slopestability

Slopestability - Slope Stability EENS 2040 Natural Disasters Tulane University Prof Stephen A Nelson Slope Stability Triggering Events Mass

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This page last updated on 11-Nov-2011 EENS 2040 Natural Disasters Tulane University Prof. Stephen A. Nelson Slope Stability, Triggering Events, Mass Movement Hazards Factors that Influence Slope Stability Gravity The main force responsible for mass movement 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. Of course if the material forming the flat surface becomes weak or fails, then the unsupported mass will move downward. 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 parallel to the slope. 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. Slope Stability 11/11/2011 Page 1 of 16

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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. z When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope. z Alternatively, if the object consists of a collection of materials like soil, clay, sand, etc., if the shear stress becomes greater than the cohesional forces holding the particles together, the particles will separate and move or flow 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 Shear strength consists of the forces holding the material on the slope and could include friction, and the cohesional forces that hold the rock or soil together. 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 movement processes, it does play an important role. Water becomes important for several reasons 1. Addition of water from rainfall or snow melt adds weight to the slope. Water can seep into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. Weight is force, and force is stress divided by area, so the stress increases and this can lead to slope instability. 2. Water has the ability to change the angle of repose (the slope angle which is the stable
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This note was uploaded on 02/01/2012 for the course EENS 2040 taught by Professor Nelson during the Fall '11 term at Tulane.

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Slopestability - Slope Stability EENS 2040 Natural Disasters Tulane University Prof Stephen A Nelson Slope Stability Triggering Events Mass

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