Mass Wasting Chapter 8 Landslides and Other Downslope Movements Falling Mountains 1 Mass wasting contributes to structural damage and loss of life Year 1916 1920 1945 1949 1954 1962 1963 1970 1985 1987 Location Fatalities Italy 10 000 China 200 000 Japan 1 200 USSR 12 000 20 000 Austria 200 Peru 4 000 5 000 Italy 2 000 Peru 70 000 Columbia 23 000 Ecuador 1 000 2 Mass Wasting Defined Mass wasting is the term geologists use to describe downslope movement of rock soil or debris directly due to gravity Most non geologists call such features landslides although the term is not specific enough to be much use to geologists This map makes a simple and clear point mass wasting is most severe in states with hilly or mountainous terrain 3 Gravity Gravity is a force that acts everywhere on the Earth s surface pulling everything in a direction toward the center of the Earth The force of gravity can be resolved into two component forces The normal force helps to hold the object in place The shear force causes a shear stress parallel to the slope and helps to move the object in the down slope direction 4 Material Resistance to Gravity Refer to pages 189 194 Material Resistance Forces that resist movement down the slope are grouped under the term shear strength Examples frictional resistance cohesion When the sheer stress becomes greater than the combination of forces holding the object on the slope the object will move down slope 5 Mass Wasting Classification Refer to page 196 1 The mechanism of movement how the material moves 2 The type of material being moved 3 The velocity of movement 6 Flows Refer to pages 204 208 Movement resembles a viscous fluid In most cases water is involved Increasing water decreases viscosity increases velocity and vice versa Fast Flows Flows are fast moving fluid mixtures of rock mud and water similar to wet concrete flowing down a slope Flows are called avalanches if they are particularly fast moving Flows move downslope as a dense fluid mass burying and or smashing most things in their path Because of the high density of the mass of moving material flows can carry 7 Flows Refer to pages 205 208 Left These debris flows in Puerto Rico wiped out paths of standing trees and hit a residential area Compare to previous slide Below Deadly debris flow between Kathmandu and Pokhara Nepal Two children were killed by this debris flow 8 Creep Slow Flows Refer to pages 204 205 Loose material on a slope creeps downhill almost imperceptibly at rates of a few inches to a few feet per year Results from processes that cause soil and loose rock to expand and contract such as alternating freeze thaw wet dry thermal expansion and contraction 9 Creep Slow Flow No threat to life cosmetic and structural damage Evidenced by bent trees tilted fences poles cracks in walls 10 doors windows stick Creep Slow Flow 11 Slides Refer to pages 201 204 Slides are defined as downslope movement of a large unit of rock or soil along a distinct surface of failure slide plane The 2 main types of slides are Rotational Slides also called Slumps and Translational Slides The difference between these two types of slides relates to the shape of the surface of failure along which the mass of rock moves Translational Slide Rotational Slide12 Translational Slides Fast Slides Refer to pages 203 204 A Translational Slide occurs on a relatively flat sloping surface of failure One very common cause of translational slides is dipping planes of weakness toward an open space These planes might be sedimentary bedding planes faults or fractures in the rock 13 Translational Slides Fast Slides Rockslide in Yosemite Rock masses move roughly as a unit in a rockslide Debris slide travels as one or more units and moves more quickly than a slump A debris slide at Thistle Utah in 1983 14 Rotational Slide or Slump Slow Slides Refer to pages 201 203 A Rotational Slide or Slump occurs along a curving concave up surface of failure often spoon like in shape see a below Because the failure surface curves upward the mass rotates as it moves downhill Rotational Slides are often slow triggered by erosion at the base of the slope 15 Rotational Slide or Slump Slow Slides Left A rotational slide that has been moving gradually for decades in San Pedro CA The rock is broken up into multiple backwardrotating blocks Below Soil slump in Sheridan WY 16 Falls Refer to pages 196 201 Falls are the simplest and most common form of mass wasting Falls are vertical or near vertical fall of rock material from a cliff or steep slope Primary causes of falls are weakening of the rock in a steep area and or undercutting and oversteepening of the area Rockfall in Zion National Park 17 Causes of Mass Wasting Refer to pages 194 196 208 209 Mass wasting processes generally result from a combination of factors that all work together to cause a slope to fail The most important factors are 1 Increase in slope steepness shear force of gravity 2 Reduction of slope strength by weathering by burrowing animals or by infiltration of water 3 Water saturation The single most important factor in most mass wasting Water saturation can result from heavy rain rapid snowmelt leaking water or sewage lines or poor drainage Water saturation weakens rock by building up pore pressure and by lubricating contacts between particles 4 Increase in weight at top of a slope 5 Removal of support at the base of a slope 6 Shaking by either natural processes earthquakes or human 18 Mitigation of Mass Wasting Refer to pages 212 215 Mitigation of mass wasting problems is accomplished by Hazard zone mapping to identify areas where movements have occurred in the past Such areas may be likely to move again Sinking foundations down into solid rock below weaker surface zones Slide 20 Building codes that limit the steepness of slopes and the types of fill used in construction Slide 21 Drainage systems that drain water from the surface and or the subsurface Slide 22 and Slide 23 Buttress fills and retaining devices to hold slopes in place Examples include retaining walls shotcrete and rock bolts Slide 24 Slide 25 Slide 26 Slide 27 For debris flows one can build deflection walls to send the flows 19 Mitigation of Mass Wasting A simple mitigation method for creep Structures will shift and slide if they are built on the creeping surface layer By sinking foundations down through the creeping surface zone and anchoring the footings of the structure in solid bedrock the structure will remain in place even if
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