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APPENDIX F

WATER MAINS IN SLIP AND POTENTIALLY UNSTABLE AREAS

F1 INTRODUCTION

Construction of water mains close to or in potentially unstable ground should be avoided if at

all possible.

Unstable ground conditions pose serious problems for any structure founded on or located

within the immediate precincts. It is of paramount importance that water mains are not

subjected to stresses as a direct or indirect result of ground movement. Any leak or rupture

can seriously exacerbate slope instability by feeding large volumes of water into the

immediate area, with consequent property damage and/or loss of supply.

Appropriate action shall be taken in both the design and construction of water mains to ensure

that construction activity (particularly trench excavation) does not instigate ground movement

and to obviate or minimise potential for subsequent damage to water mains arising from

unstable ground movement.

F2 SLIP AND UNSTABLE AREAS

F2.1 GENERAL

The term "Slip Area" applies to "talus ground” and unstable slopes. Instabi lity can, however,

occur in other than "talus ground". Appendix F applies to most forms of ground instability.

Talus material consists of a mixture of soils and rock

fragments, which range in size from pebble to large

boulder, and often forms steep slopes as a result of

the collapse of cliffs (see photo opposite). The

presence of talus usually indicates a potentially

unstable environment, where a number of factors

including construction activity could trigger ground

movement.

F2.2 GREATER SYDNEY REGION

In the Sydney area, talus is typically associated with

the Triassic Narrabeen Group of rocks, consisting of

interbeds of weaker shale and siltstone beds,

between more competent sandstones. The

Narrabeen Group weathers more rapidly than the

overlying very competent quartzose Hawkesbury

Sandstone, leading to the formation of typical “talus”

slopes.

The same sequences outcrop in the Illawarra Region,

together with the underlying Permian Illawarra Coal

Measures and the Shoalhaven Group of rocks. All contain interbedded sequences of resistant

sandstone and less resistant shales and siltstones, which leads to differential weathering,

undercutting, cliff retreat and collapse, with the formation of potentially unstable talus.

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Other potentially unstable areas in the Sydney region lie within colluvial landscapes

associated with the clayey soils of the weathered Wianamatta Group rocks. Colluvium (which

includes talus) is generally defined as unconsolidated soil and rock material moved largely by

gravity (i.e. mass movement), deposited on a lower slope and/or at the base of a slope. Soil

creep, earthflow, slumping and tension cracking occur in these areas.

F3 FACTORS INFLUENCING STABILITY OF SLOPES

F3.1 SOIL AND ROCK CONDITION

The presence of colluvial and/or talus material is indicative of a potentially unstable

environment. Shales and siltstones interbedded with more competent sandstone beds results

in a differential weathering product. The breakdown of shales especially produces clays of

varying plasticities and strengths. The combination of unconsolidated clay, steep slopes and

moisture is a recipe for failure.

F3.2 ANGLE OF SLOPE

The steeper the slope, the more prone it is to movement in the form of soil creep or major

failure. In general terms, slopes steeper than 20% are “suspect” and those greater than 35%

“extremely suspect”.

Catastrophic slides have been experienced on sites in South Coast districts of NSW where the

slope of the ground was <15% so that classification by steepness of slope should not be taken

alone in assessing the stability of talus affected areas.

F3.3 GROUNDWATER AND SURFACE WATER

The amount and movement of water through or beneath talus slopes can be critical. Water

that would normally percolate through sandstone strata tends to be abruptly stopped by

impermeable shale beds such as of the Narrabeen Group. It then moves horizontally along

the sandstone/shale interface and can supersaturate accumulated talus material, causing

failure of a slope. Drainage from roadways and domestic development can often aggravate a

quasi-stable situation.

F3.4 RESIDENTIAL DEVELOPMENT

It is common practice for home sites to be levelled by cut-and-fill methods, often cutting into

talus slopes and loading the downhill side. Altered conditions subsequently charge the

unprotected talus with greater than normal drainage water.

F3.5 VEGETATION

Obvious stabilising effects are produced by heavy vegetation. Minimal disturbance to

vegetation and revegetating of disturbed areas is mandatory for pipeline construction in

potentially unstable areas.

F3.6 ROAD CONSTRUCTION

Construction of roads by cut and fill methods disturbs the original slope, including its drainage

patterns.

Generally, where any more than one of the above influencing factors operate in a talus

environment, problems are experienced with time.

