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  1. 1. LANDSLIDES Kaustubh Sane HJD Institute of Technical Education and Research
  2. 2. What are they? Mass movements include: • Landslides • Rock falls • Avalanches • Mud flows • Debris flows • Creep
  3. 3. Anatomy of a rotational landslide
  4. 4. <1 cm/year >100 km/year 0% ~40%
  5. 5. Mass Movements • Material moves downslope due to the pull of gravity • Can happen almost anywhere • Commonly associated with other events (heavy rainfall or earthquakes, for example) and are therefore under-reported • Movements can either be catastrophic (slope failure) or slow and steady (creep) • The rate of the mass movement can be increased by various erosive agents (especially water)
  6. 6. Gravity Water Earth Materials Triggering Events Factors in Slope Stability
  7. 7. Gravity & steepening of a slope • Gravity causes the downward and outward movement of landslides and the collapse of subsiding ground. • Eventually it will flatten all slopes • The force of gravity is the mass of a body x the sine of the slope. • If can remove the initial resistance to motion the body will move. • Earthquake, heavy rain could give initial energy
  8. 8. Rotational landslide
  9. 9. Angle of Repose Varies for Different Materials
  10. 10. Water decreases rock/soil cohesion
  11. 11. Water decreases rock/soil cohesion
  12. 12. Water decreases rock/soil cohesion Water circulating underground can dissolve cements that hold sedimentary rocks together
  13. 13. Internal Causes for Slope Failure • Water (weight & interaction with clay minerals) • Decreasing rock cohesion • Incompetent/weak material • Adverse geologic structures
  14. 14. 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
  15. 15. 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
  16. 16. 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
  17. 17. 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
  18. 18. Role of Climate • Climate influences the amount and timing of water in the form of water or snow • Influences type and amount of vegetation
  19. 19. Role of Time • Physical and chemical weathering can weaken slope materials decreasing resisting forces
  20. 20. IMPORTANT CONCEPTS CREEP Movement down slope of soil and uppermost bedrock Block diagram showing the effects of creep. Most commonly seen by its effects on telegraph poles, fences and trees. The soil zone slips in ultra slow movement as particles shift in response to gravity
  21. 21. IMPORTANT CONCEPTS CREEP How creep works Surface materials expand perpendicular to slope (1 to 2) as a result of freezing, wetting or heat of sun. Upon thawing,drying or cooling they contract (2 to 3) under pull of gravity vertically. Do not go back to old position. Thus have a slow movement down slope i.e. creep
  22. 22. IMPORTANT CONCEPTS LANDSLIDE Fast moving mass-movement Causes most fatalities. Landslide is a mass whose center has moved downwards and outwards. Has a tear-away zone upslope where material has pulled away and a pile-up zone where material had accumulated.
  23. 23. IMPORTANT CONCEPTS LANDSLIDE Major topographic features Features include crown, head scarp, basal surface of rupture, transverse cracks, transverse ridges and radial cracks. All created in the downward and outward movement
  24. 24. IMPORTANT CONCEPTS EXTERNAL PROCESSES CAUSING FAILURE Three major ones On arcuate failure surfaces have balance between the driving mass and the resisting mass. Changing either can create a landslide Processes include: 1) steepen slope, 2) remove support from bottom of slope, and 3) add mass high up on slope.
  25. 25. IMPORTANT CONCEPTS INTERNAL CAUSES OF SLOPE FAILURE Clays Clays form during chemical weathering due to acidic fluids such as water, CO2 charged water and organic acids decomposing minerals created at high pressures and temperatures. Creates totally different internal structure. Clay minerals are built like books and have many unfilled atomic positions in the crystal structure. Typically can have their strength dramatically reduced by adding water which also causes expansion.
