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Physical Properties: Soil Strength: Settlement
(Gradual Compression)
While embankments and hillslopes commonly fail suddenly due to
stresses that exceed the soil's strength, most buildings and roads
are unlikely to provide loads that cause the soil to rupture. Instead,
most foundation problems result from slow, often uneven, vertical
settlement of the soil. The compactability and compressibility of
the soil indicate how easily a soil will tend to settle.
Compaction
Proper compaction ensures the stability of foundations for structures
as well as adequate impermeability for the compacted layers of a
containment structure. Foundations and structures built on them
will tend to settle when gravitational forces acting on the soil
mass exceed material strength and frictional resistance forces.
The Proctor test is the most common method used to obtain data that
can guide efforts at compacting soils. A specimen of soil is mixed
to a given water content and placed in a holder where it is compacted
by a drop hammer (weighing 2.5 kg for the standard Proctor test).
The bulk density (usually referred to
as dry density by engineers) is then measured. The process is repeated
several times with increasing water contents until enough data is
collected to produce a Proctor curve. The curve indicates the maximum
bulk density to which the soil may be compacted by a given force.
The test also indicates the water content of the soil that is optimum
for maximum compaction. When the soil is wetter or drier, compaction
is more difficult. On the construction site, water trucks will be
used, if needed, to bring the water content of the soil to the determined
optimum level (Brady
and Weil, 1999).
Compressibility
A consolidation test may be conducted on a soil specimen to determine
its compressibility - how much volume will be reduced by a given
applied force. The reduction in volume occurs due to the soil particles
rearranging themselves into a more compact form. Because of the
relatively low porosity and equidimensional shape of the individual
mineral grains, very sandy soils resist compression once the particles
have settled into a tight packing arrangement. They make excellent
soils to bear foundations. Clay soils with high porosity have a
high compressibility and soils with organic matter have the highest
compressibility. Clays and organic soils are not recommended for
foundations for this reason. Compression of wet clayey soils may
occur very slowly after a load (i.e. building, traffic) is applied
because compression can occur only as fast as water can escape the
soil pores - which for fine pores in clayey materials is not very
fast (Brady
and Weil, 1999).
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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