GEO 101 Lecture Notes - Lecture 23: Surface Tension, Mass Wasting, Revegetation
24 Jun 2018
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Prevention of Mass Wasting
Proper design during construction projects can eliminate the potential for increased
mass wasting. Human activities such as undercutting the base of the slope, adding
weight to the upper part of the slope by building large structures, removing vegetation,
and saturating the ground with water increase the risks of mass wasting. Engineering
solutions include barriers and retaining walls, drainage pipes, terracing the slope to
reduce the steepness of the cuts, and immediate revegetation. Rockfalls can be
controlled or eliminated by the use of rock bolts, cables, and screens and by cutting
back slopes to lesser gradients.
Introduction to Mass Wasting
Mass wasting is a natural result of weathering on slopes. Simply put, gravity pulls loose
rock and soil downhill. Mass wasting is the process of erosion whereby rock, soil, and
other earth materials move down a slope because of gravitational forces. It proceeds at
variable rates of speed and is largely dependent on the water saturation levels and the
steepness of the terrain. A destructive, rapid mass wasting event is called ‐
a landslide; if the movement is slow enough that it cannot be seen in motion, it is
called creep.
Three kinds of movement are generally recognized: flow, slip, and fall. A mass wasting ‐
event is called a flow if the mass moves downslope like a viscous fluid. If the mass
moves as a solid unit along a surface or plane, it is called a slip. A slip that moves along
a surface parallel to the slope is called a slide.If the movement occurs along a curved
surface where the downward movement of the upper part of the mass leaves a steep
scarp (cliff) and the bottom part is pushed outward along a more horizontal plane, it is
called a slump. Earth material that free falls from a steep face or cliff is termed a‐ fall.
Mass Wasting Controls‐
A variety of conditions affect the development of mass wasting in a particular area.
Steep slopes, widely varying altitude ranges (relief), the thickness of the loose earth
material, planes of weakness parallel to the slopes, frequent freezing and thawing, high
water content in the earth material, dry conditions with occasional heavy rainfall, and
sparse vegetation are the factors that contribute to the unstable conditions that result in
mass wasting. Movements can be triggered by the motion of earthquakes or too much
weight added to the upper part of a slope, such as snowpack.
Angle of repose. The angle of reposeis the steepest angle at which loose material will
remain in place. It is largely dependent on the size, shape, and roughness of the
particles. The angle varies from about 25 degrees to about 40 degrees. If the angle is
exceeded by additional sedimentation or tilting, a slide or disturbance will result.
Gravity and friction. Rock particles and soil move downslope because of the forces of
gravity. The gravity that acts on an object is a combination of the normal force and the
shear force. The normal force is perpendicular to the slope the object rests on, and
the shear force is parallel to the surface of the slope (Figure 1). Steep slopes have high
shear forces; the steeper the slope, the greater the chance an object will slide. Friction,
such as that from a rough bedrock surface, counteracts shear force. Rough, angular
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Document Summary
Proper design during construction projects can eliminate the potential for increased mass wasting. Human activities such as undercutting the base of the slope, adding weight to the upper part of the slope by building large structures, removing vegetation, and saturating the ground with water increase the risks of mass wasting. Engineering solutions include barriers and retaining walls, drainage pipes, terracing the slope to reduce the steepness of the cuts, and immediate revegetation. Rockfalls can be controlled or eliminated by the use of rock bolts, cables, and screens and by cutting back slopes to lesser gradients. Mass wasting is a natural result of weathering on slopes. Simply put, gravity pulls loose rock and soil downhill. Mass wasting is the process of erosion whereby rock, soil, and other earth materials move down a slope because of gravitational forces. It proceeds at variable rates of speed and is largely dependent on the water saturation levels and the steepness of the terrain.
