

These guidelines outline the principals of

erosion and sediment control and control

measures that should be used.

ARC TP124 Low Impact Design

Manual for the Auckland

Region

Approaches to site design and development

from a stormwater management context,

primarily applicable for residential land

development.

LTSA Integrated Stormwater

Management Guidelines for

the New Zealand Roading

Network

Provide guidance on a range of issues relating

to the management of stormwater run-off

from state highways and local roads in New

Zealand.

NSCC LB104 Management of

Driveway Runoff

Practice note to assist with the management of

driveway runoff in the Long Bay area including

specific requirements for stream protection

areas.

NSCC LB206 Flow Dispersers Practice note describing flow dispersers, these

are used to ensure the effectiveness of other

treatment devices and to prevent erosion.

NSCC LB301 Stormwater Treatment

for Roads

Practice note describing the on-site mitigation

of stormwater runoff that must be

incorporated into roading design to meet the

rules of the Long Bay Structure Plan Area.

NSCC LB303 Erosion – Sediment

Control Subdivision

Practice note providing guidance on the

requirements and descriptions of erosion and

sediment control practices at the subdivision

level.

NZWWA On Site Stormwater

Management Guideline

Provides guidance on the design of on-site

stormwater management devices for the

majority of applications in New Zealand.


16.4 Standards and technical documents

Table 35

Below is a non-exhaustive list of some of the more applicable standards and technical documents that

relate to sand filters:

Title Description

AS/NZS 1254:2002

PVC Pipes and Fittings for

Stormwater and Surface

Water Applications

Specifies requirements for PVC pipes and fittings for

conveyance of stormwater or surface water. The Standard

includes requirements for both plain and structured wall pipes

and fittings.

AS/NZS 1260:2009

PVC-U Pipes and Fittings

for Drain, Waste and Vent

Application

Specifies requirements for PVCU pipes and fittings for sewer,

drain, waste and vent applications above-ground or below

ground and intended to be used where the pipeline is operating

under gravity flow and the operating pressure is low.

AS/NZS 2033:2008

Installation of

Polyethylene Pipe

Systems

Specifies methods for handling, storage, installation, testing and

commissioning of polyethylene (PE) pipelines, above or below

ground, for pressure and non-pressure applications conveying

liquids.

AS/NZS 2566.2:2002

Buried Flexible Pipelines –

Installation

Specifies requirements for the installation, field testing and

commissioning of buried flexible pipelines with structural design

in accordance with AS/NZS 2566.1.

NZS 3106: 2009

Code of Practice for

Concrete Structures for

the Storage of Liquids

Sets out requirements for design, materials and construction of

concrete structures for the storage of liquids. Covers

construction in reinforced and pre-stressed concrete and in

cement mortar.

AS/NZS 4129:2008

Fittings for Polyethylene

(PE) Pipes for Pressure

Applications

Specifies requirements for fittings to be used with polyethylene

pipe manufactured in accordance with AS/NZS 4130 or AS

2698.2 or POP-009. This Standard is applicable to fittings

manufactured for the conveyance of water, fuel gas, and other

fluids including compressed air.

AS/NZS 4130:2009

Polyethylene (PE) Pipes

for Pressure Applications

Specifies requirements for polyethylene pipes for the

conveyance of fluids under pressure. Such fluids include, but are

not restricted to, water, wastewater, slurries, compressed air,

and fuel gas.

AS/NZS 4671:2001

Steel Reinforcing

Materials

Sets out specifications for Australian and New Zealand Steel

reinforcing materials (bars, coils and welded mesh).

Requirements for chemical, mechanical and physical properties

for three different strength grades and the three different

ductility classes are also covered.


Title Description

AS1831:2002

Ductile Cast Iron

Specifies the classification of ductile cast irons in accordance

with the mechanical properties of the material.

AS4087:2004

Metallic Flanges for

Waterworks Purposes

Specifies requirements for circular flanges manufactured from

copper alloy, ductile cast iron, grey cast iron, or steel. The

flanges covered are for use on pipes, pipe-fittings, valves and

other equipment that are primarily intended for the conveyance

or storage of water or wastewater.

AS4158:2003

Thermal-bonded

Polymeric Coatings on

Valves and Fittings for

Water Industry Purposes

Specifies the requirements for factory-applied thermal-bonded

polymeric corrosion protective coatings (barrier coatings). For

the purposes of this Standard, the term polymeric coating is

intended to encompass both thermoplastic and thermosetting

coating materials, which are used to coat both the internal and

external surfaces of valves and fittings.

AS4795:2006

Butterfly Valves for

Waterworks Purposes

Specifies the requirements for manually operated resilient

seated wafer, tapped lugged, and flanged butterfly valves for

waterworks purposes. The standard covers manual actuators,

gearboxes and standard spindle caps. Water supply applications

include drinking water and recycled water as well as screened

wastewater.

NZS 3101.1&2:2006

Concrete Structures

Standard

The Design of Concrete Structures specifies minimum

requirements for the design of reinforced and pre-stressed

concrete structures.

NZS 3104:2003

Specification for concrete

Production

Prescribes minimum requirements for the production of fresh

concrete.

NZS 3109:1997

Concrete Construction

Provides minimum requirements for the construction of

reinforced concrete, unreinforced concrete, pre-stressed

concrete or a combination, in elements of any building or civil

engineering structure.

NZS 3114:1987

Specification for Concrete

Surface Finishes

Categorizes by means of description and illustration various

classes of surface finish of concrete obtained off-the-form, by

exposed aggregate and on floors, exterior pavements and

inverts.


16.5 Construction considerations

16.5.1 Construction sequencing

If the sand filter is relatively small it may be able to be installed whilst other works continue.

Isolating the filter should be relatively easy depending on what provisions have been made in

the design for this. Normally the inlet and outlet can be blocked off at the incoming (usually

where the incoming diversion weir is located) and outgoing manholes.

To avoid siltation of the device, the unit should not be made operational until all works o n the

site have been completed.

16.5.2 Construction timing

Depending on the size of the filter that needs to be installed, the filter may be able to be

installed at any time throughout the year. Where earthworks consent is required, works will

be limited to between October and April.

16.5.3 Health and safety

Ensure that the finished tank level will be the same as that of the surrounding ground, and lids

flush with the ground surface so that there are no tripping hazards.

In addition, as confined space entry requires the use of harnesses and tripods, consideration

should be given to increasing the diameter of the manhole risers used for the access shafts.

16.5.4 Operation and maintenance

Ensure there is enough space for a sucker truck to be parked beside the filter otherwise

loadings for the design will need to take account of additional loadings caused by

maintenance machinery.

Consideration should be given to the installation of bollards around the tank to avoid vehicles

parking on the top of the sand filter increasing the loading on the lid (unless designed for it).

This will also ensure that the access lids are accessible for operation and maintenance.

Note that one of the major concerns with the operation and maintenance of filters is the

access into a confined space. This is particularly important for multi -chamber units where

access to adjacent chambers is limited. To ensure safety is not compromised, access shafts

should be provided for each chamber and consideration made for confined space entry barrier

requirements.


16.5.5 Existing service utilities

Check that there are no services in the location where the treatment device is to be installed.

This will need to take account of all inlet and outlet pipes and manholes associated with the

filter.

16.5.6 Trees and vegetation

Check for any trees and vegetation that may need to be removed. If there are large

specimens, it is unlikely consent will be given for their removal.

16.5.7 Existing structures

Consider any additional services that may need to be strengthened or removed for the

installation of the device.

16.5.8 Water supply

A significant water supply will be needed for the water tightness test. Check for hydrants in

the area.

16.5.9 Site access

Most filter units weigh several tons and will need to be installed by a large excavator, hi-ab

truck or cranes. This often depends on the depth the device will be installed at. The site

should be inspected to ensure that access for machinery and equipment, and their use (i.e.

turning movements) are available. It is also necessary to confirm clearance from overhead

lines and other services.

16.5.10 Excavation

The site for the device must allow enough room for the excavation of the device and

installation of the associated inlet and outlet pipes.

Where large units are installed, consider whether spoil can be used on -site for landscaping or

other site works (subject to quality and earthworks and land use consents) or whether this will

need to be removed off-site. Often, the amount of spoil to be removed is considerable.

16.5.11 Buoyancy

If the device is located in an area with high groundwater table, the device may become

buoyant when emptied. Sub-soil drainage around the base of the unit will ensure this does

not occur.


16.5.12 Foundations

It is essential the foundation of the filter unit are compacted and levelled as per the

construction plans. Any movement of the unit, potentially from base course subsidence, can

cause cracking in the unit and splitting of inlet or outlet connections. Where multiple u nits are

installed differential settlement may cause leakage between units. Because these units are

often buried leaks and structural failures may go unnoticed for some time causing

contamination of surrounding soil, groundwater and/or surface waterways.

16.5.13 Upstream pollutant traps

Upstream removal of gross pollutants prior to entry into the sand filter (e.g., gully traps, letter

screen) is highly recommended and often a requirement of the final design. Gross pollutants

create serious issues during maintenance and double the maintenance time required. For

sites where hydrocarbons may be an issue, installation of an oil and water separator up -flow

from the sand filter is also highly recommended and again may be a requirement of the final

design. Any additional items that can be installed to reduce the amount of time maintenance

crews need inside the chambers (which can be a confined space entry) should be included in

the construction of these filters.

16.5.14 Sand filter units

Consider pre-fabricated unit sizing. Sand filters are often prefabricated off site with internal

components already installed. This is often cheaper than constructing a cast -in-situ filter.

Levelling of the filter is also important to ensure that it operates correctly hydraulically.

16.5.15 Filter media

Special attention should be paid to the anticipated type and concentration of contaminants

and the media selected particularly to deal with these contaminants. It is imperative that the

correct media is installed for the filter to remove targeted pollu tants. For example, peat can

be incorporated into the sand filter to improve metals removal. As stated above, pre -

treatment using oil and water separators and gross pollutant traps are also recommended to

aid in maintaining contaminant removal efficiencies and ensure that the sand filter works as

intended.

16.5.16 Avoiding compaction of the filter media

For the filter to operate correctly, all efforts need to be made to ensure the filter media is not

overly compacted during installation. At the completion of media installation, hydraulic

compaction should be used, mechanical compaction avoided.


16.5.17 Labelling of inlet and outlet

A common error in the installation of sand filters is that these are installed with the inlet and

outlet around the wrong way. Where single filter units are installed, this can easily occur as

they are literally delivered as a ‘box’ unit. To avoid this occurring, request that the

manufacturer clearly mark out the inlet and outlet.

16.5.18 Materials

All components internally, including inlet and outlet pipes, should be able to withstand

submergence and wetting over periods of time and have chemical resistance to minor

amounts of oils and hydrocarbons.

16.5.19 Lid levels

Whatever the existing surface, consider the finished level particularly in relation to the access

lids. All underground installations must be flush with the finished level. For projects where

the installation of the separator is only part of the works, the tank may need to be installed,

sealed off, marked it necessary, and access lids installed once other works are complete.

16.6 Construction specifications

Specifications for the construction of sand filter are often provided by the suppliers of the

sand filter. However, this is not always the case. Specifications should consider the following:

Table 36

Specifications that should be taken into consideration when constructing sand filters.

Area Items to include

Excavation • Specify the need for verification of ground conditions, excavation

support and removal of excess material.

• Where the excavation is greater than 1.5 m, geotextile material,

shuttering or shoring should be specified, depending on the ground

conditions present.

• The specifications should also call for the confirmation of ground

conditions before the filter is installed.

• Fencing around the excavation will also need to be installed.



Foundations • The bedding material should be confirmed and the specification should

also include any foundation details (i.e. hard fill, blinding layer and

reinforced concrete base (if necessary)).

• Requiring that the base course hard fill is well compacted will ensure

little to no movement of the unit in combination with a small diameter

blinding layer.

• Consider measures needed for buoyancy where the filter will be

installed in areas of high groundwater.

Filter unit • The size and type of separator to be installed. Usually access lids and

other internal components are included with the pre-cast units.

• Ensure that vehicle loading requirements are clearly specified for in -

ground filters, including access lids.

• Include specifications for epoxy and other water sealing agents for

joints, risers, connection pipes, connection between multiple units and

perimeter holes.

Filter media • The type (e.g. sand, peat, zeolite, combination media), grade and

depth of filter media.

• Samples of sand should be submitted for approval prior to installation.

• Sand specifications outlined in ARC TP10 are required when sand is

used as the filter medium. The sand should be clean, washed, inert

and free of contaminants.

• Particular mention should be made to ensure the media is not overly

compacted during installation.

• Hydraulic compaction requirements should also be discussed.

Inlet & outlets • Diameter and pipe type and material.

Backfill • Type of material appropriate for backfilling.

Reinstatement • Specify the final surfacing over the top of the filter.

• Note that most pre-cast units are designed for limited loadings and the

maximum depth of cover/vehicle loadings may need to be specified.

Testing • Specify a watertightness test to be carried out once the filter

installation is complete.

Example specifications are shown in Section 16.9. These cover some of the typical aspects for

construction of sand filters, but exclude particulars such as site est ablishment, health and

safety, testing, materials, and reinstatement, as these differ greatly depending on the

application.

These example specifications do not constitute a full specification for the construction of any

sand filter, and should be used as a ‘starter specification’ for guidance only. Each sand filter


will be site specific and require careful consideration to ensure that all aspects of construction

are covered.

16.7 Construction monitoring

Table 37

Crit ical points to inspect during construction to ensure the device is installed correctly include:

Area Items to monitor

Excavation • Levels match construction plans and specifications.

• Any variation from the design plan may change the levels and

function of the sand filter separator and must be approved by the

design engineer or project manager.

Bedding/backfill

(foundations)

• Check bedding and backfilling meets specifications.

Sand Filter • Review of levels of unit and that the inlet and outlet point in the

correct direction of flow.

• All inlet and outlets are plugged during installation.

• Filter media depth is as per specifications (minimum 400 mm) and

level (must not be compacted).

Prior to lid

installation

• All levels and inverts have been checked against construction plans

and specifications, prior to chamber lids being secured.

• Photograph the filter prior to fitting the lid and include in the

operation and maintenance manual. This will aide any future

troubleshooting.

Access ladders • Access hatches installed and line up with unit ladders and ladders

line up.

Water tightness • All inlet and outlets of the chamber plugged/covered during

installation.

• Water tightness test (refer specifications) shows water loss is less

than 5% of total volume.


16.8 Photo gallery – sand filters

Figure 86

Uncovered Austin sand filter Upper Harbour Highway, note that surface biofilm on the surface which can

clog the filter bed and reduce infiltrat ion rates. [NZTA]

Figure 87

Close up view of the algal crust on the Austin sand filter media on Upper Harbour Corridor. [NZTA]


Figure 88

Sand filter lid placement showing chamber connections.

Figure 89

Sand filter Savil Drive showing pre-cast chambers and backfill.


Figure 90

Sand filter (SF1250) showing baffle wall.

Figure 91

Sand filter Ward Street showing filter media in place.


16.9 Construction specification example – sand filters

16.9.1 Site preparation

16.9.1.1 Clearance

Existing trees around the sand filter site shall be protected and those identified for removal on

the drawings shall be removed. The Contractor shall comply with any conse nt or District Plan

requirements concerning the preservation, trimming or transplanting of trees, shrubs, etc.

16.9.1.2 Topsoil

The Contractor shall strip the topsoil from all areas to be excavated, filled or otherwise

disturbed due to the construction of the contract works, and stockpile it in an approved

location. The depth of topsoil stripped shall be:

at least 300 mm in farmland, residential property or other cultivated areas; or

at least 150 mm elsewhere; or

the full depth of topsoil where less than the above depths exist.

16.9.1.3 Disposal of material and rubbish

All materials arising from site clearance which are surplus to or unsuitable for use in the Works

shall become the property of the Contractor and shall be disposed off the site.

All fences, buildings, structures and encumbrances of any character, except those that are

earmarked for removal by others, upon or within the limits of the site, shall be removed by the

Contractor and disposed of as directed by the Engineer.

16.9.1.4 Site fencing

The work area (excavations, stock-pile areas, etc) shall be adequately fenced to delineate their

extent and to fulfil the Contractor’s obligations of site safety and security, all to the

satisfaction of the Engineer.

16.9.1.5 Drainage

The Contractor shall keep excavations free of water during construction and shall dispose of

the water in an approved manner. During the placing and compacting of material in

excavations, the water level at the locations being refilled shall be maintained below the

bottom of the excavation.

The Contractor shall supply, install, operate and maintain all pumping, plant, pipework,

subdrains and sumps and other equipment necessary for this purpose and shall maintain at

the site at all times, reasonable standby plant in good working condition.


16.9.1.6 Disposal of water

Water from either sand filter installation, surface drainage of the site, or dewatering of

excavations shall be treated (e.g. passed through a silt fence, settling pond or other

treatment) in accordance with ARC requirements prior to discharge, and shall contain no more

than 100 mg L-1

of solids. Once treated to the required level, the water may be disposed of

downstream into the public road stormwater drain or public foul sewer subject to local council

and ARC approval. Where this cannot be achieved, water from excavations must be removed

off-site and disposed of in an approved manner.

Water from any lubrication system used for pipe installation may not be discharged from the

site without the approval of the Engineer.

16.9.1.7 Diversion of existing services

In the case of a sand filter, all stormwater shall be diverted from the location until post

construction and the Engineer is satisfied that the catchment contributing to the sand filter

has been completely stabilised so as to avoid contamination of the filters. This includes

stabilisation of the earthworks and related stockpiling from the installation of the sand filters.

16.9.1.8 Verification of levels

The Contractor shall take sufficient levels or cross-sections of the existing ground surfaces,

including the road surface to confirm the ground profile and levels shown on the drawings and

to ensure that the surfaces are reinstated to the levels existing at the start of the contract

works or shown on the drawings.

The Contractor shall also confirm the invert and lid levels of a ll wastewater and stormwater

lines and verify the depth of telecommunications, power, gas or other services which

transverse the sand filter location or are in the vicinity of the works. Any disagreement or

potential conflicts shall be reported to the Engineer before excavation is commenced.

16.9.2 Supply of sand filter

The manufacturer will typically supply the following components:

Sand filter bases.

Sedimentation chamber.

Sand filter risers.

Precast lids.

Manhole servicing entries.

Concrete lids.

Heavy duty frame and cover.

The contractor will typically provide/supply the following:


Installation of the sand filter on-site and all associated pipework connections.

A suitable crane to lift and install the sand filter components.

Any holes to be drilled into the inlet pipes.

Any sealing material between manhole risers/lids, etc.

All pipes between any units/manhole risers.

The excavation of the hole.

Any additional throat risers.

Sealing of all pipes.

Any site work.

The removal of water from the excavated site to allow installation.

Any fill material to bring the level of the excavation up to the correct height prior to

installation of the sand filter.

16.9.3 Excavation

16.9.3.1 Verification of ground conditions

The Contractor shall include in his price for excavation in all material s encountered, that could

reasonably be anticipated from the geotechnical information provided, and from any further

investigation undertaken by the Contractor prior to Tendering. This shall include rock and/or

tree logs or stumps if geotechnical information suggests that the presence of rock and/or tree

logs or stumps is possible. This shall also include variable material in any filled and alluvium

ground identified in the geotechnical information.

"Rock" shall be material of sufficient strength and extent that in the opinion of the Engineer it

cannot be removed efficiently using a 20 tonne size digger fitted with an appropriate narrow

"rock" bucket, and requires more intensive means for efficient excavation.

16.9.3.2 Over excavation

If the Contractor takes out any material to a greater depth or width than shown on the

drawings or specified, without the instruction of the Engineer the extra depth or width shall be

filled either with concrete, approved hardfill, or excavated material as nominated by the

Engineer, and thoroughly compacted without any extra payment.

16.9.3.3 Excavation in road reserve

Excavation in road reserve shall be carried out in accordance with the specifications and

requirements quoted in the “Code of Practice for Working in the Road” and the LTA

requirements including any additional requirements as defined in the Road Opening Notice

and LTA Infrastructure Design Standards.


16.9.3.4 Stockpiling and removal of excavated material

All excavated material required for fill shall be properly stockpiled with steep faces to allow

maximum drainage in an approved location inside the working area. Stockpiled material shall

be covered with geotextile or polythene if rain is forecast.

Stockpiled material which deteriorates and becomes unsuitable because of avoidable delays,

poor storage arrangements or other circumstances within the Contractor's control shall be

replaced with suitable material at the Contractor's own cost.

Where material is stockpiled off site the proposed stockpile site shall be submitted to the

Engineer for Approval.

16.9.3.5 Support

The Contractor shall support the sides of excavations with suitable shoring to comply with all

safety requirements and so that excessive widening of the excavation is avoided. Support may

be provided by use of shields, timber, sheet piling or other shoring systems, subject always to

the Engineer's agreement of the proposed method. Any such agreement given by the

Engineer shall not absolve the Contractor of this responsibility to minimise the area disturbed

by the works and to make the site safe.

Timbering or sheet piling shall (where possible), be drawn up and removed as the concreting

or backfilling progresses so as to ensure that all voids at the side, or in other places, are filled

as the contract works advance.

16.9.3.6 Excavation support left in place

Where the Engineer considers it necessary for shoring to be left permanently in the work he

may so order in writing, in which case payment will be made at the rate of half the new price

of such timber or sheet piles.

All shoring left in place shall be kept clear of the permanent contract works. The Engineer may

direct that shoring left in place be cut off at any specified level in which case payment will be

made only for the portion remaining in the ground.

