GEO-100 (Cain) Mass Movements Outline

GEO-100 (Cain) Mass Movements Outline - Chapter 8: Mass...

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Chapter 8 : Mass Movements Factors Influencing Slope Stability - Mass Movements occur whenever the downward pull of gravity overcomes the forces (usually frictional) resisting it. - The down-slope pulling tendency causes mass movements. Shearing Stress – Related to the mass of material and the slope angle. Counteracting the shearing stress is friction , or, in a coherent solid. .. Shear Strength – Frictional resistance to the down-slope pull of gravity that tends to cause mass movements. - When shearing stress exceeds frictional resistance or the shear strength of the material, sliding occurs. Factors that increase shearing stress, decrease friction, or decrease shear strength tends to increase the likelihood of sliding and vice versa. Effects of Slope and Materials All else being equal, the steeper the slope, the greater the shearing stress, therefore the greater the likelihood of slope failure. Angle of Repose – The maximum slope angle at which the material is stable. - Angle varies with material: Smooth, rounded particles tend to support only very low-angle slopes. Rough, sticky , or irregular particles can be piled up more steeply without becoming unstable. - Coarse fragments can usually maintain a steeper slope angle than finer ones. - In layered sedimentary rocks , there may be weaknesses along bedding planes where different rock units are imperfectly held together. Page 1 of 9
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: Mass Movements - Slopes may be steepened to unstable angles by natural erosion from water and ice. Erosion can also undercut rock or soil, removing the support beneath a mass of material and thus leaving it susceptible to falling or sliding. - Slow tectonic deformation (over long periods of time) can alter the angles of slopes and bedding planes, making them steeper and shallower . Effects of Fluid Aside from its role in erosion, water can greatly increase the likelihood of mass movements in other ways: - It can seep along bedding planes in layered rock, which reduces friction and makes sliding more likely. An increase in pore-water pressure in saturated rocks decreases the rocks’ resistance to shearing stress, tending to cause faulting and/or sliding. - The addition of limited moisture to dry soils may increase adhesion (the tendency of the particles to stick together). Saturation of unconsolidated materials reduces the friction between particles that otherwise provides cohesion and strength. The reduced friction can destabilize a slope. The very mass of water in saturated soil adds extra weight . -
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This note was uploaded on 05/09/2008 for the course GEO 100 taught by Professor Thomasboving during the Fall '08 term at Rhode Island.

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GEO-100 (Cain) Mass Movements Outline - Chapter 8: Mass...

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