June 2010 - NHBC Home

Issue 47
June 2010
3. Obtaining and testing samples of
soils once they have been placed if
no independent testing certification
is available.
Where separate subsoil and topsoil
materials are present in the cover
system, it will be necessary to confirm
the chemical quality of both of these
components.
Though British Standard BS3882:2007
contains details of sampling and
chemical testing that can be undertaken
to determine the characteristics of
topsoil, this is primarily aimed at
assessing its suitability as a growth
medium. Separate chemical testing is
normally required for any contaminants
that may be present posing possible
risks to human health (e.g. heavy
metals, hydrocarbons) and these need
to be provided to NHBC as part of the
validation.
The specific requirements for chemical
testing should ideally be discussed and
confirmed with a suitably experienced
consultant or specialist. Remember
that chemical testing of soils can take
up to two weeks to complete. Finalling
of a plot cannot be done until this
testing has been completed.
Where soils are tested off-site and are
confirmed as being suitable (i.e. where
they come from a commercial provider
Trial hole depth from ground level
or another site), it will be necessary
to provide NHBC with delivery notes
confirming that the tested and
accepted materials have been delivered
to the site.
Testing frequency
It is important that testing of the cover
system is conducted at a sufficient rate
to give adequate confidence regarding
the depth and quality of the material
used for the remediation.
ACTION
This will depend on a number of
factors, such as number of plots
being constructed and the source of
the material being used, but current
practice across the industry suggests
that for sites with more than 30 plots
testing should be conducted at a rate
of one plot in four, for sites with more
than 20 and up to 30 plots the testing
rate should be one plot in three, for
sites with between 5 and 20 plots one
plot in two should be tested, and where
fewer than five plots are being built it
will be necessary to provide validation
testing for every plot. In addition, for
sites with fewer than three plots it may
be necessary to conduct more than one
test per plot, in order to ensure that
an adequate number of samples are
tested. Again, advice should be always
sought from a suitable consultant or
specialist and NHBC staff are available
to discuss and agree acceptable testing
regimes at the early stages of any
project.
Ensure that you are aware of the requirements of NHBC Standards
Chapter 4.1 and that proposals for validating cover systems are
agreed with NHBC. Also ensure that sufficient time is allowed before
finalling to conduct chemical testing, the preparation, submission and
assessment of the validation report.
Keeping
concrete
in its place
Foundations in shrinkable soils (e.g.
clays), which will be affected by the
removal of trees, need protection from
clay heave.
Piled foundations are often used in
these situations, as they perform much
better, and can often be cheaper, than
deep trench fill foundations.
In order to protect the reinforced
concrete ground beams from the upward
and lateral swelling heave pressures
of the clay as it increases its moisture
content, heave protection materials
such as low density polystyrenes or void
formers must be used.
Placed along the side and under the
beams, the low density polystyrene or
void formers compress and absorb the
pressures, thus preventing damage to
the beams. If damage were to occur
to the ground beams it could in turn
damage the walls and floors above.
However, it has become evident
from a number of expensive claims
over recent years that void formers
and compressible materials are not
working effectively because of poor
workmanship in their installation.
Wet concrete grout can easily ‘leak’ from
poorly fitting void formers or sheets of
compressible materials when ground
beams are poured. This grout can fill the
voids in a void former and thus make it
ineffective when heave occurs.
Small gaps of 5-10mm, where two
sheets of compressible material abut
can, under a head of 500mm concrete,
allow voids to fill over a substantial
length of beam. The beam then has no
heave protection, and the heave forces
are transmitted to the building. This
causes extensive damage, which is very
expensive to repair.
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