The fact of any shoring being left in place, or not being left in place, shall not relieve the

Contractor from any responsibility for any settlement or other damage caused by his

operations. Backfill to all excavations shall meet the strength and compaction requirements

of the specification, and the reinstatement details given on the drawings.

16.9.4 Foundation

Shear strength testing of the bedding material for the foundation may be required by the

Engineer after excavation if the material is significantly different to that expected from the

geotechnical investigations provided.

Excavation for the foundations shall be 300 mm below the invert level of the sand filter boxes.

The founding material will consist of a 150 mm layer of hardfill on top of which will be a


150 mm blinding layer of site concrete with two layers of 665 steel mesh reinforcing. The sand

filters will rest on top of this concrete slab.

The foundations are to be laid to the levels indicated in the drawings to within a tolerance of

±20 mm.

16.9.4.1 Reinforcement

Reinforcing steel shall comply with NZS 3106, NZS 3402 and NZS 3109.

16.9.5 Sand filter unit installation

If not provided, the holes for the inlet and outlet pipes shall be cored prior to lowering the

sand filter unit into place. The Contractor shall use the lifting systems recommended by the

manufacturers of the sand filter units (e.g. use a crane or excavator with lifting strops). Any

minor damage in the filter unit shall be made good by caulking an approved epoxy mortar to

completely fill the voids ensuring a watertight finish. Any badly dama ged components shall

be rejected.

The units will be numbered as per their position location; the contractor shall follow the

manufacturer’s plans to ensure that the units are placed in the correct place (i.e. not back to

front).

All joints between the floor and wall surfaces and any between vertical surfaces shall be made

watertight by the use of an approved epoxy mortar. All work surfaces are to be thoroughly

cleaned before applying and all laitance and grease removed. A 10 mm layer of epoxy mortar

is to be applied evenly over the full contact surface and carefully pressed down and secured in

position.

16.9.5.1 Connections between chambers

PVC pipes shall be installed to connect the chambers at the levels indicated on the drawings.

All PVC pipes shall be manufactured in accordance with AS/NZS 1260 “PVC Pipes and

Fittings” and installed in accordance with NZS 7643 “Installing PVC Pipes” and AS/NZS 2566

"Buried Flexible Pipelines".

16.9.5.2 Epoxy mortar

Epoxy mortar shall be used to seal the filter units joints and inlet and outlet pipe connections.

All constituents of the epoxy mortar (silica sand filler, resin and hardener) supplied shall be of

a brand and specification approved by the Engineer.

Epoxy mortar shall be certified by the manufacturer as follows:

Suitable for permanent immersion in contaminated stormwater.

Suitable for curing to full strength under waterlogged conditions.

Has a service life of 100 years.

No water permeation through thin (10 mm) sections.


Suitable for adhering firmly to concrete to form durable watertight joints.

Suitable for bonding to wet concrete.

Manufacturer's instructions shall be followed strictly in storing, mixing, applying and curing.

After mixing, the mortar shall be used within the time period specified by the manufacturer.

All pre-hardened mortar shall be disposed off-site.

Do not use water and additional sand to mix.

Clean all work surfaces thoroughly before applying. Remove all laitance, free moisture and

grease from the surfaces. Avoid air entrapment by building up successive thin layers. Do not

apply in lumps.

16.9.5.3 Water tightness test

This test shall be undertaken prior to the installation of the filter components. The sand filter

units shall be plugged and filled completely and left for a period of 24 hours. A drop in water

of more than 5% of the total volume of the filter indicates that the sand filter is not watertight

and needs to be corrected before the internal components can be installed. The filter shall be

emptied of water once this test is complete.

The test shall be witnessed by the Engineer. If the filter units fail the test, the defects shall be

fixed and re-tested until the specification is met and the Engineer is satisfied.

16.9.6 Inlets and outlets

The inlet and outlet shall be constructed to connect to the sand filter from the stormwater

network as per the drawings. The diversion weir shall be constructed to the levels specified on

the drawings. The inlet connection joins shall be made watertight with an approved epoxy

mortar.

The connection shall be constructed by either drilling through the completed wall or in the

case of a fully supplied unit; through the inlet opening, subsequently sealing the annular gap

using an approved moisture compatible epoxy mortar. The pipe shall project a minimum of

25 mm and a maximum of 50 mm past the inside face of the sand filter.

Entrances to inlet and outlet pipes shall be plugged to prevent material entering the existing

stormwater system, Engineer to approve the plugging arrangement.

16.9.6.1 Concrete pipes and manhole risers

Concrete pipes and manhole risers shall be manufactured in accordance with NZS 3107:1978

“Precast Concrete Drainage and Pressure Pipes”. Installation shall be in accordance with

AS/NZS 3725 “Loads on Buried Concrete Pipes”.

16.9.6.2 Concrete

Concrete and formwork shall comply with NZS 3109 “Specification for Concrete Construction”

and subsequent amendments.


Concrete supplied to the site shall comply with NZS 3108 “Specification for Concrete

Production – Ordinary Grade”.

Sulphate resistant concrete shall be 80% Duracem, 4% microsi lica and 16% general purpose

Portland Cement.

Concrete shall be Ordinary Grade of strength 17.5 MPa or 20 MPa, as shown on the drawings,

with maximum aggregate size 19 mm and slump 100 mm.

Concrete shall be supplied to site as batched ready mixed concrete from an approved supplier.

The Contractor shall keep a delivery record for each batch delivered to site. This shall record

the supplier, date, time, quantity delivered, mix code, specified strength, aggregate size and

slump.

The Contractor shall carry out a slump test on each batch delivered, and shall allow in his price

for 1 concrete test cube to be taken, cured and tested for each batch delivered. Concrete test

cubes will be required on the instruction of the Engineer.

Concrete surfaces that will be buried shall be surface finish U1 to NZS 3114.

16.9.6.3 Other materials

Where materials or workmanship is not covered by the drawings or this specification, the

requirements of the LTAs Infrastructure Design Standard shall be followed.

16.9.7 Underdrainage

The underdrainage shall be laid in the position detailed on the drawings. The portion of

underdrain through the sand filter shall be perforated or slotted PVC. The underdrainage shall

be wrapped with geotextile material with a large enough mesh to ensure that it does not clog

yet retains the filter media.

The underdrainage shall be backfilled with GAP40 gravel to a height of 50 mm above the pipe,

placed from a maximum height of 1 m above the underdrainage.

16.9.8 Dewatering valve and overflow pipe

The dewatering valve and overflow pipe shall be installed to the levels indicated on the

drawings. The valve shall be shop-fabricated by the pipe supplier or by an approved specialist

fabricator. Fabrication on site will not be permitted.

All surfaces of the body of the valve (inside and out) shall be coated internally and externally

to comply with AS 4158:2003. The valve shall be capable of bi-directional flow of water. The

valve shall be set so that the spindle is truly vertical.

The supplier shall supply with the valves, materials certificates relating to the composition of

the casing material.


16.9.9 Filter media

The filter media shall be installed in 100 mm deep layers to the depths indicated on the

drawings. This shall be: sand, soil, gravel, peat or compost as specified in th e design. The

media shall be compacted hydraulically as follows:

Once the filter media is level with the overflow weir between the sedimentation and

filtration chamber, clean water shall be directed slowly into the sedimentation chamber

until the sedimentation and filtration chambers are completely full (i.e. just below the

overflow level).

The water will then be allowed to drain down through the filter until flow from the

underdrain ceases (this should be able to be monitored through the outlet chamber or

manhole or directly through the access chamber to the filtration chamber).

The filter will then be allowed to dry for a period of 48hrs and then shall be topped up the

filter media to the weir between the sedimentation and filtration chamber (as directed by

the Engineer).

16.9.10 Filter lids and chambers

Filter lids and chambers shall not be placed until all levels within the filter have been checked

and verified by the Engineer. This will involve shooting levels at all inlet, outlet, overflow,

floor and underdrainage levels to confirm ±20 mm accuracy from the drawings. Filter lids shall

be sealed with the approved epoxy mortar.

16.9.10.1 Chamber ladders

Chamber ladders shall be Grade 316 stainless steel and constructed in accordance with the

drawings.

16.9.10.2 Risers

Precast chamber risers shall consist of centrifugally spun sulphate resistant concrete pipes of

minimum Class 1 (S), with holes cast into the side for ladder rungs. The bottom ring shall be

seated in the rebate formed in the chamber base, and the joint sealed with a n approved epoxy

mortar.

Chamber risers shall be installed such that joints between rings are horizontal.

The Contractor shall use the lifting systems recommended by the manufacturers of risers (e.g.

chains with spreader bars). Any minor damage in riser tops shall be made good by caulking an

approved epoxy mortar to completely fill the voids ensuring a watertight joint between the

riser and the lid. Any badly damaged risers shall be rejected.


16.9.10.3 Access hatches, throat, frame and covers

Access hatches shall be heavy duty pre-cast concrete designed to HNHO72 and of a suitable

thickness for the access diameter as set out in the drawings. The joint between the access

risers and lid shall be sealed with an approved epoxy mortar.

The access hatch throat shall be cast in-situ using 17.5Mpa concrete to a smooth finish. The

height of the throat shall not exceed 300 mm. Watertight epoxy bonding shall be provided

between the manhole throat and lid.

Access hatch frames and covers shall be watertight heavy duty ductile iron to AS3996 (80kN

loading), with a clear opening of 600 mm as detailed in drawings.

The frame of the hatch cover shall be fixed to the concrete lid/throat using epoxy mortar.

Concrete (17.5 MPa) haunching shall be provided around the frame.

The height of the manhole throat shall not be greater than 300 mm. The throat and any

subsequent extensions to the throat shall be cast in-situ using 17.5 MPa compacted watertight

concrete to a smooth finish. Plastering of the throat to achieve a smooth finish shal l not be

permitted. Severely honeycombed throats shall be rejected and shall be replaced fully. Any

minor defects shall be made good using epoxy mortar. Pre -cast throats shall not be

permitted. Watertight bonding shall be provided between the throat and the lid, and

between the existing part of the throat and subsequent extension.

16.9.11 Backfilling

The Contractor shall fill in, over and around the sand filter as soon as possible after any

concrete work has attained sufficient strength and after requirements re lating to inspection

and testing have been complied with.

Filling shall, as far as possible, be made up to the previously existing surface levels or to such

other levels shown in the drawings or as agreed by the Engineer as the work proceeds.

Previously excavated fill or engineered fill shall be used for backfilling as agreed with the

Engineer. Fill shall be compacted during filling in lifts up to a maximum of 200 mm thick and

shall be compacted to obtain a minimum dry density of 95%.

Prior to backfilling, all forms and debris shall be removed from the excavation.

16.9.11.1 Engineered fill

Engineered fill shall be approved by the engineer and be free of organic material, or any other

substances including excess moisture, which prevents satisfactory placing and comp action. It

shall be free of clay lumps and stones retained on a 75 mm sieve.

16.9.11.2 Buoyancy

The groundwater levels in the area shall be determined and the completed structure shall be

checked for buoyancy under the range of operating conditions including when some or all

tanks are empty.


17 Oil and Water Separators

17.1 Introduction

Oil and water separators are only found on commercial or industrial sites and are designed to

meet the specific requirements of the site. Contractors involved in installing an oil and water

separator tend to be experienced installers with a good understanding of the functions.


Oil and water separators remove petroleum based oil and grease commonly referred to as

total petroleum hydrocarbons (TPH) from stormwater and small spills in areas where

hydrocarbon products are handled (e.g. substations, petrol stations, airports, refuelling zones,

storage terminals, industrial areas and workshops). TPH in the environment can be present in

a variety of forms including:

Free Oil - removed by separation through an oil and water separator.

TPH associated with solids: gravity separation for coarser sediments and filtration for

finer sediments.

Emulsified oils - Oils that have been associated with a detergent making it effectively

soluble in water. Difficult to remove, typically requires biological activity or caustic

reactions.

Mechanically solubilised oil - Mechanical action like vehicle movements and tyre traffic,

breaks oils into particles so small that they do not behave according to Stokes law but are

governed by electrostatic attraction and brownian motion. To remove this form of oil

requires very long-term settling, electro or chemical coagulation or filtration methods.

Oil and water separators are generally buried underground structures and can come as

package treatment plants or specifically designed for the site. Above-ground devices are also

available. Oil and water separators work by slowing down the inflow of contaminated

stormwater to allow oil droplets to rise to the surface. As the oil droplets combine they rise

faster (oil is less dense than water). Laminar flow (slow non-turbulent flow) is required for

optimum performance, so a critical component is the inlet baffle or screen which reduces the

speed of the water. Once the oil collects on the surface of the water, the clean water at the

bottom of the device is directed to the outlet by baffles and is then discharged from the

device. The oil collected on the surface of the device is generally removed by vacuum during

device maintenance.

Oil and water separators function by:

Slowing down stormwater flows from a small treatment area (e.g. service station,

refuelling depot, workshop) to allow oils, greases and other hydrocarbons to become

trapped and improve discharge water quality.


Removes 90 to 95% of oil and grease to 15 mg L-1

of oil and grease when properly sized.

Underground devices can be used in built up areas, industrial areas and car parks.

Above ground devices can be installed or retrofitted to pre-existing infrastructure.

Figure 92

Key components of an oil/water separator.


Table 38

Key components of an oil/water separator.

Device component Description

Inlet chamber with

baffle

Stormwater runoff enters the oil/water separator from the treatment

area only (via piped stormwater system). This chamber is where the

stormwater runoff is slowed down to allow the oil to start rising.

Collection chamber Oil collects on the surface, and in the case of the plate separator, is

where the plate pack is located.

Sediment baffle Sediment baffle is approximately 300 mm high from the base of the

collection chamber and located prior to the oil retaining baffle. It

makes sure sediment stays in the collection chamber to allow free

flow under the oil retaining baffle.

Plate pack (plate

separator only)

Plate packs are made of oleophilic materials that attract oil droplets

to their surface but do not adhere to them. As the oil droplets

become attracted, they collide and coalesce into bigger droplets

which increases the rate at which they separate from the water and

rise to the surface of the device.

Oil retaining baffle The oil retaining baffle leaves a 300 mm gap at the bottom to allow

clean stormwater to pass under it. This ensures all oil is retained

within the collection chamber.

Shut-off valve The valve controls the flow of stormwater exiting the separator. It

can be closed to trap spills and also for maintenance work on the unit.

Outlet chamber and

outlet pipe

This chamber collects the clean stormwater and discharges it to

either a piped system or directly to the receiving environment (i.e.

waterways such as streams and the coast).

All Auckland Regional Council (ARC) approved oil/water separators can be categorised as one

of the following:

Coalescing Collection Plate Separators (also simply called Plate Separators) – the plate

separator device slows the incoming water to a slow speed to allow time for the oil to rise

toward the surface. The contaminated water passes over a pack of closely spaced (10 mm)

plates that capture the oil. Oil droplets are attracted to the surface of the plate and then rise

to the surface of the tank where they settle out and are captured by baffles or skimmers.

Treated water passes under a baffle to ensure floating oils are retained within the storage

chamber.

API (American Petroleum Institute) Separators – the API device slows the incoming water to

allow time for the oil in the water to rise to the surface. Baffles keep the oil on the surface of

the storage section. Clean water passes under the baffle and out the outlet pipe.


Figure 93

Typical plate separator Example 1 (Source: Mbeychok, 2007)

Figure 94

Typical API separator (Source: ARC, 2003).

http://en.wikipedia.org/wiki/User:Mbeychok


Figure 95

Typical plate separator Example 2 (Source: ARC, 2003).


Guidelines relating particularly to oil and water separators include the following. Note that a

detailed review of council standards and guidelines should be carried out for every project, as

there are particular requirements for each council and these are frequently updated.


Table 39

Guidelines relat ing to oil and water separators.





Auckland Region






Region




development.

NSCC LB110 Other Technologies Practice note based on the ARC ’s TP10,

describing requirements for alternative

technologies to meet the Long Bay water

quality and quantity management objectives.






WCC Stormwater Solutions for

Residential Sites

Document providing guidance on

management practices applicable to

developments on individual residential lots

(<1000 m²). For use by engineers and

applicants for stormwater control building

permits for developments of this size.



Table 40

Below is a non–exhaustive list of some of the more applicable standards and technical documents that

relate to oil/water separators:

Title Description

AS4087:2004

Metallic Flanges for

Waterworks Purposes

Specifies requirements for circular flanges manufactured from

copper alloy, ductile cast iron, grey cast iron, or steel. The flanges

covered are for use on pipes, pipe fittings, valves and other

equipment that are primarily intended for the conveyance or

storage of water or wastewater.

AS4158:2003

Thermal-Bonded

Polymeric Coatings on

Valves and Fittings for

Water Industry Purposes

Specifies the requirements for factory-applied thermal-bonded

polymeric corrosion protective coatings (barrier coatings). For the

purposes of this Standard, the term polymeric coating is intended

to encompass both thermoplastic and thermosetting coating

materials, which are used to coat both the internal and external

surfaces of valves and fittings.

AS4795:2006

Butterfly Valves for

Waterworks Purposes

Specifies the requirements for manually operated resilient seated

wafer, tapped lugged, and flanged butterfly valves for waterworks

purposes. The standard covers manual actuators, gearboxes and

standard spindle caps. Water supply applications include drinking

water and recycled water as well as screened wastewater.

AS/NZS 1254:2002

PVC Pipes and Fittings

for Stormwater and

Surface Water

Applications

Specifies requirements for PVC pipes and fittings for conveyance

of stormwater or surface water. The Standard includes

requirements for both plain and structured wall pipes and fittings.

AS/NZS 2033:2008

Installation of

Polyethylene Pipe

Systems




liquids.

AS/NZS 2566.2:2002

Buried Flexible Pipelines

- Installation




AS/NZS 4671:2001

Steel Reinforcing

Materials

Sets out specifications for Australian and New Zealand Steel

reinforcing materials (bars, coils and welded mesh).

Requirements for chemical, mechanical and physical properties

for three different strength grades and the three different ductility

classes are also covered.


Title Description

NZS 3101.1&2:2006

Concrete Structures

Standard

The design of concrete structures, specifies minimum



NZS 3104:2003

Specification for

Concrete Production


concrete.

NZS 3109:1997



reinforced concrete, unreinforced concrete, pre-stressed concrete

or a combination, in elements of any building or civil engineering

structure.

NZS 3114:1987

Specification for

Concrete Surface

Finishes



exposed aggregate and on floors, exterior pavements and inverts.



The oil and water separator can be installed while other works continue as it is relativ ely

simple to isolate depending on what provisions have been made in the design. Normally the

inlet and outlet can be blocked off at manholes. The shut down valve should not be used for

this purpose as any sediment can transfer to the device once the val ve is open – this should

always be done in manholes where possible.

To avoid siltation of the device, the unit should not be made operational until all works on the

site have been completed.


Normally only small excavations are required for the installation of separators and this can

usually be done without the need for an earthworks consent and at all times throughout the

year.

17.5.3 Health and safety

If underground, ensure that the tank will be level with the surrounding ground and lids flush

with the ground surface on completion so that there are no tripping hazards.


17.5.4 Operation and maintenance

In an emergency, oil spill personnel will need to close the shut off valve. Consider including

signage inside the Toby box so personnel will be able to quickly determine the direction it

needs to be turned for shut off.

Valve keys should be provided close to the oil separator for this purpose.


Where the separator is to be installed underground, ensure that there are no existin g

underground service utilities where the treatment device is to be installed. The inlet and

outlet pipes, and the connection to them should also be considered.

17.5.6 Trees and vegetation

Most oil and water separators are relatively small and are predominantly located in industrial

and commercial applications, so trees and vegetation is not a problem.

17.5.7 Existing structures

Most separators will need to be lifted into place. Consider structures such as fences that may

need to be removed to allow for this.

17.5.8 Water supply

Water supply will be needed for the water tightness test.

17.5.9 Site access

Most oil and water separators weigh several tons and will need to be installed by a large

excavator, hi-ab truck or cranes. This often depends on the depth the device will be installed

at. The site should be inspected to ensure that access for machinery and equipment, and their

use (i.e. turning movements) are available. It is also necessary to confirm clearance from

overhead lines and other services.

17.5.10 Excavation

The site for the separator must allow enough room for the excavation and installation of the

associated inlet and outlet pipes.


17.5.11 Buoyancy

If the device is located in an area with high groundwater table, the device may become

buoyant when emptied. Sub-soil drainage around the base of the unit will ensure that does

not occur.

17.5.12 Foundations

It is essential the foundation of the oil and water separator unit is compacted and levelled as

per the construction plans. Any movement of the unit, potentially from base course

subsidence can cause cracking in the unit and splitting of inlet or outlet connections. Because

these units are often buried leaks and structural failures may go unnoticed for some time

causing contamination of surrounding soil, groundwater and or surface waterways.

17.5.13 Oil and water separator

Consider pre-fabricated unit sizing. Oil and water separators are often prefabricated off site

with internal components already installed. This is often cheaper than constructing a

separator on site.

17.5.14 Plate inclination and spacing

With parallel plate units the plates can clog easily if the inclination is too shallow or the

spacing too narrow. During the design phase the contaminant levels within the catchment

should be checked to ensure plate inclination and spacing is appropriate for the types and

levels of contaminates.

17.5.15 Plate slope

During construction check that the plate slope is approximately 60o as this angle will ensure

that most solids will not stick to the plates, potentially causing a blockage.

17.5.16 Materials

All components internally, including inlet and outlet pipes, should be able to withstand

submergence and wetting over significant periods of time and have chemical resistance to oils

and hydrocarbons.

