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Limited
Guidance Notes on Extensions |
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Section
D - Foundation Engineering |
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The
design of foundations for any structure is fundamental to the
success of the project as a whole and for most domestic extensions
un-reinforced trench fill concrete footings are perfectly adequate
to support the loads applied in normal soil conditions. |
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The
type of ground varies widely around the country from granite
rock in Cornwall to chalk in the South Downs and other varieties
such as sand, gravel or clay in other areas. There can also be
variations locally within a generally well known area, which
may not become obvious until work has begun on site. |
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One such
common problem soil is shrinkable clay, which changes its volume
with changes in moisture content, shrinking during dry summers
and expanding again when the winter rain returns. This may cause
subsidence and/or heave problems generating cracks in the building,
which sometimes require expensive remedial work. |
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There
is an added problem with shrinkable soils, caused by tree roots
which can ingress below shallow foundations. To avoid this risk,
the Building Control Officer, or supervising Engineer may instruct
the builder to dig deeper foundations than originally expected.
This may involve additional costs. There are recommendations
laid down by the NHBC regarding the suitable depths in relation
to the tree species and the distance of the tree from the proposed
building. |
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In areas
where the ground conditions are unsuitable for founding at a
depth reasonable for trench excavations, it becomes necessary
to consider alternatives such as piling. By comparison with deep
excavations, which involve large volumes of spoil to be removed
and also of replacement concrete, the option of piling and ground
beams may even prove to be the cheaper alternative. |
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There
are two main sources of information which can assist in determining
the type of ground on which your proposed structure will be built.
One is to consult the official geological maps that show the
solid and drift soil types for a locality and the other is to
seek local knowledge from the Building Control Officer for the
area concerned. |
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Both of
these options are useful, but by no means foolproof. If there
is any likely possibility of 'difficult' ground conditions being
encountered, requiring a more specialist design, it is best to
have a soils report prepared by a laboratory with recommendations
from the soils engineer as to the best form of foundations to
employ. Whilst there is a cost implication to this option, it
most often proves overall to be very cost effective. |
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David
Ryland will consult the local Building Control Officer for advice
and should he consider that special precautions should be taken,
he will advise you at the earliest opportunity. It must be brought
to your attention that, even with such local knowledge, sometimes
bad ground is encountered after the builder has started work
and remedial action may have to be taken at the time. You should
allow a contingency within your budget for such a situation. |
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