How to keep basements dry

How to keep basements dry
Watertight advice on construction options designed to protect structures from groundwater
Leaky basement problems are
usually due to poor workmanship, wrong assumptions on
groundwater levels or poor
detailing by designers – in particular around joints and junctions with other basements or
The standard governing this
area is BS8102, the code of practice for “Protection of structures
against water from the ground”.
It classifies basements into four
grades, with varying levels of
protection. These range from
grade one, suitable for car parks,
to grade four, where the end
user requires a totally dry environment, such as an archive.
The code also identifies the
three different forms of construction. Type A is tanked protection, with either an internal
or external membrane. Type B
is structurally integral, where
the concrete provides the protection. Type C is drained protection, where a drained cavity
and internal vapour membrane
are provided to wall and floor.
There are pros and cons to
each option. Tanked protection
(type A) traditionally took the
form of mastic asphalt, but today bonded bituminous sheet
materials are more common.
Other options include waterproof renders or polyurethane
It is important to ensure the
membrane is fully continuous,
which may be problematic when
you abut existing structures on
infill developments. Any movement joints or discontinuities
will need to be detailed carefully
and similar issues will apply to
pile heads or caps.
You also need to provide adequate temporary works and
working space to ensure relatively dry and clean conditions
for application. External membranes need protection against
puncture during backfilling or
concreting operations and internal membranes need a load coat
or screed, in the case of floors, to
prevent them being lifted off by
water pressure.
With structurally integral
protection (type B), grade three
protection can be achieved with
concrete structures designed to
BS8007, without any further precautions (membranes). You may
be tempted to use an expensive
high-specification concrete with
micro silica or other additives
An unbonded membrane and drainage
system being applied to the external
walls of a service reservoir
joints are
always a potential
weakness and you
should keep them
to a minimum”
that will impart almost magical
waterproof properties.
But I recommend you concentrate on the basics, such as a
well-compacted dense concrete
with a low water cement ratio.
Quality control on site is key.
Construction joints are always a potential weakness
and you should try to keep
these to a minimum. One joint
you cannot eliminate is the
kicker. This must be cast at the
same time as the base and be
at least 150mm high to ensure
Whether or not to use a waterbar in construction joints always
provokes debate, but they aren’t
required in BS8007. I prefer not
to use them, and I advocate that
the joint is prepared by jet washing or light scabbling to remove
laitance and expose the aggregate prior to the next pour.
If you want to use a waterbar,
take care when using hydrophilic
materials that are expansive in
contact with water. They must
be placed in the centre of the
concrete section, which must be
a least 250mm thick, otherwise
there is a high risk of spalling.
Drained protection (type
C) gives the highest protection and can be used where integral or tanked protection is
impractical, such as top-down
construction or conversion of
cellars to habitable basements.
The drained cavity can be
formed by a blockwork lining
wall or a geocomposite “egg box”
material. Relatively speaking,
these systems are more expensive and slower to construct, but
are arguably more reliable and
offer more protection. You may
have to provide pumps to remove
the collected groundwater.
A final issue with new-build
basements is the effect on the local groundwater regime. In clay,
the excavation acts like a sump
and in sands and gravels or on a
sloping site may form a cut-off.
In these cases, the groundwater conditions that the basement
has to withstand are likely to be
different from that shown in the
site investigation, and external
drainage is worth considering.
Stewart Tennant is a director at
consultant GHA Livigunn
Steel yourself for tough times
Steel price volatility threatens to damage the industry and hand victory to concrete, warns Chris Bone
Like many other sectors, the
construction market is in decline.
If it weren’t for continued levels of
public spending, particularly in the
education sector, there would be far
less work to go around. But while
the price of raw materials is sliding
fast, steel is bucking the trend.
While steel stockholders are
de-stocking, offering short-term
price relief, the price of rolled
steel has risen by around 30 per
cent in the past nine months, and
shows little sign of easing. The
de-stocking effect on price has
created a perception that steel
structures can be delivered for less
than is the case. To compound
matters, steel manufacturers are
reducing production capacity to
maintain inflated prices.
‘Contractors under pressure
to cut build-cost’
These factors are creating a gap
between the reality of steel price
movement and the expectations
of the market, causing a squeeze
on steel contractors.
Steel manufacturers must
realise the effect of this policy on
the marketplace. If they do not,
then concrete (steel’s age-old
adversary) will gain a competitive
edge, and all the work the steel
industry has done to promote steel
as a sustainable, flexible building
material will be undone.
There are fundamental
questions about the viability of
concrete as a material, namely its
performance in fire. But money
talks, and as long as the concrete
industry keeps its eye on the ball,
the balance of power will shift. It is
reported that steel accounts for
60 to 70 per cent of structural
frames in the UK. Conversations
with blue-chip contractors
suggest this may already have
swung to 50:50, if not further.
Steel contractors are under
huge pressure to cut the build-cost
of steel frames in line with
concrete, perceived global trends
and the de-stocking price effect.
That, coupled with reducing
industry credit insurance and less
demand for buildings generally,
provides a dangerous environment
in which to operate, with
potentially sliding margins and
therefore less contingency for risk
in an increasingly risky market.
If steel manufacturers could
offer a degree of pricing support to
the industry, this would help steel
contractors resist slashing
margins further. It would also help
them buy more for their pound of
credit, alleviate stockholders’
uninsured risk and kick-start the
We are all in this together and
perhaps a degree of industry
solidarity would help us all come
through the hard times.
The corporate stance (cut
capacity to keep prices high) will
end up reducing both the demand
for steel (in favour of concrete)
and potentially its supply, as steel
contractors may not be around
to buy it.
Chris Bone is group chief executive
officer of Bone Steel