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Landslides

Landslides. Learning Objectives. Understand basic slope processes and the causes of slope failure Understand the role of driving and resisting forces on slopes and how these are related toslope stability

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Landslides

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  1. Landslides

  2. Learning Objectives • Understand basic slope processes and the causes of slope failure • Understand the role of driving and resisting forces on slopes and how these are related toslope stability • Understand how slope angle and topography, vegetation, water, and time affect both slope processes and the incidence of landslides • Understand how human use of the land has resulted in landslides • Know methods of identification, prevention, warning, and correction of landslides • Understand processes related to land subsidence

  3. Mass Wasting • Definition: mass wasting refers to a downslope movement of rock or soil as a more or less coherent mass. • Comprehensive term – all inclusive term for any downslope movement of earth materials

  4. Slopes • Weak and Soft Rock • Form slope with 3 segements • Convex upper part • Straight central segment • Concave lower part • Hard Rock • Form free face with talus slope at base

  5. Important types of mass wasting • Slide – downslope movement of coherent block of earth material • Slump – is sliding along a curved slip plane producing slump blocks • Fall – rocks fall from vertical face • Flow – Downslope movement of unconsolidated material in which particles move about and mix within the mass • Subsidence is the sinking of of a mass of earth material below the level of surrounding material

  6. Landslides are commonly complex combinations of slding and flowage • Upper slump block • Lower flow

  7. Forces on Slopes • The stability of a slope expresses the relationship between resisting forces and driving forces

  8. Driving forces – forces which move earth materials downslope • Downslope component of weight of material including vegetation, fill material, or buildings • Resisting forces – forces which oppose movement • Resisting forces include strength of material

  9. Potential Slip Planes • Geologic surfaces of weakness in the slope material – bedding, foliation, fractures

  10. Safety Factor • The ratio of resisting forces to the driving forces • RF/DF SF > 1 Slope is stable SF < 1 Slope is unstable

  11. Factors Affecting Slope Stability • Type of earth material • Slope Angle and Topography • Climate • Vegetation • Water • Time

  12. Rotational Slides • Sliding occurs along a curved slip plane

  13. Translational Slides • Sliding occurs on a planar surface or on a slip plane

  14. Slump (a type of slide) • Indicators: • Scarp • “Hummocky” terrain on and below (earthflow)

  15. scarp Slump

  16. Debris Flow • Debris flows are the downslope flow of relatively coarse material • > 50% of particles in a debris flow are coarser than sand • Movement may be very slow or very fast, depending on topographic conditions • Mudflows, debris avalanches, and debris flows • Small to moderate magnitude events, occasional large magnitude events

  17. Debris Flow

  18. Rockslide • Rock moves because there’s nothing holding it back! • Generally requires a pre-existing low-friction surface...

  19. Rockslide • like a clay layer, once it’s wet...

  20. 28 deaths in 1959, triggered by earthquake “Earthquake Lake”, MT

  21. Role of Earth Materials • Slopes formed by weak rocks such as shale or have thick soil deposits typically fail by rotational slides • Slopes formed by hard rocks typically fail by translational slides • Soil slips occur above bedrock and fail by translational slides

  22. Role of Slope and Topography • Hillslope angle is a measure of the steepness of a slope = slope gradient • Steeper slope = increased driving forces • Steep slopes associated with rockfalls • Subarid to arid environments

  23. Role of Vegetation • In subhumid to humid environments, vegetation is thick and abundant • Landslide activity includes deep complex landslides, earthflows, and soil creep. • Vegetation influences slope stability by: • Providing a cover that cushions the impact of rain falling on slopes and retards erosion on surface • Vegetation has root systems that tend to provide an apparent cohesion which increases resistance to landsliding • Vegetation adds weight to the slope increasing the driving forces

  24. Role of Water • Water can affect slope stability by: • Shallow soil slips can develop during rainstorms when slopes become saturated • Slumps or translational slides can develop months or years after slope is saturated • Water can erode the base or toe of a slope decreasing slope stability

  25. Role of Climate • Climate influences the amount and timing of water in the form of water or snow • Influences type and amount of vegetation

  26. Role of Time • Physical and chemical weathering can weaken slope materials decreasing resisting forces

  27. Earthflow • basically a very viscous (thick) debris flow • slow-moving • faster in wetter weather

  28. Earthflow

  29. Creep • very slow • result of freezing and thawing

  30. Creep

  31. sandstone shale

  32. from D. Schwert, NDSU Creep

  33. Triggers for rapid Mass Wasting • Rain • Oversteepening • cutting at foot of slope • piling on head of slope • Deforesting / Devegetating • Earthquakes

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