Design of Deep Basement for Waterproofing

1.0 Introduction

Basements can provide valuable additional space without increasing the height

of the building. The meaning of “deep basement”, “waterproof”

and “waterproofing” are sometimes confusing. In the report, the definition of

the above-mentioned terms are as the followings;-

Deep Basement More than one storey building structure wholly

below ground level, which will generally be subject

to hydrostatic pressure.

Waterproof Waterproof is a different concept from “water resisting”,

which means the range of ability to resist water but not

necessarily to the degree of being impervious or absolute in

the prevention of water penetration. Waterproof means

impervious to water; not permitting water to penetrate.

According to BS 8102:2009, the target internal environment

for “no water penetration” should be Grade 3.

Table 1 Grades of Waterproofing protection extracted from BS8102:2009

Waterproofing Application of water proofing/water-resisting material

2.0 Waterproofing Protection

2.1 General

Waterproofing protection methods are generally classified into three types:-

1) Type A – Tanking Protection

2) Type B – Structurally Integral Protection

3) Type C – Drained Cavity Protection

The decision on whether to rely entirely upon Type B waterproofing

protection is essential to deep basement design. The following has to be

considered:-

1) The degree of acceptable leakage into the basement

2) Durability risks from chemical deterioration of structural elements

3) The risk of gas migration into the basement

4) Feasible methods of protection related to construction methods

5) The risk of leakage associated with protection method

6) The anticipated hydrostatic pressure

It is recommended that the structural design, overall weatherproofing

design, waterproofing design and construction processes are considered

together, as they generally interact.

2.2 Type A – Tanking Protection

Type A protection is a supplement for Type B protection for achieving a

higher grade internal environment. It provides a continuous barrier system

which excludes water and/or water vapour and may exclude gases. Based on

its installation methods, it can be classified into four types

1) External Tanking

2) Reversed Tanking

3) Sandwiched tanking

4) Internal Tanking

2.2.1 External Tanking

Sheet membrane is applied to outer face of the concrete wall. If site

conditions allows, it is preferable to use external tanking as the

hydraulic pressure forces the membrane against the concrete

structure. Special considerations should be given for ground

movement during/after the construction period as ground movement

may rupture the membrane at vertical/horizontal interface. It is also

reminded that it is almost impossible to repair the external tanking

Figure 2 Concrete Structure with External Tanking

(CIRIA report 139)

2.2.2 Reverse Tanking

Sheet membrane is applied to a surface before the construction of

the structural element against it. The method is commonly used with

floor slabs.

Figure 3 Reverse Tanking

(Sheet Membrane applied to the Inner Face of Sheet Piling)

(CIRIA Report 139)

2.2.3 Sandwiched Tanking

When external tanking is impractical, sandwiched tanking may be an

alternative. Sheet membrane should be supported by a loading coat

of sufficient resistance to prevent it being pushed away due to

groundwater pressure. It is essential that the loading cost should be

constructed as soon as possible after the membrane had been

installed.

Figure 4 Sandwiched Tanking

(CIRIA Report 139)

2.2.4 Internal Tanking

Similar to sandwiched tanking, internal tanking is also applied to the

inside of a structural walls. Unlike sandwiched tanking, which

prevents the separation of membrane by another structure, this is

achieved by providing a mechanical anchorage, adhesive sheet

membrane may be retained by sandwiching them between the

structural wall and non-structural inner skin

Figure 5 Internal Tanking

(CIRIA Report 139)

2.2.5 Waterproofing Barrier Material

Cat

.

Type Details Advantages Disadvantages

1 Bonded

sheet

membranes

Bitumen-based, sheet membranes can be:

- cold-applied (self-adhesive);or

- hot applied ("torched-on" or bonded using a hot

bitumen adhesive) - increasingly reducing in

popularity due to the potential fire / flame

hazards and additional site insurance cost.

Can be applied externally or sandwiched.

Simple

familiar

construction

Relatively

cheap

Need to form

joints between

sheets

Detailing can

be complex

Composite sheet membranes.(traditionally

bitumen based but also newer technology HDPE

– High Density Poly Ethylene, and increasingly

FPO – Polyolefin based materials).

Can be applied externally or sandwiched.

2 Bentonite

clay active

membranes

These are traditional and comprise 'bentonite' a

type of clay with a ‘carrier’ material, typically of

geotextile or high-density polyethylene. There

are two principal forms: dry bentonite and pre-

hydrated bentonite. Can be problematic when

used in variable water tables due to problems

with wash-out and between wetting and drying

out which is not reversible

Can be applied externally or sandwiched

Detailing and

installation

simple in

detailing

Detailing and

installation

can be

laborious.

Really

intended for

large scale

projects

3 Liquid

applied

membranes

There are different types of liquid applied

membranes, which include one component

materials (usually bitumen or polyurethane

based) and higher performance two component

systems. e.g. Remmers Profi Tight

Can be applied externally or sandwiched

Cheap and

simple to

apply.

Deal easily

with

penetrations

through

basement

walls

Questions

about

uniformity of

application.

Require clean,

dry sub-base

for effective

application

4 Mastic

asphalt

membranes

Applied in three coats as a hot liquid. These

systems are also now becoming less popular due

to fire risks and insurance costs etc.

Can be applied externally or sandwiched

Simple to

apply with

correct skills.

Deal easily

with

penetrations

through

basement

walls

Increasing

scarcity of

skills.

Hot process

has health and

safety issues.

5 Cement

Based

multi-coat

tanking,

renders

and

coatings

Generally used internally and applied as multi-

layer coatings incorporating waterproof add-

mixtures.

These cope well with variable surfaces and are

sometimes used as remedial treatments for

existing basements.

Can be applied internally or externally.

Simple to

apply with

correct skills.

Deal easily

with

penetrations

through

basement

walls.

Some can be

applied to

damp walls

effectively

Increasing

scarcity of

skills.

Questions

about

uniformity of

application.

Category 1 Bonded sheet membranes

Figure 6 Bitumen-based, Hot applied bonded sheet membranes

Figure 7 Application of composite sheet membranes in deep basement

Figure 8 Application of composite sheet membranes in deep basement

adjacent to starer bar

Category 2 Bentonite clay active membranes

Figure 9 Bentonite clay active membrane in dry condition (left)

and in wet condition (right)

Figure 10 Bentonite clay active membrane with mechanical anchor

Figure 11 Bentonite clay active membrane adjacent to starter bar

Category 3 Liquid applied membranes

Figure 11 Liquid applied membrane (brushing)

Figure 12 Liquid applied membrane (spraying)

Category 4 Mastic Asphalt Membranes

Figure 13 Application of mastic asphalt membrane at pile cap

Figure 13a&13b Application of mastic asphalt membrane

Category 5 Cement Based multi-coat tanking, renders and coatings

Figure 14 Application of cement cased multi-coat tanking,

renders and coatings at diaphragm wall

Figure 15 Applying cement based multi-coat tanking, renders and coatings

Figure 16 Cement based multi-coat tanking, renders and coatings