Geoscience Reference
In-Depth Information
14.3 Causes of slope failure
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The micro scale effects causing slope movement are covered in the next table.
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Slope failure occurs either due to an decrease in soil strength or an increase in stress.
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Slopes are affected by load, strength, geometry and water conditions.
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The load may be permanent, such its own weight or transient (dynamic from a
blast).
Table 14.3
Causes of slope failure (adapted from Duncan and Wright, 2005).
Decrease in soil strength
Increase in shear stress
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Increased pore pressure (reduced
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Loads at the top of the slope. Placement
effective stress). Change in water levels.
of fill and construction of buildings on
High permeability soils have rapid
shallow foundation near crown of slope.
changes. This includes coarse grained
soils, clays with cracks, fissures and lenses.
Cracking. Tension in the soil at the
Water pressure in cracks at the top of the slope.
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ground surface. Applies only in soils with
Results in hydrostatic pressures. If water in
tensile strength. Strength is zero in the
cracks for extended periods seepage results with
cracked zone.
an increase in pore pressures.
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Swelling. Applies to highly plastic
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Increase in soil weight. Change in water content
and overconsolidated clays. Generally a
due to changes in the water table, infiltration or
slow process (10 to 20 years). Low
seepage. Increasing weight of growing trees and
confining pressures and long periods of
wind loading on those trees. Vegetation has a
access to water promote swell.
stabilising effect initially (cohesion effect of roots).
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Development of Slickensides. Applies
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Excavation at the bottom of the
mainly to highly plastic clays. Can develop
slope. Can be man made or due to
as a result of tectonic movement.
erosion at base of slope.
Decomposition of clayey rock fills.
Change of slope grade.
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Clay shales and claystone may seem like
Steepening of slope either man made
hard rock initially, but when exposed to
(mainly) or by natural processes.
water may slake and degrade in strength.
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Creep under sustained load.
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Drop in water level at base of slope.
Applies to highly plastic clays. May be
Water provides a stabilising effect. Rapid
caused by cyclic loads such as freeze -
drawdown effect when this occurs
thaw or wet - dry variations.
rapidly.
Leaching. Change in chemical
Dynamic loading. Usually
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composition. Salt leaching from
associated with earthquake loading or
marine clays contributes to quick
blasting. A horizontal or vertical
clays, which have negligible strength
acceleration results. This may also result
when disturbed.
in a reduction in soil strength.
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Strain Softening. Applies to brittle soils.
Weathering. Applies to rocks and
indurated soils.
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Cyclic Loading. Applies to soils with
loose structure. Loose sands may liquefy.
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The analytical model and its interpretation influence the perceived stability.
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Shallow (surficial) failures occur often following rainfall events. An infinite slope
analysis with steady state seepage parallel to the slope applies. Note that a
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