Page 70 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 65

4.3.4 Landslides
Landslides progress more slowly than rock avalanches and faster than creep.
It must be emphasized that landslides are not the same as erosion, but involve
mass downslope movements of soil or rock masses along discrete shear zones.
As in the case of faults, landslides tend to grind and stir soils along the slip
surfaces, creating thickened slip zones.

Landslides occur when acting forces from the weight of a soil mass equal the
maximum resistance to shearing. They usually occur after prolonged periods of
rain, as frictional restraint is reduced by buoyancy from a rising groundwater
table. The causative factors, analysis, prevention, and repair are major concerns of
the geotechnical engineer. Structures involved in landslides often are a total loss,
Fig. 4.2. Insurance companies put losses from ground movements in the same
category as those from nuclear war, in that they are not covered by ordinary
homeowner insurance policies.

4.3.5 Recognizing Landslides
Engineering geologists specialize in recognizing old landslides, which is important
because an old landslide readily can reactivate whenever conditions are right.
One obvious clue to a landslide that can be overlooked by the unwary is a scarp,
which is a bare, exposed part of the shear surface that defines the upper boundary
of the slide. The scarp usually is steeply inclined and may be nearly vertical,
and forms the riser of an arcuate or curved step. Landslide scarps become less
obvious when they are overgrown with vegetation. In this case an important clue
is a row of smaller trees, bush, or shrubs that follows a contour around a hillside.
These are the newcomers that have grown since the scarp was first exposed.

Figure 4.2
Split level in
Tarzana, Cal.:
the house was
level and then it
split. A landslide
not only can pull
down structures,
the sites are
destroyed.

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