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The Foundations, Settlement and Subsidence
FAQ
What are foundations?
They are the substructure of a building. There are several types;
Ring Beams; These are a concrete casting around the periphery of the
building, normally cast into an appropriate trench. In the UK the minimum
depth is one metre, although two or even three metres may sometimes be
specified. A DPC (damp proof course) is then installed, and the external
walls are built on top of this.
Rafts; These are a concrete casting covering the whole floor area of
the building. Normally this also incorporates a ring beam around the
periphery.
Piles; These are wood (very rare nowadays) concrete or steel members
placed into the ground, by driving down to a stratum which is strong
enough to bear the superimposed load. Driving to refusal (i.e. the pile
will not penetrate any farther after repeated attempts) is also acceptable,
but necessitates trimming off the top of the pile. There are also cast-in-situ
piles (AKA side bearing piles) which are made by augering out a hole,
inserting a reinforcing cage, and pouring in concrete, which is vibrated
into place. Sometimes the hole is temporarily filled with a bentonite
slurry to prevent its collapse. The bentonite slurry is much less dense
than the concrete, so as the concrete pour takes place, the concrete
displaces the bentonite upwards, where it is recovered for re-use. This
type of pile has somewhat better bearing capacity size for size, as the
friction of the pile sides can also be allowed for. This is not the case
for driven piles.
Bolted foundation; Rare in the UK. Almost literally, when building on
solid rock (like New York) the rock has rawlbolts inserted, and the building
is bolted to the ground.
Why does settlement happen?
Settlement occurs on clay, silt, and alluvial soils. Sand and gravel
are too permeable. When a load is imposed on a fine, saturated, (almost
always the case) soil, the initial load is taken up by the pore water.
(i.e. the pressure rises.) As this pressure is not in equilibrium with
the surrounding pore water, the pressure dissipates at a rate governed
by the permeability of the soil. As the water pressure dissipates,
the load is taken up by the soil skeleton, the mineral component of
the soil, and this compresses, and the particles move slightly among
each other so that the volume decreases. If there is an underlying
permeable layer, the mean drainage path will be half as long, and as
the time to full dissipation of excess pore pressure (also to full
consolidation, depends on your outlook) varies with the square of the
length of mean drainage path, this will quarter the time. On a typical
London clay foundation, the time to full settlement will be measured
in centuries. The rate of settlement drops off asymptotically with
time. Further settlement can be caused by lowering of the pore pressure,
for example by a tree, or a land drain. Most of the clays in the UK
(those north of about Dorking) are overconsolidated, i.e. they have
previously been subject to a much greater stress than they are currently
under. The Ice Age meant that London was under about 30-40 metres of
ice, and therefore the clay was compressed quite a lot. Clay does not
fully spring back elastically after the stress has been removed, but
on being allowed free access to water after the stress is removed,
expands. (but not quite to it's original volume) When this occurs in
the ground it is known as heave. Heave can be every bit as destructive
as settlement, so cutting down a big, old tree can precipitate problems.
A leaking water main or sewer can also create problems. The problems
that occur from settlement or heave are generally only when the movement
is differential. A new extension on an old house with shallower foundations
is but one example. A tree can cause differential settlement by removing
water from only one side of a house. It is a common misconception that
deep foundations are to act as a wall against tree roots, but the real
reason is that the surface roots do not go down three metres, and therefore
the foundation will be resting on soil which will not be grossly affected
by the tree.
What is Subsidence?
Subsidence is when there is a gross failure of the rock or soil underlying
the foundations. It is often used as a synonym for settlement, but they
are two different problems. Old mine workings and cliff erosion are the
typical causes of subsidence, and sometimes poorly shored earthworks.
Often when subsidence occurs, the cost-effective solution is to scrap
the building.
I have some small cracks in the wall, what should I do?
If they are less than 1 mm. wide decorate over them. Do not forget to
check for cracks on the outside of the building and make them good, otherwise
water ingress can cause subsidiary problems. If they are wider, find
a good surveyor, ideally with some geotechnical know-how. If he starts
prating about underground streams, subsidence (assuming the problem is
settlement) or tree walls ditch him. If he takes a soil sample for plasticity
index testing, or starts talking about test pits or telltales, you have
probably found a good one. Telltales are devices stuck to the wall either
side of the crack to see if it is progressive. The best ones have a grid
for quantitative measurement, but some people use a strip of glass stuck
down either side of the crack. This is normally stuck down with car body
filler. If the glass breaks, the crack is still moving. As a general
rule of thumb, any crack you cannot get your finger into does not indicate
imminent collapse of the building. Bigger than an inch, and it is bad
news.
Copyright notice: With the growth of the net, copyright law has become
very complicated. You may reproduce this FAQ in any form from stone tablets
to putting it on your homepage. You may distribute it to absolutely anyone.
You may even add your own bits to it. However, if you remove my name
from it, or try to pass it off as your own work, I will be very, very
angry with you.
To the index
John Schmitt <john49@mdx.ac.uk>
Last updated 18/04/00
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