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slopestability - Slope Stabity EENS 204 Tulane University...

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This page last updated on 09-Mar-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Slope Stability, Triggering Events, Mass Wasting Events Factors that Influence Slope Stability Gravity The main force responsible for mass wasting is gravity. Gravity is the force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat surface the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope and a component acting tangential to the slope. z The perpendicular component of gravity, g p , helps to hold the object in place on the slope. The tangential component of gravity, g t , causes a shear stress parallel to the slope that pulls the object in the down-slope direction. z On a steeper slope, the shear stress or tangential component of gravity, g t, increases, and the perpendicular component of gravity, g p , decreases. z The forces resisting movement down the slope are grouped under the term shear strength which includes frictional resistance and cohesion among the particles that make up the object. Slope Stabity 3/9/2004 Page 1 of 12
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z When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope. Thus, down-slope movement is favored by steeper slope angles which increase the shear stress, and anything that reduces the shear strength, such as lowering the cohesion among the particles or lowering the frictional resistance. This is often expressed as the safety factor, F s , the ratio of shear strength to shear stress. F s = Shear Strength/Shear Stress If the safety factor becomes less than 1.0, slope failure is expected. The Role of Water Although water is not always directly involved as the transporting medium in mass-wasting processes, it does play an important role. Think about building a sand castle on the beach. If the sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it flows like a fluid and cannot remain in position as a wall. z Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose . The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30 and 37 o . z Slightly wet unconsolidated materials exhibit a very high angle of repose because surface tension between the water and the solid grains tends to hold the grains in place.
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