F4 IDENTIFICATION OF POTENTIALLY UNSTABLE AREAS

F4.1 GENERAL

Areas will generally fall into one of three (3) condition categories:

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(a) Unstable - evidence of past or present movement.

(b) Stable but SUSPECT - i.e. Potentially unstable.

(c) Stable.

Where water main construction is planned within unstable or potentially unstable areas,

geotechnical input shall be sought prior to and during design and construction.

F4.2 GREATER SYDNEY REGION

For the purposes of initial identification of such areas, in the greater Sydney Region any

ground with slopes of greater than 20% shall be subject to further investigation. Likewise,

areas in the Illawarra Region where ground slopes are greater than 15% shall be subject to

further investigation.

The NSW Department of Environment, Climate Change and Water lists a series of published

Soil Landscape maps available at a scale of 1:100,000 and 1:250,000. These maps address

geomorphology, and combine geology, soil type and topography (slope), to categorise typical

landscapes. Many of the landscape types are characterised by “mass movement hazard” all

of which are in areas where slopes are >15%, but most in areas where slopes are >20%.

Geotechnical input to the design of a water main shall be obtained for development in the

following Soil Landscape areas (as defined by DECCW), and to any area where, for any

reason, ground stability is considered suspect.

Code Landscape Name Comments

gw GWYNNEVILLE (for areas >15% slope)

ie ILLAWARRA ESCARPMENT

bk BERKELEY (for areas >15% slope)

vo VOLCANIC

wp WEST PENNANT HILLS

pn PICTON

kg KURRAJONG (for areas >20% slope)

hw HASSANS WALLS (Katoomba sheet)

hw HAZELWOOD (Penrith sheet)

ha HAWKESBURY

wn WATAGAN

wb WARRAGAMBA

ho HORNSBY (for areas >20% slope)

For any development lying within a potentially unstable area, a geotechnical assessment of

the site shall be made by a professionally qualified and appropriately experienced

geotechnical engineer or engineering geologist to:

(a) identify potentially unstable areas (to be shown on the Design Drawings); and

(b) recommend, in consultation with a professional structural engineer, appropriate

measures to be incorporated into the design or taken in construction of the proposed

water main.

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Design Drawings shall include references to all relevant geotechnical and engineering/design

reports and a copy of all such reports shall be provided to Sydney Water.

F5 PRECAUTIONS

F5.1 GENERAL

Design and construction requirements for a water main in unstable and/or potentially unstable

areas shall be determined and specified by appropriately qualified professionals. The design

shall consider options to improve shutdown operations (such as additional valves and/or

alternative routes for water supply to areas beyond the potential landslip area) in the event of

a leak or major water main failure. Each project shall be individually assessed in consultation

with the Water Agency.

Adopted precautionary measures shall be recorded on Design Drawings.

The following requirements and guidelines are not comprehensive.

F5.2 AVOIDANCE

Unstable and potentially unstable areas should be avoided.

A water main shall not be constructed in the fill side of a cut and fill roadway.

Water main routes should follow existing road alignments, which are generally more s table

than the adjacent areas. Where trench excavation for installation of a main is to be in a cut

and fill roadway, the trench should be located as far as possible to the cut side of the road.

F5.3 SHALLOW TRENCH OR ABOVE-GROUND CONSTRUCTION

Construction methods shall be selected so as to cause minimum disruption to the slope and

vegetation.

Reticulation mains with customer connections should normally be located in an already

formed road reserve and below ground. In other situations, it may be possible for a water

main to be constructed in a shallow trench, on the ground surface or above ground on suitable

supports. Each case needs to be decided on its own merits. The use of construction plant

should be avoided (often precluded by slope and access alone), and hand construction

methods employed as far as practicable.

Where a water main must traverse unstable or potentially unstable slopes along the contour,

the option of laying the main above ground should always be considered. The main would

have to be constructed so as to not impede the flow of surface waters. The foundations for

above-ground pipeline supports require careful design to avoid exacerbating a potentially

unstable situation

The following above-ground options may be appropriate:

(a) "Screw" type anchors appropriately protected from corrosion and which cause minimal

interference with the ground.

(b) Pipes tied to sporadic bedrock outcrop, if present.

(c) Reinforced concrete or hot-dipped galvanised steel posts founded in concrete filled

auger holes.

