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ISU GEOL 406 - SLOPE FAILURE

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1SLOPE FAILURESLOPE FAILURELandslides, Mudflows, Earthflows, and other Landslides, Mudflows, Earthflows, and other Mass Wasting ProcessesMass Wasting ProcessesRead Chapter 5 in your textbook (Keller, 2000)Read Chapter 5 in your textbook (Keller, 2000)GEOL g406 Environmental GeologyS. Hughes, 2003Gros Ventre landslide, Wyoming• There are many types of slope failure.• Slope failure, also referred to as mass wasting, is thedownslope movement of rock debris and soil in responseto gravitational stresses. Three major types of masswasting are classified by the type of downslopemovement: falls, slides, and flows.S. Hughes, 2003• In addition,another typeof groundfailure:subsidence,is importantto humanexistence.Halemaumau Pit Crater, Kilauea• Material is constantly moving downslope in response to gravity. Movement can be very slow, barely perceptible over many years.• Or, movement can be devastatingly rapid, apparent within minutes. Whether or not slope movement occurs depends on slope steepness and slope stability.SLOPE PROFILE• Some slopes are gently rounded, while others are extremely steep. Profiles of naturally-eroded slopes are primarily dependent on climate and rock type.SLOPESGEOL g406 Environmental Geology S. Hughes, 2003Common Slope ElementsSlopes common in semiaridregions or on rocks resistantto weathering and erosion.Convex-concave slopescommon in semihumidregions or in areas withrelatively soft rocks.GEOL g406 Environmental Geology S. Hughes, 2003Figure from Keller (2000)2MASS WASTING PROCESSESFlowage, or flow = downslopemovement of unconsolidated material.Particles move around and mix withthe mass.Sliding = downslope movement of acoherent block of earth material.Falling = free fall of earth material, asfrom a cliff, the free face of a slope.Subsidence = sinking of a mass ofearth material below the surroundingground level; can occur on slopes oron flat ground.GEOL g406 Environmental Geology S. Hughes, 2003Figure from Keller (2000)GEOL g406 Environmental Geology S. Hughes, 2003Common type oflandslide consistingof an upper slumpmotion and a lowerflow.Upper slumpLower flowMASS WASTING PROCESSESFigure from Keller (2000)Read Table 6.1 in Keller (2000)Type of Movement Material InvolvedROCK SOILFalls rockfall soilfallSlidesRotational rock slump block slump blockTranslational rock slide debris slide Slow rock creep soil creepearthflowFlows mudflowdebris flowFast debris avalanche Complex combinations of slides and flowsGEOL g406 Environmental Geology S. Hughes, 2003• When is a slope not stable?• Slope stability is based on the interplay between twotypes of forces:driving forces and resisting forces.• Driving forces promote downslope movement ofmaterial.• Resisting forces deter movement.• When driving forces overcome resisting forces, the slopeis unstable and results in mass wasting.• The main driving force in most land movements isgravity.• The main resisting force is the material's shear strength.SLOPE STABILITYGEOL g406 Environmental Geology S. Hughes, 20033Gravity: Does gravity act alone? NO!! Slope angle, climate, slope material, and water contribute to the effect of gravity. Mass movement occurs much more frequently on steep slopes than on shallow slopes. Water plays a key role in producing slope failure. In the form of rivers and wave action, water erodes the base of slopes, removing support, which increases driving forces. Water can also increase the driving force by loading, i.e., adding to the total mass that is subjected to the force of gravity. The weight (load) on the slope increases when water fills previously empty pore spaces and fractures.An increase in water contributes to driving forces that result in slope failure. DRIVING FORCESGEOL g406 Environmental Geology S. Hughes, 2003Resisting forces act oppositely of driving forces.The resistance to downslope movement is dependent on the shear strength of the slope material. And shear strength is a function of cohesion (ability of particles to attract and hold each other together) and internal friction (friction between grains within a material).Chemical Weathering (interaction of water with surface rock and soil) slowly weakens slope material (primarily rock), reducing its shear strength, therefore reducing resisting forces.IMPORTANT: The shear strength of the slope material is decreased by increasing the pore water pressure (pressure that develops in pore spaces due to the increased amount of water).RESISTING FORCESGEOL g406 Environmental Geology S. Hughes, 2003GEOL g406 Environmental Geology S. Hughes, 2003SLOPE STABILITYSafety Factor (SF)= The ratio of resisting forces to driving forces:Resisting ForcesSF =Driving ForcesIf SF > 1 then SAFEIf SF < 1 then UNSAFENOTE: A safety factor of ~1.25 or somewhat higher is acceptable for slope stability. A safety factor of ~10 is often used in building design to accommodate slight variances in materials and construction practices.SLOPE STABILITYW = Weight of total mass of earth material (at center of mass).D = Vector component of W parallel to potential movement.N = Vector component of W normal to slip plane.GEOL g406 Environmental Geology S. Hughes, 20034SLOPE STABILITYGEOL g406 Environmental Geology S. Hughes, 2003SLOPE STABILITYCDWNPotential slip plane(clay).ROCKACalculate the safety factorusing D to obtain driving forceand N to obtain resisting force.This is a simplified example, sothe clay layer is assumed tohave constant internal friction,i.e., the shear strength is thesame everywhere, when wet.D = W sin A= driving force• the downslope component of gravity.N = W cos A= the normal component of W• contributes to the shear strength along the slip plane• contributes to the resisting force.GEOL g406 Environmental Geology S. Hughes, 2003The safety factor involving a clay layer may be calculated by the unit thickness method using the following equation:SF = SLT/W sin AEXAMPLES = shear strength of the clay layer 9x104N/m3L = length of the slip plane 50 mT = unit thickness (assume 1) 1 mW = area (500 m2) x thickness (1 m) xunit weight (1.6x104N/m3)8x106NA = 30º, sin A = 0.5 0.5SF = 1.125 (conditionally stable)Can you think of examples where this can be applied?SLOPE STABILITYGEOL g406 Environmental Geology S. Hughes, 2003Ground material affects the pattern of slope failure:Type # 1 Æ Homogeneous material leads to rotational failure.GEOL g406 Environmental Geology S. Hughes, 20035Ground material affects the pattern of slope failure:Type # 2 Æ


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