Mini Soil Surveys
WHAT’S BELOW THE BOTTOM LINE?
Throughout this magazine you will
see superb pictures of all types of
newly built houses and extensions.
They vary from traditional
construction, to oak-framed, to
ultra modern eco-friendly slabs of
glass and rendered blockwork.
The one common feature of these
pictures is that there is a line, just
below DPC, beneath which paving,
gravel, or grass form the bottom line
of the picture.
But what is below that “bottom
line”? Above it is the “dream”. Below
it a possible “nightmare”.
Above the line is a matter of choice
and the costs can be controlled.
Below the line cannot be changed
- the sub-soil and the need for
foundations.
They represent the greatest risk
of overspending. Budget costs are
often calculated on the assumption
that “good” ground exists at
0.9m below ground level and a
contingency sum is plucked out of
the air “in case things are not as
assumed”.
If the footings are deeper than
assumed the additional cost could
cause considerable problems.
Digging a bit deeper is however
not the only potential problem.
Much more serious problems could
be lurking below the surface as
follows:-
A)
The site has been filled
or the depth to a suitable bearing
stratum is deeper than allowed.
Not only do the footings need to go
deeper, but when the depth to good
ground exceeds 2m it is usually
more economical to adopt piled
foundations.
B)
A high water table can
cause the sides of trenches to
collapse quite rapidly, making it
necessary to provide expensive
supports and pumps to lower the
water table.
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C)
Clay soils
on sites with trees
will often cause the
foundations to be
taken to far greater
depths and other
costly precautions
may need to be
incorporated. Felling
of the trees is NOT
the whole answer and
can cause additional
complications.
D) Contaminated
land can be the
worst “nightmare”.
The responsibility for
any contamination
lies with the site
owner and costs of
investigation and
remediation can be
horrendous.
E)
Coal Mining
has become less of
a problem in recent
years since active
mining has greatly
reduced, but a Coal
Mining report is essential if a site is
in a coal mining area.
F)
Methane gas emissions will
need to be investigated if a site is
within 250m of a landfill site- no
matter how ancient the site is. The
certifying Authorities, NHBC or
Local Authority Building Control
will require tests to be made to
determine whether or not a site
is affected. These tests take time
and may indicate that expensive,
precautionary works are necessary.
Anyone buying a plot of land needs
to investigate “below the bottom
line” before they buy. If this is not
practical then any offer should be
made “subject to a satisfactory
site investigation”. It must not be
assumed that because Planning
Approval has been given, it is
practical to build to the approved
plans.
If a site is already owned then the
owner should cross fingers and find
out if there are problems before
plans are taken too far.
In the 30years experience of the
author’s investigation of ground
conditions for houses, about 50% of
sites are relatively straightforward,
about 40% have above average
costs and 10% have severe
problems.
If gold bath taps are planned, make
sure at an early stage that the
foundations are not going to put
them beyond budget!
Peter Dunn BSc CEng MIStructE
M.D. Mini Soil Surveys Group
www.minisoils.co.uk