17.5.17 Lid levels

Whatever the existing surface, consider the finished level particularly in relation to the access

lids. All underground installations must be flush with the finished level. For projects where

the installation of the separator is only part of the works, the tank may need to be installed,

sealed off, marked it necessary, and access lids installed once other works are complete.




construction of oil and water separators, but exclude particulars such as site establishment,

health and safety, testing, materials, and reinstatement, as these differ greatly depending on

the application.

Table 41

Assuming a pre-fabricated API separator is to be installed, specifications will need to include:


Excavation • Specify the need for verification of ground conditions, excavation

support and removal of excess material.

• Where the excavation is greater than 1.5 m, geotextile material,

shuttering or shoring should be specified, depending on the ground

conditions present.

• The specifications should also call for the confirmation of ground

conditions before the separator is installed.

• Fencing around the excavation will also need to be installed.

Foundations • The bedding material should be confirmed and the specification should

also include any foundation details (i.e. hard fill, blinding layer and

reinforced concrete base (if necessary) including any sub-soil drainage

requirements.

• Requiring that the base course hard fill is well compacted will ensure

little to no movement of the unit in combination with a small diameter

blinding layer.

Oil separator • The size and type, and often brand of separator to be installed.

• Ensure that vehicle loading requirements (if needed) are clearly

specified, including access lids.

• Include specifications for epoxy and other water sealing agents for

joints, risers, connection pipes and perimeter holes.

• Specify the angle of the plate slope once installed to be 60°.

Shut-off valve • The diameter of the valve and type of material to be used.

• The specifications should include for the installation of a Toby box and

provision for (clearly) labeling the direction to close and open the

valve.

Outlet pipe • Diameter and pipe type and material.

Backfill • Type of material appropriate for backfilling.

• Consider measures needed for buoyancy where the separator will be

installed in areas of high groundwater.



Reinstatement • Specify the final surfacing over the top of the separator.

• Note that most pre-cast units are designed for limited loadings and the

maximum depth of cover/vehicle loadings may need to be specified.

Materials • Specify the type of materials that are allowed to be used.

Testing • Specify a water tightness test to be carried out at the completion of

the installation.


separator, and should be used as a ‘starter specification’ for guidance only. Each oil and water

separator will be site specific and require careful consideration to ensure that all aspects of

construction are covered.


Table 42



Excavation Levels match construction plans and specifications.

Any variation from the design plan may change the levels and

function of the oil/water separator and must be approved by

the design engineer or project manager.

Bedding/backfilling

(Foundations)

Check bedding and backfilling meets specifications.

Oil and water

separator

Review of levels of unit and that the inlet and outlet point in the

correct direction of flow.

Check the plate slope is approximately 60o.

Prior to lid

installation

All levels and inverts have been checked against construction

plans and specifications, prior to chamber lids being secured.

Photograph the area and separator unit without the lid for

inclusion in the operation and maintenance manual, this will

assist in any future troubleshooting.

Access ladders Access hatches installed and line up with unit ladders and

ladders line up.

Watertightness All inlet and outlets of the chamber plugged/covered during

installation.

Water tightness test (refer specifications) shows water loss is

less than 5% of total volume.


17.8 Photo gallery – oil and water separators

Figure 96

API oil separator being installed.

Figure 97

Above ground oil plate separator.


Figure 98

Above ground oil plate separator.

17.9 Construction specification example – oil and water separators


17.9.1.1 Clearance

Existing trees around the oil and water separator site shall be protected and those identified

for removal on the drawings shall be removed. The Contractor shall comply with any consent

or District Plan requirements concerning the preservation, trimming or transplanting of trees,

shrubs, etc.

17.9.1.2 Topsoil









All materials arising from site clearance that are surplus to or unsuitable for use in the Works,

shall become the property of the Contractor and shall be disposed of off-site.








17.9.1.5 Drainage

The Contractor shall keep excavations free of water during construction and shal l dispose of

the water in an approved manner. During the placing and compacting of material in

excavations, the water level at the locations being refilled shall be maintained below the

bottom of the excavation.





Water from oil and water separator installation, surface drainage of the site, or dewatering of


treatment) in accordance with ARC requirements prior to discharge, and shall contain no more

than 100 mg L-1

of solids. Once treated to the required level, the water may be disposed of

downstream into the public road stormwater drain or public foul sewer subject to local council

and ARC approval. Where this cannot be achieved, water from excavations must be removed

off-site and disposed of in an approved manner.



17.9.1.7 Diversion of existing services

In the case of an oil/water separator, all stormwater shall be diverted from the location until

post construction and the Engineer is satisfied that the catchment contributing to the

oil/water separator has been completely stabilised so as to avoid contamination. This includes

stabilisation of the earthworks and related stockpiling from the installation of the oil/water

separators.


17.9.2 Supply of oil and water separator

The manufacturer will typically supply the following components [e.g. for an API Separator]:

Oil and water separator bases.

Grill/plate pack.

Baffle.

Oil and water separator risers.

Precast lids.

Manhole servicing entries.

Concrete lids.

Heavy duty frame and cover.

Shut off valve.

The contractor will typically provide/supply the following:

Installation of the oil and water separator on-site and all associated pipework

connections.

A suitable crane to lift and install the oil and water separator components.

Any holes to be drilled into the inlet pipes.

Any sealing material between manhole risers/lids, etc.

All pipes between any units/manhole risers.

The excavation of the hole.

Any additional throat risers.

Sealing of all pipes.

Any site work.

The removal of water from the excavated site to allow installation.

Any fill material to bring the level of the excavation up to the correct height prior to

installation of the oil and water separator.

17.9.3 Excavation


The Contractor shall include in his price for excavation in all materials encountered, that could

















requirements quoted in the “Code of Practice for Working in the Road” and the local council


and local council Infrastructure Design Standards.










17.9.3.5 Support


safety requirements and so that excessive widening of the excavation is avoided. Support

may be provided by use of shields, timber, sheet piling or other shoring systems, subject

always to the Engineer's agreement of the proposed method. Any such agreement given by

the Engineer shall not absolve the Contractor of this responsibility to minimise the area

disturbed by the works and to make the site safe.









All shoring left in place shall be kept clear of the permanent contract works. The Engineer

may direct that shoring left in place be cut off at any specified level in which case payment will

be made only for the portion remaining in the ground.





17.9.4 Foundation




Excavation for the foundations shall be 300 mm below the invert level of the oil/water

separator boxes. The founding material will consist of a 150 mm layer of hardfill on top of

which will be a 150 mm blinding layer of site concrete with two layers of 665 steel mesh

reinforcing. The oil/water separators will rest on top of this concrete slab.

The foundations are to be laid to the levels indicated in the drawings to within a tolerance of

±20 mm.

17.9.4.1 Reinforcement

Reinforcing steel shall comply with NZS 3402 and NZS 3109.

17.9.5 Oil and water separator unit installation

If not provided, the holes for the inlet and outlet pipes shall be cored prior to lowering the oil

and water separator unit into place. The Contractor shall use the lifting systems

recommended by the manufacturers of the oil and water separator units (e.g. use a crane or

excavator with lifting strops). Any minor damage in the separator unit shall be made good by

caulking an approved epoxy mortar to completely fill the voids ensuring a watertight finish.

Any badly damaged components shall be rejected.

The contractor shall follow the manufacturer’s plans to ensure that the components (i.e.

baffle, grill/plate pack) are placed in the correct place (i.e. inlet and outlet pipes face the

correct way).

All joints between the floor and wall surfaces and any between vertical surfaces shall be made

watertight by the use of an approved epoxy mortar. All work surfaces are to be thoroughly

cleaned before applying and all laitance and grease removed. A 10 mm layer of epoxy mortar


is to be applied evenly over the full contact surface and carefully pressed down and secured in

position.

17.9.5.1 Epoxy mortar

Epoxy mortar shall be used to seal the separator unit joints and inlet and outlet pipe

connections. All constituents of the epoxy mortar (silica sand filler, resin and hardener)

supplied shall be of a brand and specification approved by the Engineer.

Epoxy mortar shall be certified by the manufacturer as follows:

Suitable for permanent immersion in contaminated stormwater.

Suitable for curing to full strength under waterlogged conditions.

Has a service life of 100 years.

No water permeation through thin (10 mm) sections.

Suitable for adhering firmly to concrete to form durable watertight joints.

Suitable for bonding to wet concrete.

Manufacturer's instructions shall be followed strictly in storing, mixing, applying and curing.

After mixing, the mortar shall be used within the time period specified by the manufacturer.

All pre-hardened mortar shall be disposed off-site.

Do not use water and additional sand to mix.

Clean all work surfaces thoroughly before applying. Remove all laitance, free moisture and

grease from the surfaces. Avoid air entrapment by building up successive thin layers. Do not

apply in lumps.

17.9.5.2 Water tightness test

This test shall be undertaken prior to the installation of the filter components. The oil and

water separator units shall be plugged and filled completely and left for a period of 24 hours.

A drop in water of more than 5% of the total volume of the filter indicates that the oil/water

separator is not watertight and needs to be corrected before the internal components can be

installed. The filter shall be emptied of water once this test is complete.

The test shall be witnessed by the Engineer. If the filter units fail th e test, the defects shall be

fixed and re-tested until the specification is met and the Engineer is satisfied.

17.9.6 Backfilling

The Contractor shall plug the entrance to the inlet and outlet to ensure no drainage material

or joint compound enters the stormwater system. Backfill to lid level around the oil and water

separator as soon as possible after any concrete work has attained sufficient strength and

after requirements relating to inspection and testing have been complied with.


Filling shall, as far as possible, be made up to the previously existing surface levels or to such

other levels shown in the drawings or as agreed by the Engineer as the work proceeds.

Previously excavated fill or engineered fill shall be used for backfilling as agreed with the

Engineer. Fill shall be compacted during filling in lifts up to a maximum of 200 mm thick and

shall be a minimum in-situ density as the surrounding parent material.

Prior to backfilling, all formwork and debris shall be removed from the excavation.



substances including excess moisture, which prevents satisfactory placing and compaction. It

shall be free of clay lumps and stones retained on a 75 mm sieve.

17.9.6.2 Buoyancy


checked for buoyancy under the range of operating conditions including when some or all

tanks are empty.

17.9.7 Oil and separator lid

The separator lid section shall not be placed until all levels within the oil/water interceptor

have been checked and verified by the Engineer. This will involve shooting levels at all inlet,

outlet, overflow and floor levels to confirm ±20 mm accuracy from the drawings. The

separator lid shall be sealed with the approved epoxy mortar as detailed in item 5.2.

The lid section locates onto the vault wall with a 20 mm perimeter rebate, and is placed so

that the protruding cast-in reinforcing bars sit within the corresponding holes in the lid. Once

the lid is correctly lowered onto the vault, the perimeter holes are filled with high strength

grout “Sika 212” brand or equivalent as approved by the engineer.

The access chambers shall be installed as per the drawing to line up with the existing access

ladders or rungs already installed. The oil and water separator shall then be backfilled to the

final construction levels as detailed on the drawings. Backfilling shall be as per Clause 6:

Backfilling.

17.9.8 Inlets and outlets

The inlet and outlet shall be constructed to connect to the oil/water separator from the

existing network as per the drawings. The inlet weir shall be constructed to the levels

specified on the drawings.

The connection shall be made through the inlet and outlet openings, subsequently sealing the

annular gap using an approved moisture compatible epoxy mortar. The pipe shall project a

minimum of 25 mm and a maximum of 50 mm past the inside face of the oil and water

separator.


The inlet and outlet pipe material and dimensions are detailed on the drawings.

17.9.9 Underdrainage

The underdrainage shall be laid underneath the separator in the position detailed on the

drawings. The underdrainage shall be novacoil subsoil drains backfilled with material as

specified in Clause 6: Backfilling.

17.9.10 Risers

1050ND riser sections for access shall be fitted to the pre-cored lid in the appropriate locations

as located on the drawings.

17.9.10.1 Manhole risers

Precast manhole risers shall consist of centrifugally spun sulphate resistant concrete pipes of

minimum Class 1 (S), with holes cast into the side for ladder rungs. The bottom ring shall be

seated in the rebate formed in the manhole base, and the joint sealed with an approved epoxy

mortar.

Manhole risers shall be installed such that joints between rings are horizontal.

The Contractor shall use the lifting systems recommended by the manufacturers of risers (e.g.

chains with spreader bars). Any minor damage in riser tops shall be made good by caulking an

approved epoxy mortar to completely fill the voids ensuring a watertight joint between the

riser and the lid. Any badly damaged risers shall be rejected.

17.9.10.2 Access hatches, throat, frame and covers

Access hatches shall be heavy duty pre-cast concrete designed to HNHO72 and of a suitable

thickness for the access diameter as set out in the drawings. The joint between the access

risers and lid shall be sealed with an approved epoxy mortar.

Rebated access cover transition blocks shall be cut to size and installed and fixed to the

concrete lid using epoxy mortar.

Access hatch frames and covers shall be watertight heavy duty ductile iron to AS3996 (80 kN

loading), with a clear opening of 600 mm as detailed in drawings.

The frame of the hatch cover shall be fixed to the concrete lid/throat using epoxy mortar.

Concrete (17.5 MPa) haunching shall be provided around the frame.

The height of the manhole throat shall not be greater than 300 mm. The throat and any

subsequent extensions to the throat shall be cast in-situ using 17.5 MPa compacted watertight

concrete to a smooth finish. Plastering of the throat to achieve a smooth finish shall not be

permitted. Severely honeycombed throats shall be rejected and shall be replaced fully. Any

minor defects shall be made good using epoxy mortar. Pre-cast throats shall not be

permitted. Watertight bonding shall be provided between the throat and the lid, and

between the existing part of the throat and subsequent extension.


17.9.10.3 Manhole ladders

Manhole ladders shall be Grade 316 stainless steel and constructed in accordance with the

drawings.

17.9.11 Shut off valve

The shut off valve shall be installed to the levels indicated on the drawings. The valve shall be

a butterfly valve with a 225 mm Toby box for access and inspection.

The valve shall be manufactured to AS 4795 and coated internally and externally to AS

4158:2003. The disc shall be stainless steel 316 with the flanges to AS 4087. The valve shall be

high strength ductile iron constructed to AS 1831 with stainless steel bolts. The valve shall

have a lever for manual operation; adjustable stops shall be provided to prevent over-travel of

the valve disc in either direction.

The supplier shall supply with the valves, materials certificates relating to the composition of

the casing material.


18 Permeable Paving

18.1 Introduction

There is an increasing range of porous surfaces becoming commercially available, including

concrete blockpavers, porous concrete blocks, porous concrete, porous asphalt and resin -

bound aggregate. The use of these porous surfaces for permeable paving systems is relatively

new in New Zealand, therefore construction techniques and material specifications are

constantly being improved based on experience and lessons learnt. Check with

manufacturers and experienced contractors when laying a permeable paving system to ensure

that techniques and materials specified in the construction plan are the most appropriate

given any new developments.


Permeable paving are paving systems that are specially designed to remove pollutants from

stormwater runoff and decrease peak stormwater runoff from hard surfaces (impervious

surfaces). They are designed to allow runoff to filter through a coarse graded pavement to an

underlying gravel layer where treated runoff is temporarily stored before draining slowly

through to the ground or via a collection pipe to the local stormwater system.

In New Zealand permeable paving is generally constructed of modular pavers or blocks. There

is currently little use of porous asphalt or porous concrete. Permeable paving is typica lly used

for small drainage areas (catchments) only in low volume traffic areas such as low trafficked

car parks, driveways, footpaths and sidewalks. Due to its potential to clog with sediments,

permeable pavement is not appropriate for high-traffic areas or in areas subject to heavy

sediment loads.

Permeable paving is commercially available in a number of different configurations, shapes,

colours and brand names. The paver blocks can be of an open-cell design (commonly known

as modular blocks) so that when placed they resemble a honeycomb pattern and the spaces in

between filled with coarse sand or pea gravel and is often grassed (e.g. Gobi blocks). They can

also form a tighter interlocking design (e.g. porous blocks or paver blocks). These interlocking

concrete blocks are either solid blocks (which rely on the water filtering through the gravel

filled gaps between the blocks), or porous blocks (where the water filters through the actual

block).

Table 43 outlines the key components of a typical permeable paving system. Variations may

occur depending on local conditions and site constraints. The components are listed from the

base of the system to the top.


Figure 99

Typical permeable pavement arrangement.

Figure 100

Permeable pavement system showing connection to catch pit and underdrainage system.


Table 43

Key components of a permeable paving system.

Device

component

Description

Sub-grade The sub-grade must be of sufficient strength and durability to not

degrade with the wetting and drying action over the life of the pavement.

Impermeable

liner

Impermeable liners are required in geotechnically sensitive areas where

water cannot be allowed to infiltrate through into the sub-grade, or where

the structural integrity of the pavers (due to traffic load) requires it.

In clay, low permeability sub-grades, or when an impermeable liner is

used, an underdrain is required to collect and discharge infiltrated water

to the local stormwater system. Most paving systems in the Auckland

region will have an impermeable layer and underdrain system.

Geotextile Geotextile is placed between layers to prevent the movement of fine

sediment between the layers and aid filtration. Geotextile must be

secured at edges of paving area and all joins overlapped. Note the

geotextile layer can lead to internal clogging and reduced permeability

and should only be used when additional tensile strength is required.

Base course High void ratio (up to 30% voids for water storage) basecourse material

that will not break down with the wetting and drying action over the life

of the pavement must be used.

Bedding

material

This is a high permeability material varying between a coarse sand to fine

gravel size (2 to 5 mm) depending on the type of paving system. Fine

gravel (~5 mm) is generally preferred to improve permeability and reduce

the risk of clogging.

Pavers Permeable pavers can be separated into three broad categories:

Open cell pavers – a concrete or plastic grid with spaces in between filled

with sand or pea gravel and is often grassed (e.g. Gobi blocks)

Solid block interlocking pavers – which rely on the water filtering through

the gravel filled gaps (generally 6 to 8 mm wide) between the blocks

Porous interlocking blocks – which rely on the water filtering through the

porous block itself.

Porous asphalt Porous asphalt is a standard asphalt mix from which fine aggregate is

omitted. The remaining larger sized aggregates leave open voids that

allow for porosity and permeability of stormwater.

Permeable

Concrete

There is currently no New Zealand standard for the mix, specification and

construction of porous concrete surfaces. Comprehensive standards are

available from the American Concrete Institute for design, specification

and installation which can be adopted until similar guidelines for New

Zealand conditions are developed.


Device

component

Description

Edge beams Edge beams (e.g. 300 mm x 300 mm concrete) should be provided around

all edges of the permeable paving to prevent pavers from getting

displaced.

Overflow

system

Overflow systems (e.g. catchpits etc) need to be designed into the system

to take excess flows during large storm events and as a precautionary

measure in case the pavers block up with silt over time.

Underdrainage Conveys flows that have percolated through the pavers to the stormwater

system. This is not always required or needed.


Guidelines relating particularly to permeable paving include the following. Note that a

detailed review of council standards and guidelines should be carried out for every project, as

there are particular requirements for each council and these are frequently updated.

Table 44

Guideline documents relat ing part icularly to permeable paving.





Auckland Region






Region




development.




Network

Provide guidance on a range of issues relating



Zealand.

NSCC LB102 On Site Stormwater

Mitigation

Practice note to assist choice of appropriate,

cost effective stormwater management

solutions and explains how to select pre-

approved solutions using a stormwater

migration factor or proposal alternative

technologies.



NSCC LB104 Management of

Driveway Runoff

Practice note to assist with the management of

driveway runoff in the Long Bay area including

specific requirements for stream protection

areas.

NSCC LB109 Primary and Secondary

Stormwater Systems

Practice note for design of primary and

secondary overland flowpaths.

NSCC LB201 Minimising Impervious

Areas

Practice note providing ideas for minimising

impervious areas.

NSCC LB203 Pervious Paving Practice note describing the benefits and

considerations for design and construction of

pervious paving.

NSCC LB301 Stormwater Treatment

for Roads

Practice note describing the on-site mitigation

of stormwater runoff that must be

incorporated into roading design to meet the

rules of the Long Bay Structure Plan Area.

NSCC LB303 Erosion – Sediment

Control Subdivision

Practice note providing guidance on the

requirements and descriptions of erosion and

sediment control practices at the subdivision

level.

NSCC Stormwater Management

Practice Note NSC 11:

Pervious Paving

Practice note to provide guidance in meeting

the minimum requirements proposed in Plan

Change 22 relating to pervious paving.

NSCC,

Ecowater,

RDC

Permeable Pavement Design

Guidelines, September 2004

Design guidelines for permeable pavements.








Table 45

Below is a list of some of the more applicable standards and technical documents that relate to

permeable paving:

Title Description

NZS 3101.1&2:2006

Concrete Structures

Standard

The design of concrete structures specifies minimum requirements

for the design of reinforced and pre-stressed concrete structures.

NZS 3104:2003

Specification for

Concrete Production


concrete.

NZS 3109:1997


Provides minimum requirements for the construction of reinforced

concrete, unreinforced concrete, pre-stressed concrete or a

combination, in elements of any building or civil engineering

structure.

NZS 3114:1987

Specification for

Concrete Surface

Finishes

Categorizes by means of description and illustration various classes

of surface finish of concrete obtained off-the-form, by exposed

aggregate and on floors, exterior pavements and inverts.

SNZ HB 2002:2003


Working in the Road

This handbook deals with aspects of the roles and responsibilities of

Road Controlling Authorities, principal providers, utility operators

and contractors; consents and work approvals; and details of

construction requirements; for the purpose of installation and

maintenance of utilities within the road corridor.

