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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