Page 72 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 67

unscrupulous or blissfully ignorant will cover or smooth over a landslide scarp
with fill and patch or conceal cracks in houses or other structures in order to make
a sale. Any attempt to conceal a landslide is fraudulent and gives cause to nullify
a sale and require a full refund, and bring the agent’s license into critical review.

4.3.8 Precursors of Landslides

Landslides often begin with soil creep that relieves stress at the top of a slope
and results in a vertical tension crack. Surface runoff water then enters the crack
and helps to saturate the soil and promote more aggressive sliding. The deeper the
crack, the higher the hydraulic pressure acting to move the soil. The downhill
force is a product of both the water pressure and the crack area, and therefore
is the square of the crack depth. A first step toward preventing sliding therefore
is to seal open ground cracks to prevent entry of surface water.

Case History

Creep was observed but was a futile warning for a huge landslide that
occurred in 1964 in northern Italy. The slide completely filled the reservoir
above Vaiont Dam and created such a monstrous splash that the wind broke
windows over a mile (1.6 km) away. The wave overtopped the dam by about
100 m (300 ft) and washed down through the valley, taking the lives of over
2000 people. Creep was observed and monitored prior to the slide, and an
attempt was made to drain the reservoir, but drainage did not keep up with
the rate of soil creep so the lake level kept rising. As buoyancy reduced
friction at the toe, creep turned into a landslide encompassing an area of
about 1.6 2.4 km (1 1.5 miles). Some of the engineers at the dam were
convicted of negligence because of failure to initiate a timely evacuation.

4.3.9 Landslides and Earthquakes
Earthquakes trigger landslides by adding the effect of oscillating horizontal
movements, rather like shaking peas in a pan. As the ground moves back and
forth the corresponding accelerations cause horizontal forces in accordance with
Newton’s Law, f ¼ ma: force equals mass times acceleration. The design of soil
and rock slopes in earthquake-prone areas must take these forces into account.

4.4 DEPOSITS FROM ICE

4.4.1 An Outrageous Proposal
The suggestion made in 1840 by a Swiss naturalist, Louis Agassiz, that
large portions of the northern continents at one time were covered by

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