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F5.4 ALTERNATIVE CONSTRUCTION TECHNIQUES

It may be possible to use trenchless techniques (directional drilling or microtunnelling) to go

around or under potentially unstable areas, subject to addressing the impact on drainage and

capabilities of the method proposed for a particular project.

F5.5 DESIGN FOR POSSIBLE GROUND MOVEMENT

The pipeline design shall address the following:

(a) The pipeline shall be capable of flexing / moving with minor slippage.

(b) The risk of pipe or joint failure arising from strains induced by ground movement shall be

minimised.

(c) Ground-water flow from the trench into the slip area shall be minimised.

(d) Installation of flexible main to meter services shall be undertaken at the time of water

main construction.

NOTE: Written approval and requirements shall be sought from the Water Agency.

Ductile iron pipes with restrained joints are preferred.

Polyethylene (PE) pipe requires anchorage for thermal and pressure loads and is not

recommended for water mains due to the risk that long-term ground movements would over-

stress the pipeline.

PE main-to-meter services are preferred and should be arranged to minimise risk of rupture,

especially with differential movement between the water main and the service.

A deemed-to-comply design for construction of a reticulation main in a road reserve in a

potentially unstable area is shown in Figures F1, F2 and F3. Pits shown in Figures F2 and F3

enable ready monitoring of ground movement and provide a controlled discharge point in the

event of water main failure.

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FIGURE F1 POTENTIALLY UNSTABLE/LANDSLIP AREAS MONITORING PITS FOR DN 100

AND DN 150 RETICULATION MAINS IN ROAD RESERVES

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FIGURE F2 TYPICAL TRANSITION PIT DETAIL

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FIGURE F3 TYPICAL RESTRAINED JOINT MAINS AND INTERMEDIATE AND TRANSITION

PIT DETAIL

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NOTES:

1 All dimensions in millimetres unless otherwise noted. 2 Potentially unstable areas to be identified according to the Code and be indicated on Design Drawings. Such areas

are generally where the slope is greater than 20%. 3 Main to be located on the cut side of roads. Construction techniques to include preservation of vegetation,

minimising open trenches, rapid restoration, and no alteration of existing groundwater regime. Mains to be constructed generally to the Code except that all pipes to be DICL at maximum 450 cover and have restrained joints (Tyton Loc or equivalent), monitoring pits and bulkheads as shown.

4 This Figure indicates minimum requirements for main laying in potentially unstable areas. Final design to conform to any site specific geotechnical requirements. The design is intended to restrict leakage to controlled points where it can be detected. Designs shown are not intended to withstand catastrophic landslides or other sudden large ground movements.

5 Install additional bulkheads along the trench in accordance with Code requirements (see layout sketch). 6 Secure services to the side of the main either via a tapping band, or a pre-tapped connector. 7 Main to meter services in new developments to be PE laid at the time of the water main construction and be

snaked in the trench to provide for relative movement. Minimum size to be DN 20 (De 25). 8 Provide services with ball valves at the main and dual check valves and ball valves at the meter installations. Dual

check meters should be used if available. 9 Monitoring pits to be at maximum spacing of 100 metres as detailed in the Design Drawing to suit the area

concerned. Locate pits where visible to the public and where overflow can discharge to existing stormwater system.

10 Provide indicator marks at pipe joints in pit so pipe movements can be monitored. Painting to be in accordance with System EPX5 of Sydney Water’s Painting Specification PCS105.

11 Metal access covers to be minimum 900 x 900 (two part). Class “D” for roadways, Class “B” for footways. Covers to have a single insert with ventilation holes.

F5.6 CONSIDER GROUNDWATER AND DRAINAGE

A most important factor affecting the stability of potentially unstable slopes is the nature of the

groundwater regime and the drainage network. Most slopes fail during or after periods of

heavy and/or continuous rainfall. Construction of trenches through these areas can have

either a beneficial or detrimental impact on stability by improving or worsening groundwater

and stormwater patterns (Refer Figures F4 and F5) . As most trenches act as conduits for

groundwater, it is important to ensure that they are designed and constructed to improve

drainage. However, a trench can transfer groundwater and stormwater into a previously

quasi-stable area, causing an increase in pore pressures and resulting in slope failure.