TNZ SP/SP11

Specification for

Porous Asphalt

Transit New Zealand specification for porous asphalt.

ACI 522.1-08

Specification for

pervious concrete

pavements

American Concrete Institute 2008 specification document for

pervious concrete pavements.

ACI 522R-06

Pervious Concrete

American Concrete Institute 2008 specification document for

pervious concrete pavements.




If the permeable paving system is part of other site works they should be installed l ast to

prevent clogging from other activities which may create excess sediment loads in runoff. The

surrounding area should be fully stabilised prior to work commencing to minimise this risk.

18.5.2 Permeable paving location

The topography and geology of the surrounding land plays a critical role in site selection and

design of permeable pavement system. Permeable paving works best in gently-sloping areas

(i.e. grades less than 5%) with well-drained soils and deep groundwater tables. In areas with

high groundwater tables or poorly draining soils, permeable paving may not be a suitable

treatment option.

Permeable pavement applications are commonly used to low volume traffic areas i.e. non -

commercial car parks, residential roads, and driveways as well as footpaths, but can be

designed for commercial use. However, high volume traffic areas and heavy-load areas (such

as intersections, loading docks, commercial areas, etc) tend to generate significantly larger

amounts of pollutants, especially suspended solids, oils and greases that can quickly clog the

pores in permeable paving. In addition, they do not normally meet the structural strength

criteria for heavy traffic loading.

They should also not be located in areas where spills are likely (such as docks, petrol sta tions,

etc) as this will also clog the pores. If the pavers clog then they function like a conventional

sealed road or parking area with no stormwater treatment. Spills will also contaminate the

underlying infiltration media and become a source of contaminants to stormwater discharges.

Where permeable pavements are used as part of the road carriageway (trafficked area) or

adjacent to the carriageway (such as car parking areas) it is critical to ensure that the sub-

grade is not saturated in order to protect the integrity of the road. The use of impermeable

liners and underdrain systems for permeable pavement systems within carriagew ays is

therefore recommended.

In order to prevent groundwater contamination, permeable pavement systems that rely on

infiltration should not be used in manufacturing or industrial areas where stormwater runoff is

potentially laden with dissolved pollutants and heavy metals (e.g. copper, zinc, lead) or where

spills are likely (such as petrol stations, docks, etc).

Permeable paving systems only function in specific site conditions. The wrong site will mean

that no matter how well designed or constructed the permeable paving system will not

function at maximum efficiency or not at all. Provided the site is suitable the following

aspects of the construction process are critical.


18.5.3 Protection of existing road surface

Normal road pavement design makes special provision for ensuring that water cannot enter

the road sub-base. This is perhaps a direct conflict to permeable paving which is designed to

filter stormwater run-off through the road surface.

Permeable paving is used commonly for parking areas adjoining the road carriageway. In

these situations it is imperative that water filtering through the pavers is kept away from the

main roadway. Failure to do so causes subsidence of adjacent roading areas, due to the

washing out of fines. Liners need to be specified to keep stormwater from the pavers away

from the existing road sub-base.

18.5.4 Laying

Pavers should be laid starting at the lowest point of the area and work uphill. This is necessary

because pavers can settle and slip downhill if laid from the top.

18.5.5 Sub-grade

The sub-grade and paver laying must be completed as per the construction plans to ensure

the pavers function correctly. Ensure adequate compaction of the sub-base takes place.

Where exfiltration is a desired outcome, a number of techniques can be employed to increase

exfiltration once the sub-grade has been compacted. These include the addition of

sand/gravel filled boreholes, subsoil ripping or the addition of shallow, gravel filled trenches.

Each method acts to increase the contact surface area available for exfilt ration to the

underlying soils.

Too much movement of the pavers through the wetting and drying of bedding and sub-grade

will change the levels of the area and cause ponding and channelling of flow which can erode

bedding material.

18.5.6 Bedding material

Ensure that materials used in the pavement system are free of fine sediments. Most of the

total suspended sediment coming from the Birkdale permeable paving trial site was due to

the 2 to 5 mm chip used as joint material, not the stormwater coming into the pavement. The

general preference is for use of fine gravel ~5 mm to improve infiltration and reduce

construction clogging risk.

18.5.7 Underdrain

The underdrain of a permeable paved area connects to the stormwater reticulation system to

carry away the stormwater. During construction care must be taken to ensure the underdrain

is laid with no kinks and that the bedding gravel is laid carefully over the underdrain so as not

to crush the pipe. Design details also need to consider the loading on the pipe by traffic loads

and ensure structural integrity of the pipe or protective stru cture avoid pipe being crushed.


All underdrains should have a minimum slope of 0.5% where possible and need to be

minimum 100 mm diameter.

18.5.8 Paver laying and edging

Concrete edging is essential for ensuring a secure fit of the pavers and to prevent migration of

bedding material or sub-grade. When laying the pavers they must be laid from the lowest

point to the highest point. Manufacturer guidelines for installation of pavers should be

followed.

Ensure pavers are fitted tightly together so that structural integrity of the pavers is upheld,

and to prevent weeds from growing and the bedding material from being clogged.

18.5.9 Relaying of pavers

Individual pavers may need to be re-laid after one year to allow for the effects of initial

compaction. Manufacturer guidelines for installation of pavers should be followed.


Table 46

Specifications for construction of permeable paving will need to detail:


Sub-grade If not a retrofit of an existing carriageway, the sub-grade must be

specified.

This will need to include the type and grade of material and details

relating to laying and compacting and the depth of the water table.

Include any methods used to improve exfiltration.

Impermeable

liner

Where required, the type of liner will need to be specified. If possible

the name of an appropriate liner supplier should be quoted.

Consideration should be given as to the placement of the liner in

regards to overlapping and folding, particularly at the edge of the area

where the pavers are to be placed.

Handling and inspection of the liner to ensure this is not punctured or

torn is also critical.

Geotextile The type of geotextile, weave and if possible name of appropriate

geotextiles should be specified.

Notes should be included to cover securing of the material at edges

and overlapping at joins.



Base course The size and type of base course should be documented along with

laying and compaction requirements.

Bedding

material

The bedding material is critical to the overall success of the pavers.

The specifications should include the type and size of material to be

used as this is often highly variable.

Particular attention should be given to specifying the laying and

compaction of the gravel to ensure a flat even surface.

Pavers Specify the type of paver required i.e. open cell, solid interlocking,

porous interlocking. In some cases particular pavers and colours are

required.

The specification should include the proviso that pavers are installed

from the lowest point up.

Consideration should be given to the spacing between each section as

many pavers need to be tightly compacted to provide structural

integrity and maintain the life of the pavers.

Porous asphalt Porous pavement should generally be the same as standard open

graded porous asphalt used on New Zealand roads.

The materials and mix design should meet the requirements of TNZ

P/11.

A 80/100 penetration grade is recommended.

The percent binder should be between 5.5% and 6% based on total

weight of pavement.

Porous asphalt should be rolled when it is cool enough to withstand a

ten-ton roller, normally only one or two passes are necessary as more

frequent rolling can reduce infiltration capacity.

All traffic should be kept out of the paved area for a minimum of 24

hours post-application to allow proper hardening.



Porous

concrete

The water content of porous concrete mixes is limited to a narrow

range to provide the desired strength and permeability.

Considerations include:

A porous concrete mixture should be discharged completely within

one hour after initial mixing.

Porous concrete mixtures cannot be pumped, making site access an

important consideration.

Unit weight tests should be carried out on each concrete batch.

Concrete placement should be continuous and spreading and strike-

off should be rapid.

Consolidation should be completed within 15 minutes of placement by

use of a steel roller.

Conventional floating and trowelling tends to seal surface pores, no

further finishing should be required following compaction.

Underlying construction layers should be moistened to prevent them

from absorbing moisture from the curing concrete.

After placement, fog misting followed by plastic sheeting is the

recommended curing procedure.

Curing should be started a maximum of 20 minutes after placing,

compacting and jointing.

Porous concrete pavements should not be opened to traffic for seven

days post-construction to allow sufficient curing and stabilisation

time.

Edge beams Specify the size of the edge beam required and concrete strength.

Overflow

system

Include details on the required overflow system including diameters,

invert elevation, size and type where applicable.

Underdrainage If installed, the pipe diameter and type of perforated pipe needs to be

specified.

Specify the pipe diameter, invert elevation and type of non-perforated

pipe. Ensure that this pipe type is suitable for vehicle loading.

Example specifications for the construction of permeable pavers are shown in Section 18.9.

These cover some of the typical aspects for construction of permeable paving systems, but

exclude particulars such as site establishment, health and safety, testing, materials, and

reinstatement, as these differ greatly depending on the application.


permeable paving systems, and should be used as a ‘starter specification’ for guidance only.

Each permeable paving system will be site specific and require careful consideration to ensure

that all aspects of construction are covered.



Critical points to inspect during construction to ensure the device is installed correctly include

the following. Note that a detailed review of council standards and guidelines should be

carried out for every project, as there are particular requirements for each council and these

are frequently updated.

Table 47

Crit ical points to inspect during construction to ensure the permeable paving system is installed

correctly.


Excavation/

sub-grade

Topsoil has been removed and sub-grade has been placed, compacted

and leveled as per construction plans and specifications.

Any excavations or modifications to the sub-grade.

Levels match construction plans and specifications.

Any variation from the design plan must be approved by the design

engineer or project manager.

Edge beams Concrete edge beams (to keep pavers in place) have been installed

correctly and are level.

Concrete strength and testing results, and reinforcing have been

confirmed.

Liner (if used) The water proof liner (if specified) has been laid around the entire

permeable paving area with no tears or stress points and seams

sealed.

The liner should be pinned in place and overlapped correctly

(minimum of 100 mm).

Underdrainage Underdrain is laid in minimum 300 mm of gravel bedding on a

minimum 0.5% slope.

Connection of the perforated underdrain to an approved stormwater

system is water tight.

Photograph this stage of construction for inclusion in the operation

and maintenance manual and construction progress reporting.

Perforated underdrain is backfilled with base course with no damage

to the underdrain.

Geotextile is laid over the underdrain bedding gravel to prevent

clogging of underdrain (if a filter sock has been specified this is fitte d

correctly around the underdrain).



Base course The gravel base course (with minimum 30% voids) is carefully laid over

geotextile to depth specified on construction plan.

The gravel base course must be strong enough to support the site’s

traffic loads (i.e. no scoria) and the depth is dependent on the required

storage volume in the voids.

Bedding

material

Clean, fine grade bedding gravel is laid gently over the base course

(with no compaction) as per paver’s manufacturers and construction

plan specifications.

Bedding material is leveled with a rake or straight edge.

Pavers

Pavers are laid as per manufacturers specifications and are even and

flat with joint material filling all the gaps.

If the area is on a hill pavers must be laid from the lowest point up.

If grass is required between blocks, seeds need to be densely

sown/planted to ensure complete coverage of the entire site. A water

schedule is in place until grass is established.

18.8 Photo gallery – permeable paving

Figure 101

Beachaven Reserve permeable pavement associated with car park spaces. Road stormwater directed

away from the paving to a roadside catchpit. [NSCC]


Figure 102

Barry Curt is Park permeable pavement. Note the bare soil around the landscaped margins and sediment

on the asphalt surface, both sources of sediment that could lead to accelerated clogging of the

permeable paving. [MCC]

Figure 103

Birkdale Road permeable paving. [NSCC]


Figure 104

ST001 – North Shore Events Centre permeable pavement. [NSCC]

Figure 105

ST001 – North Shore Events Centre permeable pavers (porous paver). [NSCC]


Figure 106

Kirkbride Road permeable pavement draining to rain garden.

Figure 107

Permapave resin blocks, Auckland Botanic Gardens. [ARC]


Figure 108

Unitec Sports Complex car parking, Gobi block permeable paving.

Figure 109

Permeable paving showing gravel joint material. [NSCC]


Figure 110

Permeable paving being laid over gravel basecourse. [NSCC]

18.9 Construction specification example – permeable paving


18.9.1.1 Clearance

Existing trees around the paving site shall be protected and those identified for removal on

the drawings shall be removed. The Contractor shall comply with any consent or District Plan

requirements concerning the preservation, trimming or transplanting of trees, shrubs, etc.

18.9.1.2 Topsoil



















Water from either pipe installation, surface drainage of the site, or dewatering of excavations

shall be treated (e.g. passed through a silt fence, settling pond or other treatment) in

accordance with ARC requirements prior to discharge. Once treated to the required level, the

water may be disposed of downstream into the existing water course, the public road

stormwater drain or public foul sewer subject to local council and ARC approval. Where this

cannot be achieved, water from excavations must be removed off-site and disposed of in an

approved manner.



18.9.2 Permeable paver installation

18.9.2.1 Sub-grade

The sub-grade material shall be “run of pit” sand, unless otherwise specified or approved by

the Engineer. The suitability of alternatives will need to be demonstrated. The sub-grade

shall be laid in layers not exceeding 150 mm (compacted thickness) and at optimum moisture

content.

The material shall be compacted to the specified California Bearing Ratio (CBR) as measured

with a standard scala penetrometer. Except that the standard of compaction shall not be less

than 95% of the optimum dry density of the material as specified in Test 4.1.1 of NZS

4402:1986 “New Zealand Standard Compaction Test”, or Test 4.1.3 “New Zealand Vibrating

Hammer Compaction Test”.

The entire surface of the completed sub-grade shall be made smooth, firm and uniform, by

blading, grading and rolling, approximating the crossfall required on the final surface.

Compaction shall not continue if the surface shows any signs of excessive weaving or heaving,

until the problem has been resolved.


18.9.2.2 Edge beams

Edge beams shall be constructed as per the drawings. Concrete and formwork shall comply

with NZS 3109 “Specification for Concrete Construction” and subsequent amendments.

Concrete supplied to the site shall comp ly with NZS 3108 “Specification for Concrete

Production – Ordinary Grade”.

Sulphate resistant concrete shall be 80% Duracem, 4% microsilica and 16% general purpose

Portland Cement.

Concrete shall be Ordinary Grade of strength 17.5MPa or 20MPa, as shown on the drawings,

with maximum aggregate size 19 mm and slump 100 mm.

Concrete shall be supplied to site as batched ready mixed concrete from an approved supplier.

The Contractor shall keep a delivery record for each batch delivered to site. This shall reco rd

the supplier, date, time, quantity delivered, mix code, specified strength, aggregate size and

slump.

The Contractor shall carry out a slump test on each batch delivered, an d shall allow in his price

for one concrete test cube to be taken, cured and tested for each batch delivered. Concrete

test cubes will be required on the instruction of the Engineer.

Concrete surfaces that will be buried shall be surface finish U1 to NZS 3114.

18.9.2.3 Underdrain and filter cloth

The underdrain shall be installed on top of the sub-grade in the position detailed on the

drawings. The underdrain shall be Novacoil subsoil drain with geotextile shall be laid over the

top to cover the entire area to be paved. The geotextile is to be laid and pinned as per the

manufacturer’s instructions.

An evenly placed 50 mm layer of GAP20 shall be laid along the length of the underdrain.

The Novacoil shall be connected to the existing manhole/catchpits as detailed on the

drawings.

18.9.2.4 Novacoil connection to manholes

When connecting to the existing stormwater manholes, the Novacoil shall be protected from

stress concentration and possible fracture of the pipe at the concrete face by the insertion of a

10 mm thick neoprene strip minimum 100 mm wide wrapped around and cemented to the

pipe and embedded fully in the manhole wall/concrete surround with the outer edge of the

strip at the outer surface of the concrete. There shall be sufficient concrete thickness between

the inner edge of the strip and inner surface of the manhole to ensure water tightn ess (refer

drawings). Care shall be taken to ensure that the neoprene strip is not displaced during

concreting.

In addition to the neoprene strip, a sodium bentonite strip shall be wrapped around and

cemented to the pipe and embedded fully in the manhole wall/concrete surround. Refer

drawings.


18.9.2.5 Base course

Material in this layer shall be coarse aggregate, meeting the criteria of clause 3.0: Testing.

This layer shall not be installed until the all previous layers have been completed and approved

by the Engineer.

Samples of base course shall be selected at random and tested to check compliance with

aggregate grading, ratios and compacted permeability.

Permeable aggregates are subject to separation during transport and construction. Care

should be taken to avoid separation occurring. The gravel shall be cleaned, washed, crushed

rock with a minimum of 30% voids.

The base course layers shall be constructed in layers of uniform thickness not exceeding 150

mm. Geotextile shall be installed and pinned over the base course as per the manufacturer’s

instructions.

18.9.2.6 Bedding sand

The bedding sand shall be clean sand or fine gravel spread evenly over the geotextile layer as

per the drawings. The bedding sand shall be levelled using a rake or straight edge – do not

compact the bedding layer.

18.9.2.7 Pavers

The concrete segmental pavers shall be manually laid in accordance with NZS 3116:2002

‘concrete segmental paving’. The pavers shall be installed as per the manufacturer’s

instructions.

The paver shall be a minimum of 80 mm thick and shall comply with the requirements for

application 2, residential driveways, light traffic (NZS 3116, Clause 302 Paver Selection). The

pavement layer shall achieve 95% maximum dry density.

Pavers are to be free of foreign materials before installation and installed in the pattern shown

on the drawings maintaining straight lines. Joints between pavers to be per manufacturers

design. Pavers shall be vibrated into place, dry joint sand swept into joints and vibrated until

full. Do not vibrate with 1 m of the unrestrained edges.

Final elevations shall be checked for conformance to the drawings to the satisfaction of the

Engineer.

18.9.3 Testing

The base course shall be tested for particle size distribution according to NZS 4407, Test 3.8.1

Wet Sieving Test to comply with Table 48:


Table 48

Wet sieving test complying percentages.

Sieve aperture Maximum and minimum allowable percentage weight

passing

37.5 100

19.0 60-75

4.75 3-18

2.36 0-5


19 Rain Tanks

19.1 Introduction

Rain tanks are used in rural areas for household water supply but are not as prevalent in urban

areas. This is mainly due to the public health issues associated with unfiltered tank water. In

previous years rain tanks have been discouraged or require permits if there is a mains potable

water supply.

However with the advent of a more sustainable approach to stormwater management and

particularly water supply rationing, rain water tanks are increasing in popularity for non -

potable use and rainwater detention. Rain tanks can be used for rainfall detention, water

harvesting or a combination of both (dual purpose tanks).


Rain tanks collect rainwater that runs off hard surfaces (inflow) free of substantial amounts of

sediment and debris such as roofs and car parks via a pipe network. The rain tank then stores

the rainwater temporarily to reduce stormwater runoff flows and/or permanently to be used

as a non-potable water supply (non-drinking water uses such as toilet, laundry and outdoor

uses). Rain tanks provide some minimal stormwater treatment by removing contaminants

through settlement of sediment at the base of the tank; however they are primarily used for

stormwater re-use or attenuating stormwater run-off. Rain tanks can be installed either

above or below ground.

Harvesting rain tanks should collect roof runoff only, whereas detention rain tanks are suitable

for all sources of runoff. For all rain tanks, some form of sediment control is recommended to

reduce potential clogging issues within the system.

Rain tanks are common in rural and semi-rural areas not yet serviced by a mains water supply.


Figure 111

Typical rain tank components.


Table 49

Key components of rain tanks.

Device component Description

Collection system

and pre-treatment

Collection systems can be gutter systems, kerb and channeling (if

collecting runoff from ground surfaces such as car parking areas), or

traditional stormwater piping. The collection system may be fitted

with:

Leaf guards and debris diverters.

First flush diverters (to divert the ‘dirty’ initial runoff).

Insect screens.

Sediment traps (for non-roof hard surfaces such as parking

areas).

Controlled tank inlets (to prevent stirring up of bottom

sediments).

Tank Above or below ground; concrete, steel or polyethylene; and

specifically designed tanks (i.e. for architectural purposes).

Plumbing Piped systems that supply non-potable water to the household,

including:

Pumps (in most cases).

Water supply outlet (where water exits the tank).

Backup water supply connection (connection to the existing

reticulated water supply that tops up the tank during periods of

low rainfall, usually activated by a float switch).

Filtration (a filter that removes any debris from the runoff/tank

but is not normally required).

Overflow system Piped overflow from tank that usually discharges to the local

stormwater system (often 80-100 mm in diameter), when the tank is

full.

Orifice Small diameter outlet which restricts the outflow from the tank. Only

used where detention is provided.

There are three different types of rain tank: detention, single-purpose and dual-purpose:


Figure 112

Detention tank schematic (Source: NSCC, 2008. Raintank Guidelines).

Detention tanks

Reduces peak storm water runoff flows by storing rainwater from the roof and other hard

surfaces (parking areas, etc) and slowly releasing the rainwater through a small diameter

orifice.

Detention tanks have one inlet pipe which transports the collected water to the tank. The

tanks have two outlet pipes. One outlet pipe is a small diameter orifice be tween 10 mm to 35

mm. This outlet is located near the bottom of the tank, and slowly releases the water from

the tank into the local stormwater system.

The second outlet is a large diameter overflow pipe near the top of the tank. It is usually

between 80 to 100 mm in diameter and connected into the local stormwater system. When

the tank is full, additional runoff overflows to this pipe and is discharged to the local

stormwater system.


Figure 113

Single purpose rain tank schematic (Source: NSCC, 2008. Raintank Guidelines).

Single-purpose rain tanks

Provides a non-potable water supply. Collects rainwater mostly from roof areas and supplies

rainwater for non-potable (non-drinking) household uses (i.e. toilet, laundry and outdoor use).

