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Physical Properties: Soil Strength: Sudden Failure
Sudden failure of soil involves the rupture of the soil matrix
that gives way to gravitational forces when the soil strength no
longer exceeds gravity. This phenomenon can be seen on hillslopes
that simply give way unexpectedly. A classic example of this is
during a mudslide when the soil moisture decreases the soil strength
and the slope fails and moves downhill.
Cohesive Soils
Cohesive soils (soils with a clay content of more than 15%) have
two components of strength that act against gravitational forces:
(i) the inherent electrostatic attractive forces between clay particles
and the water in very fine pore spaces, and (ii) the frictional
resistance to movement between soil particles of all sizes. The
simplest and most common laboratory test used to estimate soil strength
is the direct unconfined compression test. A cylindrical specimen
of cohesive soil is placed vertically between two flat porous stones
(which allow water to escape from the compressed soil pores) and
a slowly increasing downward force is applied. The soil column will
first bulge out a bit and then fail suddenly and collapse when the
force exceeds the soil strength. The greater the downward force
the soil can withstand, the greater ability the soil has to withstand
sudden failure. Cohesive soils become less stable when they are
wet because the particles are forced apart so neither the cohesive
nor the frictional components are very strong, such as in a mudslide
situation (Brady
and Weil, 1999).
Noncohesive Soils
The strength of dry, noncohesive soil materials such as sand depends
entirely on frictional forces, including interlocking of rough particle
surfaces. One reflection of the strength of a noncohesive material
is its angle of repose, the steepest angle to which it can be piled
without slumping. Smooth rounded sand grains cannot be piled as
steeply as can rough, interlocking sands. If a small amount of water
bridges the gaps between particles, electrostatic attraction of
the water for the mineral surfaces will increase the soil strength.
Interparticle water bridges explain why cars can drive along the
edge of the beach where the sand is moist, but their tires sink
on loose dry sand.
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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