Chapter 7. Mass wasting and Subsidence . Objectives. Basic processes, types of flow Driving and resisting forces (safety factor) and how it is related to slope stability Slope processes and the influence of slope angle, topography, vegetation, time etc Influence of human use
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Mass wasting and
Mm per year
(in the ocean,
Cy = C sin
Cx = C cos
d = h/sin
Principle of effective stress (Terzaghi, 1925 &1936)
The slope angle for each of the 145 recent landslides was determined from the U.S. Geological Survey topographic maps. The slope angle is the angle between the horizontal and the ground surface. Figure 13 relates the frequency of landslides to the slope angle. The average slope angle for landslides is 22.2 degrees with 75% of the landslides on slopes greater than 15 degrees. One landslide is on a slope of only 5.7 degrees.
Fig. 13. The frequency of recent landslides for a given slope angle. The frequencies were determined by counting the number of landslides for 5-degree intervals of slope angle. The average slope angle for recent landslides is 22.2 degrees.
Surface and subsurface drainage
Note the importance of effective stress
Grading, material from upper part of a slope is removed and placed near the base
Increased resisting forces
Cut into a series of benches
This accumulation of water high in the slope was most marked during periods of heavy rain, although the association of heavy rainfall and creep went unnoticed.
The presence of the impermeable clay layer in the bedrock was not recognized at the time and it was assumed that the movement was due to local saturation of the rocks below the level of water in the reservoir, at the toe of the creeping landslide, rather than accumulation of water pressure in the entire mountainside.
It was therefore proposed to regulate the movement of the landslide, and thus allow it to settle to a new equilibrium, by lowering the level of water in the lake when an episode of creep was in progress, until the toe of the landslide was no longer saturated and the creeping stopped.
The reservoir was then allowed to refill and the drainage cycle repeated whenever creeping movement occurred again.