They are less frequently used to collect runoff from ground surfaces such as parking areas

because the runoff includes pollutants/sediment, which are not suitable for household non -

potable water use. Additional pre-treatment and/or post-treatment may be required for

tanks collecting runoff from ground surfaces.

Single-purpose rain tanks have one inlet pipe which transports the collected water to the

tank. The tank has two outlet pipes. The tank’s first outlet pipe is a large diameter (80 –

100 mm diameter) overflow pipe near the top of the tank.

The second outlet pipe is a small (up to 25 mm) water pipe located near the bottom of the

tank which feeds the rainwater to a pump and then to household non-potable water use. All

of the water stored in the tank is used for non-potable water use.


Figure 114

Dual purpose rain tank schematic (Source: NSCC, 2008. Raintank Guidelines).

Dual-purpose rain tanks

Provides a non-potable water supply and reduces peak stormwater runoff flows. Collects

water from roof areas only. Combines collection for non-potable use and detention into one

tank by locating the small diameter orifice part way up the side of the tank. The volume

below the orifice is ‘permanently’ stored for use as household non-potable water and volume

above the small diameter orifice is ‘temporarily’ stored and released through the orifice

during and after each rainfall event.

Dual-purpose rain tanks comprise one inlet which transports the collected water to the tank,

and three outlet pipes. One of the outlet pipes is a small diameter orifice (between 10 mm to

35 mm) located midway down the side of the tank. This outlet slowly releases the top section

of water (detention) from the tank into the local stormwater system.

The second outlet pipe is a small (up to 25 mm) water pipe located near the bottom of the

tank which feeds the rainwater to a pump and then to household non-potable water use.

The third outlet pipe is a large diameter (80 to 100 mm diameter) overflow pipe near the top

of the tank.



Table 50

Guidelines relat ing part icularly to rain tanks include:





Auckland Region






Region




development.



Residential Single Purpose

Rain Tanks

Practice notes giving general information on

the minimum design requirements and

maintenance of single purpose rain tanks.


Practice Note NSC 06: Dual

Purpose Rain Tanks for

Residential Applications



maintenance of rain tanks for residential

applications.



Detention Tanks



maintenance of detention tanks.


Practice Note NSC 08: Single

Purpose Rain Tanks for Non-


Practice note developed to give general

information on the minimum design

requirements and maintenance of single-

purpose tanks for non-residential activities.


Practice Note NSC 09: Dual

Purpose Rain Tanks for Non-


Practice note developed to give general

information on the minimum design

requirements and maintenance of dual-

purpose tanks for non-residential activities.

NSCC LB102 On Site Stormwater

Mitigation

Practice note to assist choice of appropriate,

cost effective stormwater management

solutions and explains how to select pre-

approved solutions using a stormwater

migration factor or proposal alternative

technologies.

NSCC LB103 Rainwater Harvesting Practice note to assist with the design of

rainwater tanks in the Long Bay area.



NSCC LB107 Long Bay Water Supply

Systems

Practice note to be used in conjunction with

NSCC Infrastructure design standards for the

design and construction of the Long Bay water

supply system.

NSCC LB110 Other Technologies Practice note based on the ARC’s TP10,

describing requirements for alternative

technologies to meet the Long Bay water

quality and quantity management objectives.

NSCC LB209 Worked Examples Some worked examples showing the steps for

design of stormwater management devices

such as filter strips and rain gardens.

NSCC North Shore City Raintank

Guidelines

Provides guidance on different types of rain

tanks, layouts, considerations, etc.







Residential Sites








Table 51

Below is a non-exhaustive list of some of the more applicable standards and technical docum ents that

relate to rain tanks:

Title Description

AS/NZS 1254:2002

PVC Pipes and Fittings

for Stormwater and

Surface Water

Applications

Specifies requirements for PVC pipes and fittings for conveyance

of stormwater or surface water. The Standard includes

requirements for both plain and structured wall pipes and fittings.

AS/NZS 1260:2009

PVC-U Pipes and

Fittings For Drain, Waste

and Vent Application

Specifies requirements for uPVC pipes and fittings for sewer,

drain, waste and vent applications above-ground or below ground

and intended to be used where the pipeline is operating under

gravity flow and the operating pressure is low.


Title Description

NZS 3101.1&2:2006

Concrete Structures

Standard

The design of concrete structures, specifies minimum



NZS 3104:2003

Specification for

Concrete Production


concrete.

NZS 3109:1997



reinforced concrete, unreinforced concrete, pre-stressed concrete

or a combination, in elements of any building or civil engineering

structure.

NZS 3114:1987

Specification for

Concrete Surface

Finishes



exposed aggregate and on floors, exterior pavements and inverts.

NZS 7643:1979

Code of Practice for the

Installation of

Unplaticised PVC Pipe

Systems

Sets out the methods of installation of uPVC pipelines above or

below ground for pressure and non-pressure applications using

pipe and fittings complying with NZS 7641, NZS 7642 and NZS

7649.

New Zealand Drinking

Water Standards 2005

The DWSNZ2005 details how to assess the quality and safety of

drinking water using the revised water quality standards and

compliance criteria. The drinking water standards apply to

drinking water, that is, water intended to be used for human

consumption, food preparation, utensil washing, oral or personal

hygiene.

Building Code Covers the requirements under the Building Act (1991).


Rain tanks that do not have a pumping system installed or connection to a mains supply are

normally very easy to install. Some of key considerations for the installation of tanks include

the following:

19.5.1 Aesthetics

Consider the visual impacts of the building platform and tank for aesthetic reasons. The tanks

can be supplied in a variety of colours and shapes, which will assist to blend in with the

surrounding environment or fit beneath decks or the house.


19.5.2 Tank location

Select an appropriate location for the tank. Consider aspects such as:

Lifting in the tank (i.e. clear of overhead lines, trees, etc).

Permanent installation (i.e. clear of overhanging branches, whe re vermin will not be able

to enter the tank).

Accessibility for maintenance, proximity to downpipes, etc.

Sufficient access available around the rain tank for operation and maintenance purposes.

Groundwater levels should be considered for installation of underground tanks.


Where the tank is to be installed underground, ensure that there are no existing underground

service utilities. Consider overhead telecommunications and power lines. The tank may need

to be relocated to suit.

19.5.4 Catchment area

Runoff from other impervious surfaces, such as driveways or paving will need to be managed

by other stormwater management methods or as part of a treatment train (e.g. swales or

raingardens) to remove large sediments which can clog the filters and tank.

19.5.5 Above or below ground tanks

Generally tanks should be above ground unless approved otherwise by council. If

underground tanks have to be used, consideration of vehicle loading and the structural

capacity of the tank will need be confirmed. Also, consider the type of tank material that is

appropriate for the purpose i.e. do not bury HDPE tanks that are designed for above ground

purposes.

Underground tanks often crack allowing surrounding groundwater into the tank. DO NOT put

underground rain water tanks for drinking purposes near septic tanks or effluent disposal

lines.

If the tank is located in an area with high groundwater table, the device may become buoyant

when emptied for maintenance. Sub-soil drainage around the base, or some form of

anchoring will ensure that does not occur.

19.5.6 Foundations

The tank foundation needs to be structurally sound to support the weight of a tank full of

water. It is also critical that the foundation is level such that the tank will not tip over. Means

of securing or anchoring the tank into the base should be considered if the tank base is small,


and tank sides high. A ring beam around the base of the tank, or struts to support the tank

can be used in this situation.

Where the tank is on a sloping site, specialist geotechnical input should be used for retaining

walls and foundations.

19.5.7 Inlet and outlet levels

Prior to installing holes in the tank for the inlet and outlet pipes (if not pre -drilled), confirm the

inlet and outlet levels. This is an important aspect as these vary considerably depending on

the type of system that will be operating i.e. detention tanks, single-purpose tanks, and dual-

purpose tanks.

19.5.8 First flush diverters

Normally the first flush will contain the majority of contaminants from the ro of area. Where

practicable, it is preferable to prevent this entering the tank and indeed where tanks are used

for drinking water this is essential. First flush and debris diverters can be installed to achieve

this. Sediment traps and controlled tank inlets can also be used for this purpose.

19.5.9 Insect screens

The tank should be protected from leaves, other debris and vermin entering the tank.

Consider the installation of leaf guards and insect screens. A fine mesh should be used to

prevent entry of mosquitoes.

19.5.10 Access hatches

All tanks should have hatches which are easy to access i.e. consider overhead clearance if

under buildings/structures, height of the tank, etc. These should be fully secure and should be

lockable.

Where possible fully removable hatches should be specified as this will make maintenance

easier (when the tank is cleaned out). Ensure that these are lightweight enough to be lifted

and/or removed (consider that most will be accessed by a ladder).

Access hatches are provided for access to the tank for inspection and maintenance purposes

but under no circumstances should personnel (except by qualified and experienced personnel)

enter the tank as it is a CONFINED SPACE.

19.5.11 Outlet orifices

Note that detention tanks often require the installation of a small diameter outlet to act as an

orifice. These can often become blocked and consideration should be made during

installation in regards to accessing the outlet for cleaning.


19.5.12 Connections to water supply and plumbing

In many situations a substantial amount of plumbing from the tank to the existing water

supply, or the installation of a pump, floats and control box will be required. It is important to

remember that a building consent is required under the Building Code and that this work can

only be carried out by a certified plumber and electrician.

Where pumps are required, consideration of power supply to the site is needed. Safety

considerations should given to electrical safety where electrical supply for pumps is required

19.5.13 Overflows

The location of the overflow from tanks fitted with overflow devices should be carefully

considered. This is particularly important in urban areas where incorrect discharge may cause

localised flooding on neighbouring properties.

In rural areas, the overflow should be clear of existing infrastructure such as septic tanks and

effluent disposal lines.


Table 52

The following aspects should be specified when constructing a rain tank:


Collection

and pre-

treatment

system

• Specify the type, size and material for the collection system. If

included, also specify the size and type of additional fittings:

o Leaf guards and debris diverters.

o First flush diverters (to divert the ‘dirty’ initial runoff that falls on the

roof).

o Fine mesh insect (or vector) screens.

o Sediment traps.

o Controlled tank inlets.

Tank

foundation (if

required)

• Specify the sub-base material, concrete strength, depth and reinforcing

required for the concrete foundation. If a blinding layer is necessary,

specify the grade and depth of material required.

Tank • Specify the volume of the tank to be installed and required tank

material.

• Specify tank dimensions and height above ground.

Plumbing • Specify in detail the following (where included):

o Pumps (brand, pump rate and head required).



o Water supply outlet diameter and any additional pipe work.

o If a backup water supply connection is required and where backup

water would be supplied.

o Filter size and type.

Overflow

system

• Specify the type of overflow system. Normally this includes the pipe

diameter and material required.

Orifice • If an orifice outlet is included, the pipe diameter and material should be

specified.


construction of rain tanks, but exclude particulars such as site establishment, health and


application.


rain tank, and should be used as a ‘starter specification’ for guidance only. Each rain tank will

be site specific and require careful consideration to ensure that all aspects of constructio n are

covered.


Critical points to inspect during construction to ensure the device is installed correctly are

listed in Table 53 It is a good idea to keep a photographic diary of construction activities for

the operation and maintenance manual and to assist in any future troubleshooting of issues.

Table 53

Crit ical points to inspect during construction of a rain tank:


Excavation/

foundations

• Check levels match construction plans and specifications.

• Any variation from the design plan may change the levels and

function of the rain tank separator and must be approved by the

design engineer or project manager.

• Good founding material present and foundation is level.

• Note any groundwater interaction.



Plumbing • Make sure a registered plumber and electrician are used for all plumbing

and electrical work. Check that all plumbing and electrical work is

certified as required by the Building Code.

• Check elevations of the different piping details checked to ensure they

perform as per specifications.

• Make sure all fixtures are water tight and there are no leaks on

completion.

• Ensure no backflows occur in the system.

Overflow • Rain tank overflow is NOT connected to the wastewater network (this is

illegal).

Signage • All taps and outlets supplying non-potable water have required symbol

and wording “Not Suitable for Drinking”.

19.8 Photo gallery – rain tanks

Figure 115

Maybury Street rain tank.


Figure 116

Glencourt Place rain tank.

Figure 117

Waitakere Hospital rain tanks.


Figure 118

Waitakere Hospital rain tanks.

Figure 119

Low impact urban design incorporating rain tanks.


Figure 120

Glencourt Place rain tank. Note good solid foundation.

Figure 121

Glencourt Place rain tanks.


Figure 122

Glencourt Place rain tank on unstable foundation, tank is now leaning.

Figure 123



Figure 124

Glencourt Place rain tank with overhanging branches.

Figure 125

Glencourt Place rain tank with diff icult access.


Figure 126


Figure 127



Figure 128

Royal Road rain tank part ially buried. [WCC]

19.9 Construction specification example – rain tanks

19.9.1 Excavation


The Contractor shall include in his price for excavation in all material s encountered, that could








"rock" bucket, and requires more intensive means for efficient excavation. Check water table

elevation.



substances including excess moisture, which prevents satisfactory placing and compaction. It

shall be free of clay lumps and stones retain on a 75 mm sieve.
















19.9.1.5 Support


safety requirements and so that excessive widening of the excavation is avoided. Support may

be provided by use of shields, timber, sheet piling or other shoring systems, subject always to

the Engineer's agreement of the proposed method. Any such agreement given by the

Engineer shall not absolve the Contractor of this responsibility to minimise the area disturbed

by the works and to make the site safe.








All shoring left in place shall be kept clear of the permanent contract works. The Engineer

may direct that shoring left in place be cut off at any specified level in which case payment will

be made only for the portion remaining in the ground.






19.9.2 Foundation




Excavation for the foundations shall be 300 mm below the invert level of the rain tank. The

founding material will consist of a 300 mm layer of GAP7 bedding. The rain tank will rest on

top of this bedding.

The foundations are to be laid to the levels indicated in the drawings to the satisfaction of the

Engineer.

19.9.3 Tank installation

All plumbing and drainage work to the Building Act 2004 and the NZ Building Code.

19.9.3.1 Tank material

The tank material shall be polyethylene, complying with AS/NZS 4766:2006 “Polyethyle ne

Storage Tanks for Water and Chemicals”, or concrete complying with NZS 3106: 2009 “Code

of practice for concrete structures for the storage of liquids”.

19.9.3.2 Tank installation

The tank shall be installed to the manufacturer’s instructions.

19.9.3.3 Buoyancy


checked for buoyancy under the range of operating conditions including when the tank is

empty.

19.9.3.4 Plumbing and piping

The coarse litter screen (leaf screen) shall be installed in the location indicated on the

drawings to the manufacturer’s specifications.

The existing spouting shall be connected to the rain water tank by a PVC pipe as detailed on

the drawings. PVC pipes shall be manufactured in accordance with AS/NZS 1260 “PV C Pipes

and Fittings” and installed in accordance with NZS 7643 “Installing uPVC Pipes”.

Connection to the mains for top up shall be as detailed on the drawings and shall comply with

the local council’s standards for connection to water mains and the NZ Building Code.

The tank overflow connection to the stormwater system shall comply with NZS4404:2004 and

as per the local council’s standards.


19.9.3.5 First flush diversion

The first flush diverter shall be installed as detailed on the drawings as per the manufacture r’s

instructions.

19.9.3.6 Pumps

Pumps be installed in accordance with manufacturer’s instructions and shall be plumbed as

per NZS 3500.5:2000 by a certified and registered plumber. All electrical work shall be

completed by a registered electrician. Pump and enclosure shall be of silent type.

Mains water feed shall be controlled by a float operated shut off ball valve.

19.9.3.7 Backflow prevention

A non-return valve shall be installed as detailed on the drawings for backflow prevention as

per AS3500.1.

19.9.3.8 Access hatch

The access hatch shall be watertight, lockable and finish flush with the ground surface as

detailed on the drawings.

19.9.3.9 Labeling of pipes

Labelling of pipes and fittings shall comply with NZS 3500.5:2000, clause 2.16.6 – coloured

lilac in accordance with AS 2700 colour code P23 (for greywater). All non-potable water

supply outlets shall be clearly and permanently labelled “CAUTION NOT FOR DRINKING”.

Pipes shall be labelled every 500 mm.

19.9.4 Testing

The system shall be tested to ensure the pump works, the backup water supply comes on at

the correct time and that all surfaces drain to the tank that are supposed to and that the

overflow pipe goes to the stormwater system. This Engineer shall verify that the system is

operating correctly before sign-off.


20 Living Roofs

20.1 Introduction

Living roofs are gaining in popularity, with property owners either modifying existing

buildings (retrofitting) or including them at the outset of design for new buildings. Living

roofs are also referred to as green roofs, eco-roofs, vegetated roofs or roof gardens. Living

roofs can be designed to assist in a building’s stormwater management, air quality, building

insulation, roof longevity and reduction of the heat island effect.


Living roofs are built on top of a human-made structure (e.g. house, office building, or

underground car park) and can be located below, at, or above ground. Living roofs have been

categorised into three basic types: extensive, semi-extensive, or intensive, depending on their

substrate depth, design characteristics and core purpose. Table 54 provides definitions

targeted to a stormwater engineering audience from an international task committee on

living roofs for stormwater control, which falls under the umbrella organisation of the

American Society of Civil Engineers (ASCE).

The key components of a living roof system are presented in Figure 129 and described in Table

55.

Table 54

Broad categorisation of living roof systems

Living Roof

Type

Description1

Extensive Low profile with thinner layers (drainage, substrate, and plants) than

semi-extensive and intensive living roofs

Low growing plants are established in 20–150 mm of substrate

Usually less expensive and lower maintenance when compared to other

types of living roofs

Roof structural requirements are lower than other living roof types, with

saturated weights reported from 70–170 kg m-2

Thin substrate depth limits how much water can be retained in the

system, and hence the diversity and height of plants that can be grown

in the absence of irrigation

1 For more detail, see Fassman, E., Simcock, R., & Voyde, E. (2010). Extensive Green Roofs for Stormwater Management: Part 1

Design and Construction. Auckland: Prepared by Uniservies for Auckland Regional Council. Auckland Regional Council Technical Report 2010/017.


Living Roof

Type

Description1

Generally not meant to support foot traffic, other than for occasional

maintenance

Focus on function over form

Semi-

extensive

Designed to be low maintenance, but with deeper layers (drainage,

substrate, and plants) than extensive roofs but not as deep as intensive

roofs

Typical substrate layers range from 100–200 mm

Larger variety of plants can grow on this roof type when compared to an

extensive roof

Irrigation is typically infrequent or absent

Provide function plus a wider plant biodiversity, but are not typically for

occupied spaces

Intensive Have the deepest layers (drainage, substrate, and plants) and a wider

plant variety, including herbaceous plants to shrubs or trees

Substrate is typically >200 mm, which promotes deeper potential root

depth, and hence accommodates a wider height and variety of plant

species

Associated high saturated weight (300–1000 kg m-2

) requires significant

structural support for the roof

Regular irrigation is usually a required design element

Many intensive roofs are designed to be at least partially accessible,

hence design emphasises form and accessibility

Rooftop

Garden

Accessible areas on the roof with containerised plants instead of layers

of membranes and growth media that are installed directly on the roof

deck

May sometimes be considered intensive living roofs

Usually irrigated, where aesthetic requirements are high or particularly

important


Figure 129

Typical living roof.

Table 55

Key components of a living roof system.

Device

component

Description2

Plants Species selected to withstand extreme climatic conditions including solar

radiation, wind, drought, and rain. The primary role of the vegetation is to

ensure the surface of the substrate is stable – resistant to rain, wind and

animals. Stems and leaves physically protect the surface from impact while

roots bind and hold the substrate in place. Vegetation also reduces

absorption of solar radiation and provides stormwater attenuation volume

by direct interception of rainfall and through uptake of water from the

substrate, then release to the atmosphere via evapotranspiration.

Growing

media/

substrate

Usually composed of 80-90% light weight aggregate (eg. pumice or zeolite)

and up to 20% organic material. The growing medium should be stable

(resistant to rot, shrinkage etc) and well draining to promote ongoing plant

health and structural integrity. The use of fertiliser shall be minimised,

however if necessary shall only be applied as per landscaping plan. The

growth medium supports plants both physically and nutritionally, stores

precipitation up to field capacity, extends flow path to reduce runoff

velocity, and provides thermal mass/insulation.

2 For more detail, see Fassman, E., Simcock, R., & Voyde, E. (2010). Extensive Green Roofs for Stormwater Management: Part 1

Design and Construction. Auckland: Prepared by Uniservies for Auckland Regional Council. Auckland Regional Council Technical Report 2010/017.


Device

component

Description2

Root barrier/

geotextile

material

The geotextile (filter fabric) physically supports the substrate, separating it

from the drainage layer and preventing migration of substrate fines, to

maintain a free-flowing drainage layer. It can be either a separate layer, or

attached to a commercially available drainage layer.

A root barrier is required to prevent root penetration into the waterproof

membrane. There are a number of options for this including chemical

compositions in the water proof membrane or geotextile, or a laminated

layer.

Some root barriers contain chemicals toxic to fish – these should not be used

for living roof applications.

Drainage

layer

The drainage layer provides free drainage for rainfall in excess of the system

storage capacity and provides air circulation for plant roots. When using an

aggregate drainage layer, the drainage layer should extend to the surface

around the roof for a width of ≥200 mm. When using a drainage mat, a

gravel edge should be included, extending from the drainage mat to the

surface, around the perimeter of the living roof (and around any protrusions)

for a width of ≥200 mm. This provides additional drainage, fire control and a

visual cue for maintenance access.

Insulation

layer

An optional insulation layer may be included in the living roof design.

