Lecture 20 - SL1 STABILITY OF EARTH SLOPES Types Of Slopes...

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SL1 STABILITY OF EARTH SLOPES Types Of Slopes o Open Cuts o Embankments o Levees o Earth Dams o Natural Slopes Failure may occur suddenly without warning, or may get warning cracks and settlements. Failure occurs when the shear stress exceeds the shear strength along a continuous surface. This surface is termed the failure surface, slip surface, or surface of rupture. Estimated cost of landslide damage in the U.S (direct and indirect) – $1.5 billion Causes of Slope Failures (Landslides) A. Increased Shear Stress o External loads (buildings, roads) o Unit wt. increase due to water content increase o Water pressure in tension crack o Seepage forces o Seismic forces (earthquakes, vibrations) B. Decreased Resistance o Removal of lateral support at toe (stream erosion, excavation) o Undermining caused by tunneling, collapse of caverns or seepage o Tension cracks C. Decreased shear strength o τ = c + ( σ - u ) tan φ o Increased pore water pressure, u o Deterioration of cementing material, c o Loss of capillary tension on drying, c and -u o Swelling of clays by adsorption of water, c o Earthquakes reduce shear strength of sensitive silts and clays, c and φ↓ Crown Toe Base H τ τ Intact Failed
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SL2 Types of Failures Circular Wedge Base Toe Slope or Face (rare) Common Analysis Assumptions o plane strain – 2D failure (approximately true for large masses) o failure mass moves as a rigid body o isotropic soil: shear strength parameters same in all directions o factor of safety calculated by comparing average developed shear stress with shear strength along failure plane (stress at any point may be higher/lower than average) o failures on long (~infinite) slopes tend to be face failures with a failure surface essentially parallel to the ground surface o failures on steep slopes tend to be single plane wedge failures o failures of finite slopes in cohesive soils have been observed to be nearly a circular o failures along planes of weakness (esp. layer interfaces) may be analyzed using a series of straight lines (wedges) o the actual failure surface will be a combination of the above and three dimensional o analyses are necessarily iterative to find the weakest failure surface o determination of soil properties has a bigger impact on safety than defining the actual failure surface o both short and long term stability should be considered s short term depends on undrained soil parameters and total stresses s long term depends on drained soil parameters and effective stresses
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This note was uploaded on 09/12/2011 for the course CEG 4012 taught by Professor Staff during the Fall '08 term at University of Florida.

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Lecture 20 - SL1 STABILITY OF EARTH SLOPES Types Of Slopes...

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