Page 69 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
64 Geotechnical Engineering

The marginally stable slope angle of gravity deposits is called the angle of repose.
It typically is around 358, but may be as high as 458 in coarse, angular rubble.
The angle of repose also may be observed where sand, gravel, rock, or grain are
dumped onto a pile from a conveyor belt, and occurs in nature on the back sides
of active sand dunes.

4.3.2 Rockfalls and Rock Avalanches
Rockfalls are as the name implies, and pose obvious dangers for anybody or
anything that lies in their path. Methods of protection include cutting benches to
catch the falls, covering steep slopes in loose rock with steel mesh, or building a roof
to support sliding rocks so that the end appearance is that of an open-faced tunnel.

A rock avalanche involves a mass movement of loose rock. A rock avalanche
is analogous to a snow avalanche but is potentially bigger and more devastating.
Velocities can attain 100 km/hr (60 mph). An advance warning can be obtained
by monitoring the creep rate of a potentially unstable rock mass.

4.3.3 Creep

Creep is an imperceptibly slow downhill movement. Creep occurring in advance
of a rock avalanche can be monitored by careful measurements of the ground
movement, or may be indicated by fences that gradually move out of line. Special
microphones may be used to detect and monitor subaudible ‘‘rock noises’’ that
come off as minute clicking noises associated with stick-slip. The more scientific
name is acoustic emissions, and an increase in the rate of occurrence is a precursor
to mass movement.

Curiously, rock noises apparently are audible to animals, which become
visibly agitated and may attempt move off of a slope before it fails.

Creep of soil instead of rock is common on hillslopes and is not necessarily
a precursor of a landslide. Creep is aided by wetting and drying cycles, and by
freezing and thawing of near-surface soil. Visible indications of soil creep are slow
tilting of retaining walls, fence posts, and grave markers. Another common
indication is curving tree trunks because a tree corrects for early tilting by growing
upright. Then as a tree becomes large enough to become firmly anchored,
soil flows around it, piling up on the uphill side and leaving a shallow cavity in
the tree shadow on the downhill side.

Creep is important to recognize because it increases lateral soil pressures
against retaining walls and foundations.

Colluvium is soil that has been moved to the toe of a slope by creep or by
a combination of creep and periodic alluvial activity.

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