Waterproof

membrane

At least a double-ply waterproofing membrane of high quality is strongly

recommended. Alternatively, a purpose-made living roof heavy-duty single

ply membrane with felt layer can be used. Care must be taken during

installation to prevent any damage to the waterproof membrane (by

footwear, installation tools etc). Particular care must also be taken around

protrusions/penetrations (roof vents, air-conditioning units etc), typical

waterproofing problem areas. It is recommended to install gravel edging

around protrusions for protection and easy identification for maintenance.

Outlet Excess water in the drainage material is directed to the outlet. Access is

required for regular maintenance; gravel edging should be installed around

the outlet for ease of identification and access.

20.2.1 Living roof functions

Living roofs reduce the overall impervious coverage of a site, which results in a number of

benefits. In a stormwater management context, living roof systems provide volume reduction

and peak flow attenuation. By converting a usually impervious roof into a permeab le area the

volume of stormwater reaching the outlet during a rainfall event is greatly reduced compared

to that from a conventional rooftop. Water is intercepted by the vegetation, stored within the

porous substrate and returned to the atmosphere via evapotranspiration. In addition, the

path water must take through the living roof system to exit the rooftop results in greatly


reduced peak flow rates. Additional environmental, economic, amenity and aesthetic benefits

are also provided by living roof systems:

Reduced energy consumption

o Enhance building insulation (when dry) and thermal mass (when wet)

o Improve air conditioning efficiency and reduce operational cost w here vents are

located above the vegetation, as intake air temperature is cooler than ambien t air

Extend the useful life of a roof surface by protecting it from damaging UV rays which

cause mechanical breakdown

Mitigate the “urban heat island effect” (i.e. lower ambient temperatures)

o Reduce the amount of solar energy absorbed by building materials

o Create a cooling microclimate by evapotranspiration

o Reduce reflected heat

Create urban habitat

o Mitigate removal of habitat from modifying existing land use

o Provide “green corridors”

Absorb and filter airborne pollutants, including dust, while releasing oxygen

Reduce sound/noise transmission

Provide amenity value as a recreational space

Provide aesthetic value and/or blend a building into a sensitive landscape.

The relative degree of each benefit from an individual living roof varies dependant on system

configuration (substrate depth, composition, and vegetation characteristics).


Guidelines relating particularly to living roofs are included in Table 56. Note that a detailed

review of council standards and guidelines should be carried out for every project, as there are

particular requirements for each council and these are frequently updated.


Table 56

Guideline documents relat ing to living roofs.


ARC TP124 Low Impact

Design Manual for the

Auckland Region

Approaches to site design and development from a

stormwater management context, primarily

applicable for residential land development.

ARC TR2010/017 Extensive

Green (Living) Roofs for

Stormwater Mitigation:

Part 1 Design and

Construction

Report documenting four years of research into

developing green roof design specifically for

stormwater management. The research project

centres around design and retrofit of an extensive

green roof constructed on the roof of the UoA Faculty

of Engineering building

ARC TR2010/018 Extensive

Green (Living) Roofs for

Stormwater Mitigation:

Part 2 Performance

Monitoring

Report documenting performance monitoring of up to

five years of the extensive living roof constructed on

the roof of the UoA Faculty of Engineering, the four

“mini” roofs at Landcare research, the Waitakere City

Civic Centre and the Auckland Botanic gardens living

roofs.

NSCC LB102 On Site

Stormwater Mitigation

Practice note to assist choice of appropriate, cost

effective stormwater management solutions and

explains how to select pre-approved solutions using a

stormwater migration factor or proposal alternative

technologies.

NSCC LB109 Primary and

Secondary Stormwater

Systems

Practice note for design of primary and secondary

overland flowpaths.

NSCC LB201 Minimising

Impervious Areas

Practice note providing ideas for minimising

impervious areas.

NSCC Stormwater

Management Practice

Note NSC 12: Green

Roofs

Practice note with information provided specifically

on most aspects of living roofs.




stormwater management devices for the majority of

applications in New Zealand.

WCC Stormwater Solutions

for Residential Sites

Document providing guidance on management

practices applicable to developments on individual

residential lots (<1000 m²). For use by engineers and

applicants for stormwater control building permits for

developments of this size.



Table 57


relate to living roofs:

Title Description

AS/NZS 2566.2:2002

Buried Flexible Pipelines -

Installation




AS/NZS 3750.10:1994

Paints for Steel Structures

Covers paints for steel structures – Full gloss epoxy (two-pack).

ASTM E2396-11 Standard Test Method for Saturated Water Permeability of

Granular Drainage Media [Falling-Head Method] for Vegetative

(Green) Roof Systems

ASTM E2397-11 Standard Practice for Determination of Dead Loads and Live

Loads Associated with Vegetative (Green) Roof Systems

ASTM E2398-11 Standard Test Method for Water Capture and Media Retention

of Geocomposite Drain Layers for Vegetative (Green) Roof

Systems

ASTM E2399-11 Standard Test Method for Maximum Media Density for Dead

Load Analysis of Vegetative (Green) Roof Systems

ASTM E2400-06 Standard Guide for Selection, Installation, and Maintenance of

Plants for Green Roof Systems

FLL 2008 Guidelines for the Planning, Construction and Maintenance of

Green Roofing: Green Roofing Guideline

NZS 3104:2003


Production


concrete.

NZS 3109:1997



reinforced concrete, unreinforced concrete, pre-stressed

concrete or a combination, in elements of any building or civil

engineering structure.

NZS 3114:1987


Surface Finishes



exposed aggregate and on floors, exterior pavements and

inverts.

NZS 7643:1979

Code of Practice for the

Installation of

Unplasticised PVC Pipe

Systems

Sets out the methods of installation of uPVC pipelines above or

below ground for pressure and non-pressure applications using

pipe and fittings complying with NZS 7641, NZS 7642 and NZS

7649.


Title Description

Building Code Covers the requirements under the Building Act (1991).


20.5.1 Siting a living roof

Living roofs are used around the world in densely developed urban environments. Living roof

use for stormwater attenuation is increasing throughout Auckland City.

Living roofs can be installed on any sized roof, on either new buildings or retrofitted onto old

ones, dependant on the structural support available and roof accessibility. Each system will be

specifically designed to account for the variations and constraints of each site (i.e. structural

support, roof pitch, accessibility for installation, climate, shade, living roof purpose, desired

aesthetic etc3) (AC TR2012/012) however the general arrangement will remain the same.


A number of steps are required to ensure a roof surface is ready to retrofit a living roof:

Clear the roof of unused equipment or debris

Prepare the existing surface

o Ensure the surface is free from debris, such as gravel ballast

o Test to ensure the waterproof membrane has no leaks

Living roof configuration will be influenced by roof slope

o For flat or near flat roofs, where slope is not clear, a roof survey on a 1 m x 1 m grid

will identify flow paths to the outlets and local depressions that may cause ponding

within the drainage layer

20.5.3 Structural integrity

The single most important consideration is the ability of the existing or proposed roof to

support the additional weight of a living roof at maximum water capacity. In new buildings

this will be accounted for in the design and will be a requirement of the building consent. In

retrofitting existing roofs however, structural integrity is a key consideration and in many

situations either an extensive living roof (shallow substrate depth) and/or structural

improvements may be required.

3 See Living Roof TR


The design phase of a living roof project should have ensured the structural integrity of the

structure, however it is important that throughout the construction process atten tion is paid

to the building and checks are regularly made of its structural integrity.

20.5.4 Building consent

Most councils require building consent for the installation of a living roof, for either new or

retrofit situation. It is also a requirement under the building code. A structural assessment

should have been carried out by a registered Professional Engineer as part of the design and

subsequently submitted with the consent application.

20.5.5 Roof slope

Living roof installation is not limited to flat or non-pitched roofs, although low slope provides

easier installation and configuration. Roof slope will influence living roof design, with respect

to substrate stability, and vegetation selection. Sloped roofs will drain faster than roofs with a

shallow slope due to the laws of gravity. Plants growing near the bottom of a slope will likely

experience higher substrate moisture content relative to the top, and thus must be selected

accordingly.

Design and installation on pitched roofs should follow best practice fo r erosion control and

slope stability, and considerations for working with low-cohesion soil (the substrate). For new

buildings, it is recommended to provide a minimum roof slope of 2% to ensure free drainage.

For retrofits with roof slope less than 2%, a formal drainage layer is required. For living roofs

with greater than 5° (8.8%) pitch, a biodegradable mat to hold plants in place until

establishment is recommended. Living roofs up to 15° (26.8%) pitch are unlikely to require

special structural anti-shear/slip protection measures. Structural anti-shear/slip protection

measures should be installed for a roof pitch greater than 20° (36.4%).

Roofs with pitch in excess of 30° (57.7%) require greater structural control still, and a different

approach to construction. Roofs with such a high pitch are not considered typical for living

roofs and are not discussed further.

20.5.6 Waterproof membrane

The waterproof membrane is the most critical aspect of the living roof system. At least a

double-ply waterproofing membrane of high quality is strongly recommended. Alternatively,

a purpose-made living roof heavy-duty single ply membrane with felt layer can be used. The

membrane must be installed correctly ensuring seams are sealed and breaks for equipment

(e.g. air vents or air conditioning units) are sealed, while still allowing access for maintenance.

The waterproof membrane should be tested prior to any other material being placed upon it

to ensure its water-tightness, particularly where external equipment is fitted to the roof.

Other stages of construction, such as laying drainage material or planting media, have the

potential to rip or puncture the membrane so extreme care should be taken with tools and

activities on the roof once the membrane has been sealed. Any faults in this membrane will


result in significant delay in the installation process and additional costs to repair the

membrane. If not identified immediately, faults may result in leaks into the building.

The key considerations for installing the waterproof membrane are:

Protect the membrane throughout construction

o Most damage to waterproof membranes occurs during the construction phase

o Nails, screws, or cutting implements should not be present on the rooftop when the

membrane is being laid

o Drainage mats provide a physical block for shovels or other gardening implements

which could cause damage to the waterproof membrane

o Some drainage mats are designed with a geotextile on the bottom (can also contain a

root barrier) which helps protect the membrane from the sharp edges of the mat

itself

Test the integrity of the waterproof layer in place before installing any other features

o It is much more cost-effective to spend more up-front to ensure the waterproofing

achieves its purpose, rather than having to repair it once the substrates and plants

have been installed

o Testing should be performed by an unbiased third party, rather than the membrane

installer

Flashing, substrate, or a gravel edge should completely cover the waterproof membrane

o Any exposed membrane is susceptible to premature UV damage

o Even small defects can cause a leak in the future

Extra caution should be used when sealing around protrusions

o For new construction, roof design should minimise protrusions when a living roof is to

be built

o Protrusions typically provide significant opportunity for leaks

20.5.7 Leak detection

Several methods for leak detection have been developed:

Flood test

o Standard approach for assessing integrity of a newly installed waterproof membrane

o Basic approach is to fill the roof with several centimetres of water and measure water

level drop over a period of approximately 24 hrs

o The method only works for flat roofs before installation of the drainage mat,

substrate, or vegetation


o Accuracy is subject to relatively coarse measurement l imitations; very small

penetrations may not be detected

o The method itself can cause damage to the roof if the membrane integrity was

compromised

Electric field vector mapping (EFVM)

o Relies on electrical conductance - a low electrical voltage is applied over a thin layer

of water which has been spread over the surface to be leak tested

o A watertight membrane will prevent detection of electric potential using a

potentiometer; compromised membrane integrity is indicated when voltage is

detected

o Enables isolating the location of the breach, and may also identify potential future

failures (e.g. small punctures which may not have yet fully penetrated the membrane

surface)

o Non-destructive or invasive method which may be performed on a sloped roof, and

may be conducted between layers of living roof installation (e.g., to verify that

drainage mat installation has not compromised the integrity of the waterproof

membrane) and/or any time after the living roof has been installed

o For some roof surfaces (e.g. ply) it is necessary to include a mesh layer beneath the

surface at the time of installation in order to perform EFVM testing, but for most roof

surfaces no additional layers are required

The best approach is to take extra care with installation of the waterproofing system, and

subsequent layers above the waterproofing, as prevention is virtually always less costly than

repair.

20.5.8 Drainage layer

When using a drainage mat with geotextile attached:

Inspect all products for defects prior to installation

100 mm of overlap is recommended between each layer of the drainage mat and the

adjacent sheet

The ease and speed of laying drainage mat improves with:

o use of wider drainage mat cores to allow fewer sheets to be cut to cover the roof area

o products with a plastic overlap zone without cups

o products with an additional width of geotextile above the width of the drainage

board to allow for overlap between sheets

o avoidance of irregular living roof shapes (e.g. curves or a V shape) as this makes

cutting and joining the drainage mat difficult


o installation of the drainage sheets on sloped roofs parallel to the length of the slope

(not crosswise) to prevent slippage before the substrate is installed

When using a separate drainage board/aggregate drainage layer and geotextile layer:

Installation may be complex in windy conditions as components are difficult to keep in

place until covered by substrate

20.5.9 Substrate

Typical extensive living roof substrate is comprised of 80-90% (by volume) light-weight

aggregate (LWA) and 10-20% (by volume) organic matter. LWA provides pore space for air,

water, and gas exchange, and ensures rapid drainage.

The main substrate considerations are safety, substrate weight, and stormwater control. It is

essential that the substrate be tested post-mixing, prior to installation, to ensure the

parameters do not violate design criteria. Table 58 identifies what tests need to be

performed, their purpose, methods of testing, and the minimum requirement. Refer to AC

TR2012/010 for detailed discussion. If the substrate does not meet any one of the test criteria,

in the interests of safety, it should not be used until modifications have been made to rectify

the shortfall.

Table 58

Substrate specifications to test post-mixing

Characteristic Purpose Method Minimum Standard

Dry bulk density Structural loading Standard geotechnical

test, FLL, ASTM E2399-

11

Depends on roof

structure design

Weight at field

capacity

Structural loading FLL or

ASTM E2397-11

Depends on roof

structure design

Saturated weight Structural loading FLL, ASTM 2397-11

or equivalent

Depends on roof

structure design

Permeability Structural loading,

plant health

FLL or

ASTM E2399-11

> 1800 mm h-1

> 3600 mm h-1

(if no

dedicated drainage layer)

Particle size

distribution

Structural loading,

plant health

Dry sieve, e.g.

ASTM C136-06 or

AS1289.3.6.1-1995

Check this if there is a

problem with weight or

permeability

Readily available

water + stress

water

Stormwater control,

plant health

Tension test

10-1500 kPa

@ finished depth > water

quality storm depth


Physically getting living roof materials onto a rooftop provides several challenges. Substrate

installation may be independent of planting. As substrate installation likely requires

mechanised equipment, new construction should coordinate (at least) substra te installation

with the presence of other mechanised equipment (e.g. crane or blower) on site to save cost.

Two primary options are available for installing substrate:

Crane hoist of bags

o Bagging of substrate in 1 m3 bags is typical for most media suppliers

despite being characterised as “light-weight”, a 1 m3 substrate-filled bag

may weigh in excess of 800 kg m-2

bags should not be emptied as a point load directly onto the roof, unless

the roof is designed to manage such a load

o Cranes are usually available on-site for new construction

As the building shell is often the first task for completion, a living roof can

be installed as soon as the waterproof membrane is installed and leak -

tested

Plants are allowed time to become established while the building interior

is completed; the living roof is then “ready” when the occupants take up

residency

o For retrofit installations, the crane must be able to get close enough to the building

limiting factor in specifying crane size may be reach, rather than lift

(weight) capacity in order to avoid damage to street trees or neighbouring

buildings

Blowing or spraying substrate to height

o Specialty pneumatic blowers are often used in erosion control to rapidly seed bare

surfaces, the same type of equipment can be used to transport substrate to a

rooftop

o Length of the hose may limit the height to which substrate can be blown, and is

specific to the service provider

o Contingency costs should be incorporated, as substrate may coat building facades

and thus require post-installation cleaning

o Operation of blowers may be compromised in cold weather, as the system may

clog (in which case bag hoists would be a preferred option)

Extensive living roof substrates should be hand-spread using rakes (with small tines so as not

to break the geotextile) or other methods which do not promote compaction.

Nonetheless, some compaction will occur over time due to planting and weeding activities.

The substrate materials will also settle due to self-weight and plant mass. It is recommended

to anticipate compaction of 10%; substrate depth should be increased accordingly during

installation.


When installing substrate on sloped roofs, install the substrate from the base of the slope back

towards the ridge to prevent slippage.

20.5.10 Access

Ensure the project site has space in which the equipment needed to store (if needed) and

install the substrate and plants can operate, as large trucks and cranes may be needed.

Substrate can rarely be stored on the roof without overloading the structure. If the substrate

is not bagged, it will need to be protected from weeds and insect colonisation. The water

content of the substrate also needs careful management: dry substrates may create a dust

and health hazard (associated with use of composts); very wet substrates are heavy, slower,

and more difficult to spread, may be susceptible to degradation through compaction.

It is important to minimise foot traffic over the living roof, unless it is designed for such.

Excess foot traffic will result in compaction of the substrate and thus reduced permeability;

water in excess of the system storage capacity may not be able to exit the roof during large

rainfall events (a potential safety issue).

Allow for access paths between plantings to access air vents and air conditio ning units and

maintain plants. Gravel edging around living roof edges, outlets, and protrusions allow s for

easy identification of, and access to, these potential weak points in the water proofing.

Furthermore, they prevent plant material overgrowing and clogging installations such as

outlets or air conditioning units.

Worker safety must be considered as for many living roof installations the work will be

conducted at height. Appropriate harnesses, restraints and/or barriers, in addition to

appropriate training for working at height, may be required.

20.5.11 Planting

Living roofs plants are selected based on their ability to withstand the harsh conditions found

on a living roof. Plant selection for living roofs should be based on an assessment of each

individual living roof, and where the features listed below vary across a roof, sections of the

living roof for:

Minimal irrigation or fertiliser needs

Shallow root depth

Low nutrient requirements (for extensive living roofs)

Dense, self-repairing foliage cover

Low mature plant height (to reduce effects of wind exposure which may dessicate plants)

Drought and wet season tolerance

Full sun tolerance

Unlikely to spread off roof to neighbouring properties


The predominant plant species used on extensive living roofs overseas are sedums, which are a

variety of readily available, but non-native, succulent plant. As native plants are more likely to

also promote habitat creation for native invertebrate and bird species, various New Zealand

plant species have been identified (AC TR2010/012)4.

In Auckland, the ideal planting time is from mid-autumn to early-spring (April to September).

If plants are established using seed or cuttings, summer should be avoided. If substrate must

be installed in summer, it is recommended to cover the substrate with an erosion control

netting or mat, and delay planting until the appropriate season. Outside Auckland, in areas

where frost, snow, or frozen substrates could occur, planting in winter is best avoided. The

feasibility of delaying planting depends on building configuration –plants can generally be

carried to the roof in an elevator (perhaps in multiple lifts), but the method would not be

viable for buildings without direct roof access from the lift.

In terms of construction sequencing, containerised plants or pre-grown mats will take two to

12 months to be grown (the longer times generally for growing plants from seed) and may

only have limited viability before becoming root-bound or over-mature. Plant suppliers

therefore need to be kept informed of construction sequencing, particularly within a month of

delivery, to allow adequate hardening off and timely pest control. Once plants are delivered

to site, they should be planted within several days to reduce stress. Plants at ground level are

at risk of damage from drought stress, pest invasion, and cannot be stored for more than a

few days without adequate light or air circulation, particularly when stacked in pallets. Plants

must therefore arrive on site just before planting. Plants should be watered immediately

following installation. Refer to ARC TR2010/053 Operation and Maintenance of Stormwater

Treatment Devices5 for more detail on living roof operation and maintenance.

In New Zealand, most roofs are established using plugs or root tra iners, however, four other

methods are also used overseas: seed, cuttings, pre -grown mats, and pre-planted modules.

Plugs and Root Trainers

Planting using plugs or root trainers offers the most design control in terms of visual effect.

Most ground covers should be planted at 10–25 plugs m-2

. Higher densities are used where

growth rates are expected to be slower, or the maintenance period is required to be shorter,

e.g., due to a high potential for weed invasion or short time to building completion. Wood y

plants and tussocks such as Festuca matthewsii and Poa cita can be planted at a maximum of

about 5 plugs m-2

, as these species should reach 300 to 600 mm diameter in 24 months.

On a roof subject to high winds, initial plantings can be held in place using erosion netting or

fabric. These can be biodegradable (e.g. coir and wool), which will also hold some moisture,

or permanent (e.g. some plastics and metal lattices). Straight-sided, or gently tapering plugs

should be specified as this shape is resistant to wind; bowl-shaped or pyramid-shaped plugs

should be avoided as they are prone to being blown out, are difficult to plant deeply, and the

roots, being close to the surface, lose moisture quickly. Matching plug length with the depth

of the media ensures maximum root contact and stability.

Trays of plants may be delivered to the roof stacked in pallets and in a relatively dry state to

reduce their weight. Upon delivery to the roof, plants should be thoroughly watered. Each

5 Ref to ARC TR2010/053


plug should be visually inspected before planting so any weeds can be removed to disposal

containers and out-of specification plants separated. This process should occur on a hard

surface (without substrate) or groundsheet on top of the substrate to minimise the

establishment of unwanted insects, lizards, slugs, snails, and weeds to the roof. Empty trays

should also be stored off the substrate as the underside of trays and gap between potting mix

and pot are common places for invertebrates to hide. Where biosecurity is extremely

important, e.g. for creation of NZ native invertebrate habitats, plants should be removed from

their containers on the ground.