  26. 26. IMPORTANT CONCEPTS INTERNAL CAUSES OF SLOPE FAILURE Quick Clay: Ontario, Canada 1993 Fine grained rock flour left behind during the retreat of the glaciers and deposited in a nearby sea. The clay and silt particles are loosely packed and held together as a rock by sea salts. When the sea retreats, the sediments are uplifted and the glue removed by fresh water. Anything can cause the house of cards to collapse
  27. 27. IMPORTANT CONCEPTS INTERNAL CAUSES OF SLOPE FAILURE The five roles of water 1) Sediments have high porosities. When these void spaces are filled with water the sediment is much heavier and the driving mass increased. 2) Water is easily absorbed and attached externally to clay minerals with a major decrease in strength. 3) Water flowing through rocks can dissolve the minerals that bind the rocks together. The removal of the cement makes the rock easier to move or a slope easier to collapse. 4) Water can physically erode loose material creating caverns. 5) Pressure builds up in water trapped in the pores of sediments being buried deeper and deeper. Sediments can compress but water does not compress. Get abnormally high pore-water pressures which “jacks up” the sediment and makes it very easy to move.
  28. 28. IMPORTANT CONCEPTS INTERNAL CAUSES OF SLOPE FAILURE The role of flowing water Schematic cross section of ground water flowing through poorly consolidated rock. The water will carry sediments to the stream creating a series of caverns that seriously weaken a hill.
  29. 29. IMPORTANT CONCEPTS INTERNAL CAUSES OF SLOPE FAILURE Slope stability Addressed by use of Coulomb/Terazaghi equation where s = c + (p - hw) tan ø Where s = resistance due to shear, c = the cohesion of the sediment layer p = load of sediment and water above a slide surface hw = weight of water above the potential surface. F = internal angle of friction. Strength comes from cohesion + the weight of the sediment. Weakness from the pore water pressure and the internaL angle of friction. Clays have high cohesion but a very low failure angle. Sands have poor cohesion. Granites have very high failure angle.
  30. 30. IMPORTANT CONCEPTS INTERNAL CAUSES OF SLOPE FAILURE Quick sand Example of the Coulomb-Terazaghi equation. The pore water pressure hw equals the weight of the sands p. Leaves cohesionless sand with no shear stress. With no shear stress you will sink into the sand when you walk on it.
  31. 31. IMPORTANT CONCEPTS ADVERSE STRUCTURES 1) Ancient slip surfaces are weaknesses that tend to be reused over time. These surfaces are especially slippery when wet. 2) The orientation of the sediment layers can create strong or weak conditions. Sediment layers dipping into the hill are very stable, dipping in the same direction but shallower than the slope have daylight bedding. Potentially dangerous condition. 3) Rocks have inherent weakness that set-up slope failure. Lack of cement, clay layers, soft rocks, splitting joints, faulting surfaces. TRIGGERS Basic causes bring slopes close to failure. Rain, earthquakes or humans create trigger.
  32. 32. MASS MOVEMENTS CLASSIFICATION Speed of movement and water flow. On left have mass movement speed versus moisture content. On right have rates of travel for mass movements
  33. 33. MASS MOVEMENTS CLASSIFICATION Falls, Flows, Slides and Subsides. Falls and subsides involve vertical drops. Slides and flows involve downward and outward motion. Sliding involves a coherent mass. Flowing involves the moving mass behaving like a viscous fluid.
  34. 34. MASS MOVEMENTS FALL 10 July 1996 Yosemite
  35. 35. MASS MOVEMENTS SLIDES Rotational Downward and outward movement on a curved surface. Note the rotation of head and the up movement of the toe. Swedish circle analysis of slope stability has a compass set at the center of rotation. Use this to compute driving and resisting forces. Note backward tilted head and bulged toe. Toe helps stability, tilted head catches water.
  36. 36. MASS MOVEMENTS SLIDES Rotational: 1976 Ensenada slide The slide was associated with a pronounced head scarp shown here. A rotation of coherent beds downwards and outwards and the formation of a pronounced bulge. Not all rotational slides are as simple as this because of discontinuities on the surface.