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FIGURE F4 EFFECTS OF CONSTRUCTION TRENCHES ON GROUNDWATER MOVEMENT

The design should consider where existing groundwater seepage paths exist and should allow

seepage to continue along the same paths after trenches have been constructed through the

area. This can be achieved by matching, as far as possible, the density and permeability of

the pipe embedment and trenchfill with that of the original ground material and by providing

bulkheads at regular intervals along the trench. Where this is not feasible, consideration

should be given to providing trench drainage to prevent groundwater from collecting in the

unstable or potentially unstable area.

Where rock is encountered along the trench, the frequency and openness of joints, bedding

partings, shears, etc will have an influence on the pattern of groundwater movement and the

type of trench backfill used. The position and frequency of bulkheads shall be assessed,

based on these and other topographical features. As necessary, drainage shall be provided

along the trench and through bulkheads, discharging to an appropriate and authorised

location. In some circumstances it may be more appropriate to allow for cross trench

drainage (with impermeable bulkheads) and to restrict along-trench drainage to short

distances of maybe only a few metres.

Groundwater regime on a steep slope prior to trenching.

PRIOR TO DEVELOPMENT

Groundwater level rises from damming behind impermeable backfilled trench.

Backfill subsides collecting surface water, more water transported along and out of trench raising downslope groundwater levels.

TRENCH BACKFILL LESS DENSE WITH HIGHER PERMEABILITY

TRENCH BACKFILL DENSER WITH LOWER PERMEABILITY

Groundwater level remains at pre-construction levels.

TRENCH BACKFILL AT SAME DENSITY AND PERMEABILITY

Groundwater regime unaffected by construction.

PIPE PLACED ABOVE GROUND

ON SCREW ANCHORS

DESIRED OUTCOME

UNDESIRED OUTCOME

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FIGURE F5 EFFECT OF CONSTRUCTION TRENCHES ON GROUNDWATER FLOW

PATHS

Pipe laying and backfilling shall be completed with full support being given to the sides of the

trench during excavation and until completion of backfilling, as recommended by geotechnical

advice.

Where conditions have been assessed as “stable”, normal design and construction practices

can be adopted, but always with an “awareness” of potential problems, particularly if unstable

areas are known to exist in the general precincts.

F5.7 PRECAUTIONS DURING CONSTRUCTION

In all areas of UNSTABLE and POTENTIALLY UNSTABLE ground, the following extra

precautions shall be taken:

(a) Ensure minimal interference with vegetation, especially large trees.

(b) Use hand excavation methods to minimise disturbance to the general environment.

Operation of mechanical equipment on steep slopes is often impracticable and may also

pose hazards to operator safety and slope stability.

(c) Complete excavation, pipe laying and backfilling in an upstream direction, with full

support being given to the sides of the trench during excavation through until backfilling

is complete.

Slope direction

Proposed trench

showing drainage

direction

Existing groundwater flow

paths

Slope direction

Excavated trench with no

bulkhead controls showing

drainage direction

Groundwater flow paths diverted along trench

and escaping elsewhere

from the trench.

Slope direction

Trench constructed

with impermeable

bulkheads.

Slope direction

Groundwater flow paths diverted along trench and

into drainage system

away from unstable area.

Existing

groundwater flow paths essentially

maintained

Trench constructed with

bulkheads fitted with

drainage pipes.

Unstable

area

Unstable

area

Unstable

area

Unstable

area

PRIOR TO DEVELOPMENT UNDESIRABLE OUTCOME

MAINTAINING EXISTING

GROUNDWATER REGIME

IMPROVING GROUNDWATER

DRAINAGE

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(d) Remove all surplus material from the site - do not stockpile surplus material or use as

land or embankment fill on site.

(e) Where buried mains are to be constructed across the slope, reinstate the original cross

fall.

NOTE: Subsidence of the backfill can lead to ponding of water and subsequent slope failure.

(f) Carry out trench excavation, pipe laying and backfilling expeditiously in short sections

i.e. to reduce the risk of instability of slopes due to excessive entry into the ground of

rainwater or water from any other source during construction.

(g) Backfill and compact trenches and complete restoration on a daily basis to, as near as

possible, the original conditions of soil density and surface permeability.

(h) Where pipelines are constructed above ground and especially when across the slope,

ensure that surface water is not able to pond behind the pipe i.e. ensure there is

sufficient clearance between the pipe and the ground to allow surface water to flow down

the slope in an unimpeded manner, as it would have prior to construction.

(i) Have a geological or geotechnical specialist inspect the site and mains six months after

completion and specify any appropriate remedial measures.