CONCRETE IN THE GROUND
Structural waterproofing
of contiguous piles
Contiguous piles are often an efficient and cost-effective means of creating retaining
structures. However, while benefits include avoiding excessive excavation and assisting in the
control of ground movement, a significant disadvantage is the risk of groundwater ingress
between the piles. Therefore, in order to preserve and maintain internal walls and finishes,
it is extremely important to ensure that contiguous piles are correctly and competently
waterproofed. David Bucknell of Newton Waterproofing Systems considers the best, as well
as an example of the worst, methods for effectively waterproofing such a structure.
W
hen considering the
waterproofing requirements
of any structure, there are two
fundamental questions that
must be asked before anything else:
• What is the structure in question
constructed from?
• What is the expectation of the
waterproofing?
It is only once the answers to these
questions have been established that the Type
of waterproofing can be agreed upon – be
it Type A, B or C as defined within British
Standard 8102:2009(1) – or a combination of
two or three of these.
At Newton Waterproofing we frequently
see designs that propose the application of
a cuspated drainage membrane directly to
the surface of the contiguous piled walls.
Without question, this is an extremely
dangerous practice and should never be
contemplated, due to the fact that:
• There is (technically) no structure, piles
could have been chosen as the
construction method for a number of
reasons, such as a tight construction line
or in order to achieve the desired depth.
However, from a waterproofing
perspective contiguous piles are not, and
cannot be considered as, a structure.
• Simply applying a cuspated membrane
to the piles does not create Type C
waterproofing or ‘drained protection’.
The British Standard makes it explicitly
clear as to the standards that should be
achieved by a retaining structure, stating
that, “the outer leaf of the exterior wall
should be capable of controlling the
quantity of water that can pass through
it, in order not to exceed the drainage
50
concrete
Figure 1: The cuspated drainage membrane
has bowed away from the contiguous piled
wall, causing the complete collapse of both the
inner wall and the inner spine wall.
•
capacity of the system. Water entering a
drained cavity system is regulated by the
structure, so defects that might result in
unacceptable leaks should be remedied
before the system is installed”.
A contiguous piled structure makes it
impossible to achieve any of the three
internal environmental grades, as defined
within BS 8102, because of the potential
for large volumes of water to pass
through the gaps between the piles.
In short, it impossible to achieve a
successful waterproofing system by simply
applying a drainage membrane to the piles.
Such an approach is not only inadvisable
in theory; in practice it can also lead to a
complete structural failure, as is evident in
both Figures 1 and 2.
What should be done?
In the project shown in Figures 1 and 2, the
aim was to achieve a completely dry and
habitable Grade 3 internal environment.
However, in order to achieve this, the
first thing that needs to be created is an
acceptable and capable structure. In the case
of contiguous piles, first and foremost this
www.concrete.org.uk JǬǣǰ ǻǹǺǿ
CONCRETE IN THE GROUND
Figure 2: With the membrane cut
away, it is easy to see how the earth,
which should have been retained by
the structure and probably carried
by groundwater, has slumped to
the bottom of the piles, causing the
internal wall to collapse.
means that the gaps between the piles should
be of suitable dimension for the type of
ground they are retaining and then grouted in
order to prevent the sort of slumping that is
clearly visible in Figure 2.
Correctly designed
With correctly designed and constructed
contiguous piles in place – which, once
excavated, have been well prepared – a
combined approach to waterproofing
protection, with two or more of the Types, is a
vital consideration. This combined approach
is a significant recommendation of the British
Standard, especially when, as can be the case
with contiguous piles, “the likelihood of
leakage is high” and even more so when trying
to achieve a Grade 3 internal space where “no
water penetration is acceptable”.
Therefore to begin with, hydrophilic
polymer membranes such as Newton 403
HydroBond provide an effective and selfhealing Type A waterproof barrier. Such
membranes are pre-applied to the contiguous
piles as well as being laid across the floor and,
once placed, the locking fleece on the internal
surface of the membrane forms a mechanical
bond with the cast-in-place concrete raft
reinforced liner walls.
www.concrete.org.uk JǬǣǰ ǻǹǺǿ
Figure 3: Newton 403 HydroBond is a
self-healing, hydrophilic polymer Type
A waterproofing membrane that can be
applied directly to contiguous piles.
When placing a concrete structure
that conforms to BS EN 1992(2), Type B
waterproofing can also be achieved by the
inclusion of waterbars at the kicker and/
or construction joints, thereby providing
a combined solution of Types A and B.
Interestingly, a well-designed and wellplaced reinforced concrete structure, with the
inclusion of waterbars in the construction
joints to preclude water, should not require
the inclusion of admixtures in order to
achieve Type B waterproofing. Good-quality
concrete is inherently water-resistant and
the addition of an admixture is unlikely to
prevent leakage at the joints.
With both Types A and B waterproofing
already in place, the final and failsafe
barrier to water ingress is to apply Type C
waterproofing internally in the form of a
cavity drainage system. This method not
only accepts that water could enter the
structure, it does nothing to resist the ingress,
instead depressurising the water upon entry
and harmlessly managing it away from the
structure.
Waterproofing design specialists
The effective waterproofing of below
ground structures always requires a well-
Figure 4: Newton System 500 cavity
drainage being applied to a reinforced
concrete structure and so providing
combined Type B and Type C
waterproofing.
defined strategy and contiguous piles are no
different. This approach is most effectively
co-ordinated by a waterproofing design
specialist, a role that the British Standard
suggests should be included as an essential
part of the design team, to ensure that an
integrated waterproofing solution is realised.
Such a specialist will be capable of devising
a tailored solution to accommodate the
constraints and requirements of individual
projects, as well as providing the design
team with the necessary information to
assist with the design, installation and future
maintenance of the waterproofed structure.
In the case of contiguous piles where
an internal Grade 3 environment is being
sought, the recommendation of such a
specialist will always be for a combined
approach that is correctly and competently
installed, and not just a directly applied
cuspated membrane. ■
References:
1. BRITISH STANDARDS INSTITUTION, BS 8102. Code of
practice for the protection of below ground structures
against water from the ground. BSI, London, 2009.
2. BRITISH STANDARDS INSTITUTION, BS EN 1992-3.
Eurocode 2. Design of concrete structures. Part 3 – Liquid
retaining and containing structures. BSI, London, 2006.
concrete
51