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 its 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, over a period of time, 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.
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John Schmitt <john49@mdx.ac.uk>
Last updated 19/12/00