Mass_Wasting

Mass_Wasting - Mass Wasting Process by which material moves...

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Unformatted text preview: Mass Wasting Process by which material moves downslope under the force of gravity Mass Movement is Classified on the Basis of: • Nature of material • Velocity of movement • Nature of movement • • • • Factors Influencing Mass Movement Nature of slope materials Steepness of slope Water content Slope stability Table 12.1 Factors Influencing Mass Movement Nature of Slope Material Steepness of Slope Water Content Slope Stability Mass Movement Depends on Nature of Material, Water Content, & Slope Fig. Story 12.1 1 Mass Movement Depends on Nature of Material Mass Movement Depends on Nature of Material (35o) Angle of Repose: the maximum angle at which a pile of unconsolidated particles can rest Fig. Story 12.1a (40o) (45o) The angle of repose increases with increasing grain size Fig. Story 12.1a Origin of Surface Tension Water molecules in a liquids interior are attracted in all directions… Origin of Surface Tension Water molecules in a liquids interior are attracted in all directions… …whereas surface molecules have a net inward attraction that results in surface tension… Fig. Story 12.1b Fig. Story 12.1b …that acts like a membrane, allowing objects to float. Mass Movement Depends on Water Content Surface tension in damp sand increases cohesion Fig. Story 12.1b Fig. Story 12.1c 2 Mass Movement Depends on Water Content Mass Movement Depends on Water Content Surface tension in damp sand increases cohesion Dry sand is bound only by friction Fig. Story 12.1c Surface tension in damp sand increases cohesion Dry sand is bound only by friction Saturated sand flows easily because of interstitial water Fig. Story 12.1c Steep slopes in damp sand maintained by moisture between grains Loss of vegetation and root systems increases susceptibility of soils to erosion and mass movement Fig. 12.2 Yellowstone National Park Yellowstone Fig. 12.3 Weathered shale forms rubble at base of cliff Weathered shale forms rubble at base of cliff Angle of Repose Fig. 12.4 Fig. 12.4 3 Before the 1964 Alaska Earthquake After the 1964 Alaska Earthquake Water saturated, unconsolidated sand Cyclic loading during the “Good Friday” Earthquake caused the sand beneath Turnagain Heights Subdivision to liquefy. Fig. 12.5 Fig. 12.5 Turnagain Heights Subdivision, Alaska Classification of mass movement is based on dominant material, fluid content, and velocity of movement. Fig. 12.5 Fig. 12.6 Types of Rock Mass Movement • rock fall • rock slide • rock avalanche Rockfall A very rapid mass movement in which newly detached blocks of rock suddenly fall from a steep slope or cliff. 4 Rockfall Rockfall in Zion National Park Fig. 12.7 Fig. 12.7 Rockslide The rapid movement of large blocks of detached bedrock sliding more or less as a unit. Rockslide Fig. 12.8 Rockslide Rock Avalanche The rapid* mass movement of broken rock material, often riding on a cushion of trapped air. Usually triggered by an earthquake. * 10’s to 100’s of km/hr! Fig. 12.8 5 Rock Avalanche Rock Avalanches Triggered by an Earthquake (Nov. 3, 2002) in Alaska Fig. 12.9 Fig. 12.9 Types of Types Unconsolidated Mass Movement Unconsolidated Flows Creep Earth flow Debris flow Mud flow Increased velocity Creep The downhill movement of soil and other debris, typically at rates of about 1 to 10 mm/year. Evidence of Creep Fig. 12.10 Fig. 12.10 6 Earthflow A fluid movement of relatively fine-grained material, e.g. soils, weathered shale, and clay. Earthflow Fig. 12.11 Debris flow A fluid mass movement of rock fragments supported by a muddy matrix. May move a speeds of up to 100 km/hr! Earthflow Fig. 12.11 Debris flow Fig. 12.12a Fig. 12.12a 7 Mudflow A flowing mass of material (mostly finer than sand, along with some rock debris) containing a large amount of water. It may travel large distances and high speeds, and carry particles as large as a house! Mudflow Fig. 12.12b Types of Unconsolidated Mass Movement Unconsolidated Slides and Falls Slump Debris slide Debris avalanche Fig. 12.12b Increased velocity Slump A slow slide of unconsolidated material that travels as a unit. Slump Fig. 12.14 8 Slump Debris Slide A type of mass of material in which rock material and soil move largely as one or more units along planes of weakness. Fig. 12.14 Debris Slide Debris Slide Fig. 12.15 Fig. 12.15 Debris Avalanche Mt Huascaran, Peru Huascaran, (before 1970) Fig. 12.13 Fig. 12.13 9 Possible Triggers for Mass Movement Mt Huascaran, Peru Huascaran, (after 1970) • over-steepened slope: – erosion / lateral erosion – volcanic ash – excavation (manmade) Towns buried by debris avalanche Fig. 12.13 • increased water content: – intense rainfall – rising water table (e.g. behind dam) • cyclic loading: – earthquakes – storms Effect of Tectonic Setting • high relief, steep slopes • fractured, tilted rocks • frequent earthquakes (triggers) Landslide Caused by Undercutting Inclined Strata Box 12.1 Note dipping beds undercut by excavation for house Failure occurs when watersaturated strata slide along slippery clay unit, breaching thin retaining wall Box 12.1 Box 12.1 10 1925 Gros Ventre Slide, Wyoming 1925 Gros Ventre Slide, Wyoming Fig. 12.16 Fig. 12.16 1925 Gros Ventre Slide, Wyoming 1925 Gros Ventre Slide, Wyoming Fig. 12.16 Fig. 12.16 1925 Gros Ventre Slide, Wyoming 1925 Gros Ventre Slide, Wyoming Fig. 12.16 Fig. 12.16 11 Vaiont Reservoir Slide Gros Ventre Slide, Wyoming Fig. 12.16 Fig. 12.17 Ways to Reduce Losses Due to Landslides Include: • avoid construction in areas prone to mass movement • build in a way that does not make naturally stable slope unstable • engineer water drainage to prevent strata to become water saturated and prone to fail 12 ...
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