Cuttings

Sedum living roofs are commonly established in North America by spreading cuttings during

favourable seasons at a density of about 12 to 25 kg 100 m-2

. Many sedum species will readily

sprout from stem segments or leaves that break from the parent plant, and spread this way on

established roofs. Sedum cuttings do not need to be inserted into the substrate, but they

should be held down until plants take root. An erosion mat, netting or tacifier (sticky

substance) promotes contact of the cuttings with the substrate. Under Auckland autumn

conditions, Sedum cuttings root in two to four weeks and remain viable for several months.

Broadcasting cuttings generally gives low design control in terms of visual effect, but the

installation cost is greatly reduced compared to planting if successful, i.e., if post -

establishment conditions allow the cuttings to anchor into the substrate an d grow; temporary

irrigation increases the likelihood of good establishment.

Some native NZ plants can be established from rooted cuttings and divisions planted into the

surface. Rooted cuttings have been used to successfully establish Coprosma (C. acerosa incl.

‘Hawera’) and Libertia peregrinans on a few living roofs. Hebes and Disphyma (iceplants)

should also be readily-established from rooted cuttings at favourable times of the year, while

some Arthropodiums are readily divided; however, few roofs have used these techniques to

establish native plants on living roofs to date. Bromeliads and many succulents can be similarly

propagated by removing partly-rooted ‘pups’ from adult plants. An advantage of using ‘pups’,

divisions or rooted cuttings is the greater resilience once established compared to mature

plants.

Cuttings are most successful when the time to establishment of roots is reduced - this is

influenced by temperature, moisture availability, and extent of contact of the stems/rooted

sections with substrate. Generally, wetter and warmer conditions favour establishment from

cuttings.

Seed, straw and bulbs

Living roofs can be vegetated by broadcasting seed, hydroseeding seeds, moss and plant

fragments and by hand-planting bulbs. Very few roofs are established solely with seed

because plants would take too long to establish, leaving a roof vulnerable to weeds and

erosion. Extensive living roofs with their coarse media and high evaporation rates (low

humidity, high wind and sun exposure) are relatively hostile environments for seed

germination; optimum autumn or spring sowing is a key success factor, as is provision of

irrigation.

Small, early-flowering, summer-dormant bulbs in the onion (Allium), tulip (Tulipa), and Iris

genera are used in Europe and North America to add seasonal interest and height to extensive

living roofs. The main disadvantage to use of bulbs is dead and dying foliage in summer that


is both unsightly (close up) and can create bare patches that are available for weed

establishment. It is recommended that bulbs are therefore best planted singly or in small

groups combined with plants that cover the ground throughout the year. A minimum

substrate depth of 100 mm is recommended for bulb species other than chives.

A variety of biodegradable coir (coconut fibre) and wool-based blankets that can be pre-

seeded have become available in New Zealand over the last few years. These products have

been used primarily for establishing grasses in erosion-prone areas, but could be adapted for

use on living roof installations. A key factor is ensuring the blanket can be securely fixed to

the roof, to avoid being blown from the roof, as traditional pegs cannot be used without

potential damage to the waterproofing layer. Such mats can be used onl y in wind-prone areas

or to temporarily reinforce surfaces where water runs onto a roof from higher surfaces.

In Switzerland and in London, on sites where the design objective is a local ruderal (‘wasteland

or river-gravels’) or meadow flora and insect fauna, substrates may be left uncovered at

ground level to naturally ‘inoculate’ with local seeds and non-flying insects. Storing substrate

on pallets enabled it to be craned onto the roof with minimal disturbance to invertebrates.

Hay containing seed heads, spread at about 20 mm depth is also being used on some projects.

Both approaches require vigilant, informed weed maintenance to prevent establishment of

very aggressive species. An extended maintenance period is also likely unless ‘placeholder’

species such as sedums are used to reduce the area of bare ground into which weeds can

establish. In urban and rural New Zealand, this approach is unlikely to favour establishment of

native species.

Pre-grown mat, sod and containers

Pre-grown mat, sod and containers offer an instantaneous effect upon installation and are

therefore particularly useful for sites, or parts of sites, with a high erosion risk and for

temporary installations. Such mats require 4–8 months to grow before installation. Mats and

sods are often rolled up for transport and installation. In this form plant roots are exposed and

must be protected from drying out (wind and sunshine); they also need to be protected from

sunlight to minimise the risk of ‘composting’. Effective coordination be tween the mat or sod

supplier, living roof installation team and main building contractor is therefore required to

avoid storage by ensuring access for delivery trucks to the loading site and lifting equipment is

available (e.g. a crane or forklift) when plants arrive. Ideally, the mat or sod should arrive on-

site on the day of installation to avoid storing for more than a couple of days.

Sods should be grown in the living roof substrate to avoid layering and ensure consistency

with roof loading and infiltration rate specifications. However, there is also potential to

design living roofs to receive sod cut from suitable native ecosystems with shallow rooting

depths, if hydrological conditions can be matched. Such roofs would be most likely where

structures are ‘inserted’ into highly sensitive natural landscapes, using sod transfer techniques

developed in the mining industry. Both sods and mats generally have thin substrate, and

plant roots are cut prior to transportation – both increase plant stress. Irrigation is necessary

until roots grow into underlying substrate. If mats are prone to shrinkage on drying, irrigation

is needed to avoid exposing underlying roof components.

Containerised, or modular, systems are commonly planted and grown before instal lation.

Modular systems have recently become available in New Zealand; the most resilient and

aesthetic systems have substantial substrate contact between adjacent modules, enabling


plant roots to travel between containers, reducing edge effects and minim ising the potential

for ‘stripes’ to develop below tray edges when plants are drought-stressed. Biodegradable

containers also reduce the edge effect. Containers are particularly suited to risk -averse clients

as they can be removed (provided plastic edges are not exposed and degraded by UV),

allowing easier maintenance of membranes – they therefore are common in North America, a

relatively new living roof market, and likely to be popular in New Zealand. Modules can also

be carried up stairs and lifts, and placed over existing membranes, thus suiting small retrofit

applications. However modular systems are uncommon in Germany and Switzerland where

living roofs are common and extensive roofs have been installed for decades.

Pre-grown mats, sods and containerised systems have a high potential for introducing both

desirable and undesirable invertebrates and plants. On the University of Auckland living roof,

the sedum mat was the source of some ‘weeds’ (narrow leaf plantain and dandelion), the

beneficial earthworm population, as well as four slug and snail species (all non-native).


Table 59

Specifications for the construction of living roofs should consider the following:


Insulation

layer

• Type, manufacturer, and thickness of insulation required.

Waterproof

membrane

• Type, supplier, and minimum thickness of the membrane required

Drainage

layer

• Type of drainage layer required: mat/board or aggregate

• If mat/board: supplier, product type, whether it will be installed “cups

up” for more water retention or “cups down” (where applicable), and

depth.

• If aggregate: the grade, material type, and installed depth.

Root barrier/

geotextile

• Type of non-woven geotextile material or root barrier (both may be

included with the drainage mat/board).

• Details should include the required overlap and whether the root barrier

is sealed.

Growing

media/

substrate

• Detailed description of the composition of the growing media will

typically be included with the design drawings (aggregate and organic

component source, particle size distribution, mixing method)

• Specifications should detail the quality of the media required, including

tests to ensure the minimum requirements are met prior to installation

• Final depth, including allowance for compaction/settling).



Plants • A planting schedule is normally included with the drawings and will

provide details on the type (genus and species), and size (e.g. plugs,

cuttings etc.) of the plants required as well as spacing and planting

method.

• Specifications should enforce compliance with this planting schedule.

Outlet • The diameter, size, type and material of the outlet.

Edging • Material type, and aggregate grade

• Width of perimeter edging, fire breaks, and edging around outlets

and/or protrusions


construction of living roofs, but exclude particulars such as site establishment, health and


application.


living roof, and should be used as a ‘starter specification’ for guidance only. Each living roof

will be site specific and require careful consideration to ensure that all aspects of construction

are covered.


Table 60



Insulation

layer

• Insulation layer installed by a qualified contractor as per the

manufacturer’s specifications.

Waterproof

membrane

• Waterproof membrane installed by qualified contractor as per

manufacturer’s specifications.

• Impermeability test shows no leaks (if flood testing, over a minimum

period of 24 hours but preferably 48 hours).

• Post installation, ensure none of the waterproofing is exposed, to

prevent weathering or damage.

• If using EFVM, perform impermeability tests on the waterproof

membrane post installation to ensure no damage occurred during

installation of subsequent layers.

• Photograph the membrane and application process for inclusion in

the operation and maintenance manual. This will help when

troubleshooting problems in the future.



Drainage

layer

• Check that the drainage media is spread evenly over entire living roof

area.

• Ensure all connections to the downpipes or outlets are watertight.

• Take caution not pierce or damage the waterproofing layer below.

Root barrier/

geotextile

material

• The root barrier should be installed as per the manufacturer’s and

design specifications.

• Geotextile material should be laid over the drainage layer, if it is a

separate layer

• Geotextile should overlap at least 100 mm, or as per manufacturer’s

specifications.

• Take caution not pierce or damage the waterproofing layer below.

Growing

media/

substrate

• Check that the growing media composition matches design

specifications.

• Inspect the growing media after installation to ensure that distribution

matches design specifications. In many cases substrate should be

spread evenly over the roof area, some designs may specify variable

depth zones to support different vegetation types and habitat

development.

• Ensure growing media has no compaction (caused from moving heavy

equipment or supplies over recently laid media).

• Take caution when laying media not to pierce or damage the geotextile

or drainage layer below.

Plants • Check that the plants match the planting plan in regard to size, plug

shape (if using plugs), and species.

• Ensure that plants are planted deep within the growing media, so as not

be blown out from high winds and retain moisture content.

• Ensure an erosion mat is in place, if specified.

• Take caution when planting to not pierce or damage the geotextile or

drainage layer below.

Irrigation • If fixed irrigation is included in the design, check installation matches

specification.

• Ensure plants have been watered immediately post installation.

Outlet • Test to ensure positive drainage

Edging • Test to ensure positive drainage


20.8 Photo gallery – living roofs

Figure 130

Victoria Ave living roof.

Figure 131

Furneaux Lodge house living roof.


Figure 132

Kawakawa’s Hundertwasser Toilets living roof.

Figure 133

Maori Bay Toilets living roof. [ARC]


Figure 134

Placement of growing media at Waitakere City Council living roof. [WCC]

Figure 135

View of completed living roof at Waitakere City Council building. [WCC]


Figure 136

Set-out of Auckland University’s living roof. [UoA]

Figure 137

Completed Auckland University living roof. [UoA]


Figure 138

NZI Building living roof. [ARC]

Figure 139

NZI Building living roof. [ARC]


Figure 140

Botanic Gardens Toilet Block living roof: waterproofing membrane and stainless steel edging

Figure 141

Botanic Gardens Entrance living roof: drainage mat installat ion, note overlap of the geotextile (f ilter

cloth)


Figure 142

Botanic Gardens Entrance living roof: temporary wooden edging for placement of gravel edging stones,

drainage mat extends under the zone in which edging stones were placed, bags of stones keeping joins in

drainage mat in place.

Figure 143

Substrate installat ion: Left - Botanic Gardens Entrance living roof, installing substrate from edging

inwards and upslope; right- an alternative method of spreading substrate straight from the truck (private

garage with the same substrate).


Figure 144

Botanic Gardens Toilet Block living roof: upper edge of the roof was completed last, f ilter cloth overlaps

drainage mat that extends almost to top of slope, but underneath irrigation hose along ridge to prevent

substrate slipping underneath drainage mat, gravel edge provides visual cue for location of irrigation

hose and connections.

Figure 145

Botanic Gardens Toilet Block living roof: plants inserted through coir nett ing, addit ional nett ing was used

to protect the surface from wind- and water-induced erosion, and to help deter birds from pulling out

smaller plants.


Figure 146

Botanic Gardens Toilet Block living roof: watering the roof immediately after planting helps plant

establishment, cleans stones, and tests drainage and gutters

20.9 Construction specification example – living roofs

20.9.1 Living roof components

20.9.1.1 Insulation layer

[This is optional] Installed to the manufacturer’s instructions.

20.9.1.2 Waterproof membrane

The membrane shall be installed as per the manufacturer’s instructions. The membrane shall

be waterproof and after installation an electric field vector mapping (EFVM) impermeability

test shall be undertaken. Any identified weaknesses, punctures, or leaks must be repaired

before any other layers are installed.

20.9.1.3 Drainage layer

If using a specialty product with integrated geotextile (filter fabric), and root barrier, the

drainage layer shall be installed as per the manufacturer’s instructions. The dra inage layer

should be installed such that the geotextile is placed with overlaps of at least 100 mm.

Installation of drainage sheets on sloped roofs should be parallel to the length of the slope

(not crosswise) to prevent slippage before the substrate is installed.

If using aggregate, the drainage layer shall be laid on the living roof as detailed on the

drawings.


20.9.1.4 Root barrier

The root barrier can be combined with the geotextile layer as a laminated upper layer; as a

separate layer above the waterproof membrane, or by chemical additives in the waterproof

membrane.

20.9.1.5 Geotextile layer

The filter layer can be a single layer of non-woven geotextile, installed separate to the

drainage layer (aggregate or mat), or can be affixed to a proprietary living roof drainage mat.

20.9.1.6 Growing media

Growing media composition shall comply with the design specification.

Construction specifications should identify the tests required to confirm adherence to the

minimum design specifications prior to installation, and the method of installation.

The installed depth of substrate should be increased to anticipate 10% compaction to achieve

the final design depth.

20.9.1.7 Planting schedule

Planting shall comply with the planting schedule. Plant spacing shall comply with the plan

and shall be at the spacing intervals defined for each species with holes dug for plants deep

enough so as to cover the root ball of the specimen.

Caution must be taken during planting not to damage the geotextile and drainage layer

below.

20.9.1.8 Outlet

The outlet shall be connected to the spouting by a PVC pipe as detailed on the drawings. PVC

pipes shall be manufactured in accordance with AS/NZS 1260 “PVC Pipes and Fittings” and

installed in accordance with NZS 7643 “Installing uPVC Pipes”. The outflow connection to the

stormwater system shall comply with NZS4404:2004 and as per the local council standards.

20.9.1.9 Irrigation and water supply

If a permanent irrigation system is specified, connection to the mains for irrigation shall be as

detailed on the drawings and shall comply with the local council standards for connection to

water mains and the NZ Building Code.


21 Infiltration Trenches and Wells

21.1 Introduction

There are vast array of infiltration devices in use in the Auckland region, often designed for

specific site and geological conditions. Infiltration devices are often known as soakholes. Rain

gardens, swales and permeable paving systems can also be constructed for infiltration of

stormwater, rather than discharge to stormwater reticulation systems. This chapter only

looks at infiltration trenches and dry wells, which are also known as private soakage.

Soakholes and larger public infiltration systems are often made of precast concrete chambers

with perforations allowing water to percolate to the next chamber and ultimately the

surrounding ground. Some also include bores that extend into underlying aquifers to

maximise soakage and disposal rates.


All infiltration devices direct stormwater runoff to the underlying geology and ultimately

groundwater. The key function of infiltration devices is to reduce the volume of stormwater

runoff, reduce surface water contaminant loadings, and extend baseflow in streams. Runoff is

stored in an open chamber or the void space between the storage medium (aggregate) and

slowly permeates into the surrounding ground where natural biological processes can improve

water quality. Infiltration devices are most effective in permeable natural soils with water

tables a minimum of 1 m below the base of the device.

Infiltration devices can have high failure rates if careful consideration is not given to site

selection and maintenance. Catchment characteristics, particularly potential sources of

pollution (e.g. oil, grease, high sediment loads) need to be identified and suitable pre -

treatment included in the design of the infiltration device. There is also significant risk of

polluting groundwater if runoff is not sufficiently treated prior to infiltration. Geotechnical

stability also requires consideration to avoid ground failure caused by too much water

permeating the ground.

Infiltration trenches: Trenches can be buried and receive inflow either from pipes or surface

trenches that receive sheet flow runoff. Pre-treatment is essential to prevent the trench from

clogging. Infiltration rates should be between 3 mm hr-1

and 1 m hr-1

in order to achieve

attenuation and treatment, and their suitability is therefore highly dependent on geology.

Trenches with a piped inflow point often have a dispersion pipe the length of the trench to

evenly disperse runoff. In this situation a trench may be grassed, however clear delineations

are required to protect the trench from any activities which could compact the soil and trench

(e.g. vehicle or pedestrian traffic).


Dry wells: Operate by receiving runoff from roofs only. Pre-treatment is not required

however gutter guards and screens are essential to prevent organic material entering the dry

well. Dry wells are also known as private soakage.

Figure 147

Typical infiltrat ion trench components (cross-sect ion and long sect ion).

Figure 148

Typical dry well components (cross-sect ion).


Table 61

Key components of infiltrat ion trenches and dry wells.


Pre-

treatment

Pre-treatment is strongly recommended to remove larger sediments as this

helps prevent clogging of the infiltration media. For example swales,

forebay type structures or surface sand filters are often used. Dry wells do

not normally require pre-treatment as they only accept roof runoff, however

gutter guards or an inline leaf/debris diverter are essential to prevent organic

material entering the dry well (organic material is a source of

contamination).

Inflow points Where the stormwater enters the infiltration trench or dry well. Trench:

inflow can be piped or via a swale or other pre-treatment device. Dry Well:

infow via a pipe, usually downpipe from roof.

Overflow

controls

Piped inflows will have an overflow pipe which will discharge directly to the

stormwater system or overland flow paths. Trenches that rec eive inflow

from the surface may have an overflow weir on the downstream side of the

trench. When the weir is overtopped runoff will flow to a nearby catch pit or

overland flow path. Overflow must be diverted safely to prevent flooding

and erosion of surrounding land.

Observation

well

A vertical perforated PVC pipe that is anchored to the bottom of the trench

or dry well, with lockable cap (100-200 mm diameter). Allows for

maintenance inspections (to check infiltration rates) and if necessary to

remove standing water that may indicate clogging or high groundwater

levels. Larger trenches may require an inspection entry portal, however

confined space health and safety measures should always be adhered to

before entering the portal for inspection.

Surface layer Visible surface of the infiltration trench or dry well. Trench: this layer is

often pea gravel (depth of 300 mm) and the first stage of filtration. Dry Well

(or buried trench): this layer will be topsoil and grass or paving (depth of 300

mm) with no function other than aesthetics.

Dispersion

pipe

A perforated horizontal pipe that runs the length of the trench or the width

of the well. Disperses runoff from a piped inlet to the whole area. The

dispersion pipe is covered by a filter sock to prevent clogging of storage

media.

Geotextile Non-woven geotextile material lines the walls (and in some cases the base)

of the trench or dry well to prevent migration of surrounding soil into the

infiltration device and blinding of the surrounding soil by fine sediments.

Geotextile can also be used between layers of media (e.g. sand and gravel)

and to cap the dry well or trench before finishing the surface (e.g. grass,

pavement). A non-woven geotextile is preferred due to its filtration ability

and it is less likely to ‘open’ with pressure from aggregates or sediment.

Note that geotextile can become a clogging point leading to failure of the

device by preventing water infiltration.



Storage

media

The component of the trench or dry well that provides storage of runoff

(depth typically 1-2 m). Usually aggregate, runoff is stored in the voids

which are usually 30%-40% of the total volume. Aggregate size will vary

with the necessary infiltration rate but are often graded between 25 to

75 mm.

Base sand

filter

At the bottom layer of the trench (not usually present in dry wells) and

provides enhanced treatment of runoff. Depth of 150 mm to 300 mm. Can

add fibric or hemic peat to target particular contaminants, however the

addition of peat will reduce infiltration rates so must be considered during

design phase. Geotextile can be used between the sand and the storage

media.


Guidelines relating particularly to infiltration devices are listed in the following table. Note

that a detailed review of council standards and guidelines should be carried out for every

project, as these are frequently updated.

Table 62

Guideline documents relat ing part icularly to infiltrat ion devices.


ACC On-site Stormwater

Management

Guidelines for the reduction of impacts of

stormwater runoff (water quantity) and

pollution (water quality) resulting from

intensive development throughout the city.

Information on specific stormwater

management devices.

ACC Soakage Design Manual This manual solely addressing infiltration

devices, including percolation testing, types of

devices, design procedures, construction, and

operation and maintenance.




Auckland Region






Region




development.



ARC TP95 Long-term Baseline

Groundwater Chemistry

Provides a database of groundwater quality of

the primary aquifers in the region. This is

useful when determining pre-treatment water

quality objectives if discharge is to an aquifer.

ARC Regional aquifer allocation,

management plans and

resource assessment reports

Plans and reports for specific regions with

groundwater resources. Provides information

on allocation, availability, specific quality or

quantity issues and threats.




Network

Provides guidance on a range of issues relating



Zealand.







Residential Sites








Table 63


relate to infiltrat ion trenches and dry wells:

Title Description

AS/NZS 2033:2008

Installation of

Polyethylene Pipe

Systems




liquids.

AS/NZS 2566.2:2002

Buried Flexible Pipelines –

Installation





Title Description

NZS 7643:1979 Code of

Practice for the

Installation of

Unplasticized PVC Pipe

Systems

Details requirements for the installation of uPVC pipes.

SNZ HB 2002:2003


Working in the Road

This Handbook deals with aspects of the roles and

responsibilities of Road Controlling Authorities, principal

providers, utility operators and contractors; consents and work

approvals; and details of construction requirements; for the

purpose of installation and maintenance of utilities within the

road corridor.

