Chapter 8: Unstable Land
Chapter 8: Unstable Land GEOL 1005
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This 3 page Class Notes was uploaded by Kerrigan Unter on Monday October 19, 2015. The Class Notes belongs to GEOL 1005 at George Washington University taught by Brown, C in Summer 2015. Since its upload, it has received 22 views. For similar materials see Environmental Geology in Geology at George Washington University.
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Date Created: 10/19/15
GEOL 1005 Chapter 8 Unstable Land 8 Slope stability basics Driving force gravity Resisting force oppose gravity and work to maintain slop stability A slope may become unstable and fail if the driving force exceeds the resisting forces Talus piles of angular rock fragments at the bases of cliffs Friction resistance to movement along a contact between two bodies Cohesion force created by attractions between grains of material Slope failure the result if the resisting forces are less than the slopeparallel component of gravity Unconsolidated materials loose materials soil sand and broken rock debris Angle of repose when gravity and friction are in balance the pile assumes a characteristic maximum slope Increased amount of water in slope materials increases their weight Increasing water content can decrease internal resisting forces Pore pressure the pressure of water at any particular place in the material proportional to the amount of water above it Vegetation tends to stabilize slopes 82 Types of Unstable Land How to categorize different types of mass movements How quickly the materials move down the slope The type of earth material involved Type of movement Falls fastest movements characterized by tumbling rolling or free fall of materials down a steep slope or cliff Slide when material moves downslope along a sloping surface as opposed to free falling tumbling or bouncing Transitional slide the slide surface is a twodimensional planar surface Rotational slideslump unstable material slides downward and outward along a concave surface Scarps form along the surfaces at the upslope head of the slump Flow materials move like liquid Debris flows involve relatively coarse material mixed with water Debris avalanche extremely rapid type of debris ow Earthflow debris ows composed of finegrained material Mudflow wet slurries of soil and other fine material Quick clay can rapidly turn into a owing mass if disturbed composed of silt grains surrounded by a jumble of thin platy clay materials forming a chaotic structure filled with water Creep very slow type of earth ow The most vigorous slope creep is driven by cycles of freezing and thawing Solifluction when a hillside in a permafrost region has a thawing uppermost layer that refreezes Types of mass movements can evolve into other ones Karst terrain areas where underground cavities such as caves are common Sinkholes circular depression Regional subsidence occurs when land gradually becomes lower over an extended area Natural subsidence occurs where watersaturated sediments become buried and lose the water they contain Sinkholes develop when the land surface collapses into underground caverns and other cavities 83 Causes of Land Failure The typical cause of a debris ow is a drenching rainstorm or series of storms that saturates unconsolidated slope materials Hurricanes rank high as causes of mass movements Ground shaking during earthquakes frequently triggers mass movements by temporarily reducing friction and cohesion in slope materials Wildfires consume tree cover surface leaf litter and organic material within soil Erosion at the base of slopes by rivers or ocean waves can trigger mass movements People can signi cantly in uence a slope s stability Constructed facilities add weight to the slope increasing driving forces Excavating terraces and roads steepen slopes and interrupt normal drainage patterns precipitation causes erosion or increases the driving force through saturation Septic systems lawn sprinklers and leaking water pipes increases the amount of water the infiltrates a hillside increasing driving force and reducing resisting forces Clearing trees brush and other vegetation at that has welldeveloped deeper roots can decrease a slope s resisting forces Aquifers groundwater reservoirs 84 Living with Unstable Land Landslide hazard maps delineate areas with different likelihoods of slope failure Landslide susceptibility map produces through relationships among bedrock geology previous landslides and slope steepness Avoidance not living or building near hazard areas Grading codes require builders to obtain a permit before altering a slope Decreasing slopes increases stability by decreasing the driving force increasing slopes decreases stability by increasing the driving force Preventing regional subsidence due to groundwater withdrawal requires regulating aquifer use
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