AS/NZS 1547:2000 Onsite

Domestic Wastewater

Management (Appendix

4.1F Soil Permeability

Measurement – Constant-

Head Test)

Appendix 4.1F outlines the methods and formula for testing

permeability of in-situ soils.

AS/NZS 4411:2001

Environment Standard for

Drilling of Soil and Rock

This Standard sets out the minimum national environmental

performance requirements for drilling of soil and rock, the

design, construction, testing and maintenance of bores, the

decommissioning of holes and bores, and record keeping.

Required for any rock bore infiltration devices.


21.5.1 Infiltration trench and dry well location

The catchment for an infiltration trench should be about two hectares (maximum four

hectares) while a dry well catchment can be significantly smaller. Infiltration devices are

usually only used in areas where the natural soil is free draining and the water table is dee p or

well below the base of the infiltration device. Test pits or bores on site can be used to

estimate infiltration rates of the surrounding geology of the site. Given the risk of

contamination of groundwater from infiltration of stormwater runoff it is essential that

comprehensive investigations are made into the suitability of a location for an infiltration

device particularly where water supply bores are found.

Trenches should follow the same contour of the site to ensure they are level. Dry wells sh ould

be located at least 3 m from any building foundations to prevent subsidence. Allow for

downpipe extension to enter dry well.



The trench or dry well may be excavated during the earthworks stage of construction (if part

of a larger site development). If this is the case, the excavation needs to be protected from

sediment laden runoff from other construction activities and clean runoff diverted until the

catchment area has been stabilised. Impermeable geotextile can be used to ‘seal’ the

excavation. The area surrounding the trench or dry well will also need to be protected from

vehicles as any compaction will greatly reduce the infiltration rates of the natural ground.

Sediment ponds or decanting earth bunds should not be used for infiltration devices unless

the area is thoroughly dug out including any blinded natural soils.


If the dry well or trench is buried and the surface is to be grassed consideration should be

given to when grass strike will be at its greatest i.e. autumn or spring. If the infiltration device

is to be constructed during the summer months, in line with the earthworks season, then

watering at regular intervals should be specified.

21.5.4 Permeability

Permeability of the natural soil must be maintained in order for the device to continue

functioning. At no stage during construction should the natural soil be compacted either from

tracking machinery across the area or using the area for stockpiling. When selecting the site

for the infiltration device consideration should be given to long-term protection from

activities which could compact the natural soil.

21.5.5 Site slope

Infiltration trenches should not be placed on slopes greater than 15% as the water movement

in the natural soil can cause stability problems. A comprehensive geotechnical investigation

would be required to allow an infiltration trench on a slope greater than 15%. The surface and

base of the infiltration trench or dry well must be level. If water naturally flows in a particular

direction check dams can be constructed on the bottom of the trench to prevent water

ponding in one corner.

Trenches operate best when the up gradient is 5% or less and the down gradient is up to 15%.

If the infiltration trench is a surface trench the level can be slightly lower than ground level to

capture runoff or an overflow berm can be constructed on the down gradient side of the

trench.

21.5.6 Geology

Hard rock or impermeable layers may impact on the infiltration rates of the surrounding soil

and the effectiveness of the infiltration device. During the design phase test pits or boreholes

should be constructed at least 1.5 m below the bottom of the infiltration device. This will


determine infiltration rates for design. The seasonal high water table should be a minimum of

1 m from the base of the infiltration trench or well.

In areas where the slope is greater than 15% a comprehensive geotechnical investigation will

be required to determine suitability of site.

21.5.7 Pre-treatment

All infiltration devices must have some form of pre-treatment to reduce sediment loads to

help keep the device free from clogging. The extent of pre-treatment will depend on whether

the runoff is injected into an aquifer. If runoff is injected, the runoff must go through water

quality pre-treatment to achieve water quality that is geochemically compatible with the

receiving groundwater. Dry wells only receive runoff from roofs so while pre -treatment is not

strictly required, gutter guards and downpipe screens should be fitted to prevent organic

matter (e.g. leaf litter) entering the dry well.

21.5.8 Equipment

The correct equipment is very important when excavating the trench or well as the natural soil

surfaces can be sealed with certain types of machinery. Backhoe excavator or ladder type

trenchers are most suitable for excavating the trench or well (rather than front loader or

bulldozer). When excavating, care needs to be taken not to compact surrounding natural soil

with the heavy machinery. In areas with fractured rock, rock breakers may be required.

21.5.9 Structural integrity

When constructing an infiltration trench care must be taken when it is located beside hard

surfaces which potentially have heavy loads, like car parks or road ways. Retaining walls or

other reinforcing such as edge beams may be required to ensure the walls of the infiltration

trench do not collapse under the load, or the surface of the road or car park does not subside.

An alternative is to set the infiltration trench back far enough from the car parking area or

road way so as not to undermine the walls of the trench with the extra weight or the surface of

the car park or road way.

21.5.10 Surfacing

Trenches can either be buried or exposed at the surface. Buried infiltration devices (whether

trench or well) are usually topped with 300 mm of topsoil and grass with a layer of geotextile

between aggregate and topsoil for improved treatment. Consider using instant turf rather

than grass seed or hydroseeding for immediate effect and treatment.

If the trench is exposed at the surface and receives sheet inflow, consider finishing the trench

with 300 mm of pea gravel with geotextile between the storage aggregate and the pea gravel.

The pea gravel maximises filtering ability and allows for easier maintenance. If an infiltration

trench or well is not performing, check the state of the geotextile fabric as it can be a point of


clogging. Refurbishing the geotextile or underlying pea gravel layer will often remedy any

infiltration problems.


Table 64

Infiltrat ion trenches and wells construction specifications will need to detail:


Excavation • Specify the need to verify ground conditions and soil characteristic

encountered when excavating (e.g. rock or clay layer).

• Only backhoe excavator or trencher to be used to protect natural soil

from sealing and compaction. Lateral planks can also be used to

prevent compaction of surrounding soils.

• Using teeth of the excavator to scarify the base of the infiltration

device can improve performance. Be careful not to “smear” the base

during excavation.

• Ensure levels and dimensions of the excavation including any slope

are clearly marked on construction drawings as this is critical to the

operation of the infiltration device.

• Specify health and safety procedures (e.g. Department of Labour

Code of Practice for Excavation and Shafts for Foundations).

Geotextile • Specify non-woven geotextile to be used, include length for overlap

of seams etc. If possible the name of appropriate geotextiles and the

supplier should be specified.

• Allow enough geotextile to wrap over the top of the trench or dry well

with overlap of material from either side.

Inflow and

overflow

• Inflow structure or method must be shown on construction drawings

and details of inflow pipe diameter, type and connections must be

specified.

• Show clearly on construction drawings location of overflow structures

and details of such, including diameter of any pipes, connections to

stormwater systems or erosion protection measures on overland flow

paths.

Observation

well

• The observation pipe diameter and type of perforated pipe needs to

be specified including details of connections.

• Specify footplate material and how to anchor pipe to footplate and

footplate to base of trench or well.

• Detail the observation well cap requirements particularly any locking

mechanism.



Storage layers • Size and type of gravel or sand to be specified including details of

supplier if appropriate. Construction drawings should show exact

depths and level of compaction (if any) required for each layer.

• If a storage chamber of vault is included in the design, specify its

details.

• Specify non-woven geotextile for placement between layers. If

appropriate name the product and provide details of supplier.

Dispersion pipe • The dispersion pipe diameter and type of perforated pipe needs to be

specified. Only part of the dispersion pipe is usually perforated so

lengths of perforation and sealed sections must be stated along with

details of connections.

• The depth of the dispersion pipe and connection to inflow pipes must

to be noted.

Surfacing • For wells or buried trenches, specify the depth of the finished level

from ground level. Consider instant turf rather than grass seeding for

immediate effect and functionality.

• Specify the surface finish required for trenches open at the surface.

This includes size of aggregate and depth of surface aggregate.

Pre-treatment • Pre-treatment devices are necessary and may be of sufficient scope

to require their own construction specifications. Refer to the relevant

chapter of this document for specific construction information (e.g.

swales, oil/water separator or proprietary device).

Example specifications for the construction of infiltration trenches are shown in Section 21.9.

These cover some of the typical aspects for construction of infiltration trenches, but exclude

particulars such as site establishment, health and safety, testing, materials, and

reinstatement, as these differ greatly depending on the application.

These example specifications do not constitute a full specification for the construction of an

infiltration trench, and should be used as a ‘starter specification’ for guidance only. Each

infiltration trench will be site specific and require careful consideration to ensure that all

aspects of construction are covered.


21.7 Compliance monitoring

Table 65

Crit ical points to inspect during the construction of infiltrat ion trenches and wells to ensure the device is

installed correctly include:


Excavation • Before breaking ground ensure erosion and sediment control measures

are set up (e.g. catch pit protectors, diversion bunds).

• Excavation to be carried out by a back hoe or ladder trencher. Check

trench bottom is level and uniform (as per drawings). Ensure bottom

and walls have not been sealed by excavation.

• Check all protrusions (e.g. tree roots and rocks) from walls and bottom

of trench have been removed.

• Natural soils must not be compacted, ensure construction machinery is

kept as far as practicable from excavation and prevent traffic movement

within 3 m of the excavation.

Geotextile • The geotextile material matches construction specifications and has

been laid, with necessary overlap as specified. Ensure there are no rips

or stress points in material.

Inflow and

overflow

• Check inverts and levels match drawings.

• Connections from inflow pipes must be watertight.

• Piped overflow connections must be watertight; overland overflow must

have erosion protection.

• If inflow is via a grass filter strip, verify the filter strip edge is le vel to

allow even, diffuse flow into the trench.

Observation

well

• Perforated observation well matches specifications and is securely

fastened to foot plate.

Storage

material

• Aggregate is washed and clean before laying in loads of 200 mm at a

time.

Dispersion

pipe/level

spreader

• Connection to inflow pipe is water tight and perforated dispersion pipe

or level spreader matches specifications.

Surfacing • Geotextile is secured over top of trench or well.

• If pea gravel is specified ensure it is washed and clean before laying.

Once laid depth should be 300 mm or as per drawings.

• If surface is grass, check topsoil depth is 300 mm (or as specified) and

turf is well watered and densely sown. Ensure a schedule of watering is

established.



Pre-

treatment

• Pre-treatment constructed as per construction drawings. See respective

device chapter depending on pre-treatment device specified e.g. swale,

oil/water separator or proprietary devices.

21.8 Photo gallery – infiltration trenches and dry wells

Figure 149

Buried infiltrat ion devices showing inspection/access ports (ACC, 2003, Soakage Design Manual).

Figure 150

Surface infiltrat ion trench along roadway (ARC, 2007, Infiltrat ion Trenches).


Figure 151

Surface infiltrat ion trench

Figure 152

Infiltrat ion trench under construction showing observation well, footplate, and fabric (ARC, 2003).


Figure 153

Infiltrat ion soakhole (private soakage).

Figure 154

Infiltrat ion soakholes, surface and interior view examples.


21.9 Construction specification example – infiltration trenches


21.9.1.1 Topsoil















extent and to fulfil the Contractor’s obligations of site safety and security, all to th e


21.9.1.4 Drainage

The Contractor shall keep excavations free of water during construction and shall dispose of

the water in an approved manner.





Water from either infiltration trench installation, surface drainage of the site, or dewatering of


treatment) in accordance with council requirements prior to discharge, and shall contain no

more than 100 mg L-1

of solids. Once treated to the required level, the water may be disposed

of downstream into the existing water course, the public road stormwater drain or public foul


sewer subject to council. Where this cannot be achieved, water from excavations must be

removed off-site and disposed of in an approved manner.



21.9.1.6 Verification of levels

The Contractor shall take sufficient levels or cross-sections of the existing ground surfaces,

including the road surface to confirm the ground profile and levels shown on the drawings and

to ensure that the surfaces are reinstated to the levels existing at the start of the contract

works or shown on the drawings.

The Contractor shall also confirm the invert and lid levels of all wastewater and stormwater

lines and verify the depth of telecommunications, power, gas or other services which

transverse the infiltration trench location or are in the vicinity of the works. Any disagreement

or potential conflicts shall be reported to the Engineer before excavation is commenced.

21.9.1.7 Protection of existing assets and services

The Contractor shall:

Ensure that no damage beyond fair wear and tear is caused to public roads and paths.

Comply with the requirements of the Land Transport Authority transport services road

opening notice regarding protection and cleanliness of all roads and paths adjacent to the

site. Meet all costs arising from non-compliance.

Keep approaches to the site clear of mud and debris.

Comply with Local Authority requirements regarding protection of paths, kerbs and

channels.

Comply with Local Authority requirements regarding restrictions on truck movements

during peak traffic periods.

21.9.1.8 Street furniture

The Contractor shall protect all existing road signage and similar street furniture. The

Contractor shall seek approval from the LTA and the bus company if any bus-stops made

unavailable by the work. Where agreed, the bus-stops shall be replaced by temporary stops as

close as possible to the permanent site. All such furniture disturbed, damaged or removed,

including road markings, shall be fully reinstated.


21.9.2 Stormwater system

21.9.2.1 Overflow pipe connection to stormwater system (if required)

Measures to divert the stormwater during construction if required, shall be installed (as

specified in the site Erosion and Sediment Control Plan) and will only be removed upon the

direction of the Engineer.

21.9.3 Excavation


The Contractor shall include in his price for excavation, excavation into all materials

encountered, that could reasonably be anticipated from the geotechnical information

provided, and from any further investigation undertaken by the Contractor prior to Tendering.

This shall include rock and/or tree logs or stumps if geotechnical information suggests that the

presence of rock and/or tree logs or stumps is possible. This shall also include variable material

in any filled and alluvium ground identified in the geotechnical information.






drawings or specified, without the instruction of the Engineer the extra dept h or width shall be

filled with excavated material, (being native soil) as nominated by the Engineer, care must be

taken not to compact.



requirements quoted in the “Code of Practice for Working in the Road” and the LTA


and LTA Infrastructure Design Standards.











21.9.3.5 Equipment

Care shall be taken during excavation to ensure that the surrounding natural soil is not

compacted. Backhoes or ladder type trenchers are most suitable for excavating the

infiltration trench (rather than front loaders or bulldozers).

21.9.4 Infiltration trench installation

21.9.4.1 Pre-treatment

Refer to the relevant pre-treatment device construction specification for details.

21.9.4.2 Inflow points

Inflow may be piped into the infiltration trench or enter via a swale or other pre-treatment

device. Where the inflow is piped it shall be via a PVC pipe manufactured in accordance with

AS/NZS 1260 “PVC Pipes and Fittings” and installed in accordance with NZS 7643 “Installing

uPVC Pipes” and AS/NZS 2566 "Buried Flexible Pipelines".

21.9.4.3 Surface layer

The surface layer of the infiltration trench shall be pea gravel placed to a depth of 300 mm

above the storage media. Do not compact the surface layer.

21.9.4.4 Geotextile layers

Non-woven geotextile material lines the walls and base of the trench to prevent migration of

surrounding soil into the infiltration trench and blinding of the surrounding soil by fine

sediments. Geotextile shall also be used between layers of media (e.g. sand and gravel) a nd

to cap the trench before finishing the surface (e.g. grass, pavement, or pea gravel).

The geotextile is to be laid and pinned as per the manufacturer’s instructions.

21.9.4.5 Storage media

Gravel is typically used as the storage media. The aggregate size will v ary with the necessary

infiltration rate but is often about 50 mm.

Permeable aggregates are subject to separation during transport and construction. Care

should be taken to avoid separation occurring. The gravel shall be cleaned, washed, crushed

rock with a minimum of 30% voids.

The storage media shall be placed in layers of uniform thickness not exceeding 150 mm.

Geotextile shall be installed and pinned around the sides of the storage media as per the

manufacturer’s instructions. Do not compact the storage media.


21.9.4.6 Base and sand layer

The bottom layer of the trench provides enhanced treatment of runoff; the depth shall be

150 mm (or up to 300 mm) as specified. Geotextile shall be placed between the sand layer and

the storage media.

The sand shall be clean sand or fine gravel spread evenly over the geotextile layer as per the

drawings unless otherwise specified or approved by the Engineer. The suitability of

alternatives will need to be demonstrated. The sand layer shall be levelled using a rake or

straight edge: do not compact the sand layer.

21.9.4.7 Dispersion pipe

A perforated horizontal PVC pipe may run the length of the trench or width of well, the

dispersion pipe shall be covered by a filter sock to prevent clogging.

The PVC pipe shall be manufactured in accordance with AS/NZS 1260 “PVC Pipes and

Fittings” and installed in accordance with NZS 7643 “Installing uPVC Pipes” and AS/NZS 25 66


21.9.4.8 Observation well

A vertical perforated PVC pipe that is anchored to the bottom of the trench o r dry well shall be

installed as specified, with a lockable cap (100-200 mm diameter).

The PVC pipe shall be manufactured in accordance with AS/NZS 1260 “PVC Pipes and

Fittings” and installed in accordance with NZS 7643 “Installing uPVC Pipes” and AS/NZS 2566


The foot plate shall be shall be Grade 316 stainless steel and constructed i n accordance with

the drawings.

21.9.4.9 Overflow control

Infiltration trenches with piped inflows shall have an overflow pipe which will discharge

directly to the stormwater system or an overland flow path. The level of the overflow pipe

shall be checked after installation to ensure compliance.

The overflow pipe shall be fitted with a screen and shall be PVC pipe manufactured in

accordance with AS/NZS 1260 “PVC Pipes and Fittings” and installed in accordance with NZS

7643 “Installing uPVC Pipes” and AS/NZS 2566 "Buried Flexible Pipelines".

Trenches with inflow on the surface will have an overflow weir on the downstream side of the

trench. When the weir is topped runoff will flow to a nearby catch pit or overland flow path.

The overflow path shall be protected as specified on the drawings to prevent erosion of the

surface.


21.9.5 Cover and infiltration pit delineation

The surface layer of the infiltration trench shall not be placed until all levels have been

checked and verified by the Engineer.

Trenches with a piped inflow point may have a dispersion pipe the length of the trench to

evenly disperse runoff. In this situation a trench may be grassed, clear deline ations shall be

installed to protect the trench from any activities which could compact the soil and trench

(e.g. vehicle or pedestrian traffic).

21.9.6 Topsoil and grassing

Finishing of the site shall be as specified on the plans, if instant lawn or hydroseeding is not

used then topsoil and grass seeding shall be installed in accordance with this clause. Topsoil

shall be good quality loam of a workable consistency stripped from its original location to a

minimum compacted depth of 150 mm, free of pernicious weeds, straw, stones, sticks, clay

lumps and any foreign matter exceeding 25 mm dimension and evenly spread and graded

over the area. It shall have a pH value between 6.5 and 7.5, with a humus content greater than

50%.

Grassed areas shall be sown with a suitable seed to suit the local site conditions, free of

weeds, with a high germination rate and fungicide and bird-repellent treated, given a dressing

of fertiliser and lightly rolled.

Super phosphate fertiliser shall be uniformly applied at the rate of 1,400 kg ha-1

. The surfaces

shall be inspected at intervals and any dangerous or undue settlement shall be made good

The literature review of erosion in cohesive soils indicated that for compacted clay soils, the

erosion rate is linearly related to excess shear stress. Departures from this relationship occur

when soil properties vary with depth or when the characteristic of the soil surface changes,

such as by armouring. The relationships between shear stress and erosion rates in the NIWA

Auckland study (Elliott et al. 2005; Debnath et al. 2007) tended to support a linear

relationship, although exponential relationships were better in some cases. The NIWA studies

also showed erosion rates varying from a few millimetres per hour to about 0.5 m per hour

depending in the cohesive strength of the soil. These erosion rates were consistent with other

reported studies. The critical shear stress of Auckland soils as measured by jet tests (Elliott et

al. 2005) were also consistent with reported studies, but were not consistent with the flume

tests (Elliott et al. 2005; Debnath et al. 2007) that showed low or zero critical shear stresses.

The NIWA studies showed that the critical shear stress and erosion rates were very variable

and this is consistent with international experience of cohesive soils. The variability relates to

differences in soil mineralogy, texture, degree of compaction, and organic matter content.

However, channel sizes will have adjusted to natural variations in soil properties, with wide

sections where the soil is weak and narrow and/or steep sections where the soil strength is

high. Thus, because the channel characteristics vary with soil strength, it is possible to use an

average value of critical shear stress and average cross-section shape for design purposes.

The method of estimating shear stress in streams is as important as the erosion equations. We

recommend that relationships between shear stress and flow be derived from measured


relationships between flow and stage. If uniform flow equations are used to predict water

levels, then some variation of Manning’s n with flow should be incorporated in the analysis.

We recommend that an erosion equation for cohesive Auckland streams follows that most

commonly used in modelling studies cME 3 . The parameters in this equation will

apply to both pre- and post-development scenarios. An assessment of erosion rates resulting

from urbanisation should take account of both the erosion threshold c and the extent to

which this threshold is exceeded. We recommend that site specific studies be carried out to

determine relationships between soil properties, as determined by the relatively simple jet

tests, channel morphology, bank vegetation, and total shear stress at the channel forming

(bank full) discharge. In particular, shear stress/flow relationships can be calculated for stable

stream reaches and shear stresses during past high-flow events, such as the channel forming

(bank full) discharge, used as a guide to the critical shear stress (eg, Julian and Torres 2006). If

specific parameters are not developed for a stream, we suggest using the median critical shear

stress (c. 33 N m-2

) and a value of 0.005 to 0.01 kg m-2

s-1

for the coefficient M3.

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