  37. 37. MASS MOVEMENTS SLIDES Rotational slide: 1976 Ensenada Top) View northward along Highway 1 showing seaward shift of the highway. This was created by the downward and outward movement of the center of the slide. Bottom) Toe of the Ensenada slide. Note that the ocean floor was lifted above sea level stabilizing the slide.
  38. 38. MASS MOVEMENTS TRANSLATIONAL SLIDES Three types 1) Move as coherent blocks. Pt Fermin 1929 2) May deform and break-up as a debris slide. 1963 Viaont, Italy 3) Involve lateral spreading where the underlying material fails and flows, Anchorage, Alaska 1964
  39. 39. MASS MOVEMENTS SLIDES Translational slide: Pt Fermin, Ca 1929 Cross section showing block on top of a inclined slippery layer which day lighted under the ocean. Block moves towards the unsupported offshore. Though triggered by watering from yard irrigation seeping down to layers of weak clays which expand and lose strength and sliding begins.
  40. 40. MASS MOVEMENTS SLIDES Translational: Head of Point Fermin 1929 slide
  41. 41. MASS MOVEMENTS SLIDES Translational Debris Slide: Vaiont Italy 1963 Reservoir built on a) sedimentary rock layers with beds dipping towards the reservoir, b) fractures formed in the rocks due to expansion after retreat of glaciers and river cutting Canyon, 3) weak clay layers and numerous Limestone caverns. After very heavy rains the slide slumped down the fracture surfaces into the reservoir.
  42. 42. MASS MOVEMENTS SLIDES Translational slide: Debris Slide, 1963 Vaiont Italy Huge area of slope slid into the reservoir. Debris at 150 M above the water level. A huge wall of water climbed over the dam and swept down the river bed as a 70m high wall of water causing extensive fatalities in the towns down stream. The Dam stood. Has been called world’s worst dam disaster
  43. 43. MASS MOVEMENTS FLOWS Movements that behave like fluids with internal reorganization Loess flows - the flow of loose silt, Gansu Province China, 1920. The mountains walked. Earth flows - wet flows moving slowly on slick surface, Portuguese Bend Ca, 1958 Debris flows (Sturtzstroms) - massive rock falls that convert into highly fluidized rapidly moving Debris flows. Elm, Switzerland, 1881
  44. 44. MASS MOVEMENTS FLOWS Earth flow: Portuguese Bend 1958 and following. Cross section through Portuguese Bend showing the seaward dipping reflectors, bentonitic clay layers, a preexisting slide surface and waves eroding the base of the slope. An ancient earth flow site which was unstable but not moving. 1950’s put development on area by beach. No sewers. Within a few years lower slope started to move. Watering and sewage disposal seeped down to bentonite layers. They expanded, lost strength and started to move.
  45. 45. MASS MOVEMENTS FLOWS Earth flow: Portuguese Bend 1958 View from the sea. Note the pier and Crenshaw Boulevard.
  46. 46. MASS MOVEMENTS FLOWS Earth flow: Portuguese Bend View of the toe of the bulged up earth flow in 1959. Note the remains of the houses and the roads and the damaged pier.
  47. 47. MASS MOVEMENTS FLOWS Slumps and Debris flows, La Conchita Heavy rains led to failure at depth triggering a slump and earthflow in 1995 which moved very slowly during day. No loss of life. Heavy rains led to a faster moving debris flow in 2005 which killed 10 people. These flows typical of coastal flows near coast in southern California.
  48. 48. MASS MOVEMENTS FLOWS Debris flow: Elm, Switzerland 1881 Three stages, rock fall, a jump and fluidization of the debris, and then a flow of the disintegrated rock mass down the valley floor using air and dust as the internal fluid. Many fatalities in Elm 2.25 km away.
  49. 49. MASS MOVEMENTS FLOWS Submarine flows: Hawaii Left) Slump and debris-avalanche deposits cover more than five times the area of the islands. Right) Created by the collapse of flanks of the Islands. The zone of normal faults associated with the injection of magma at Kilauea appear to be head scarps for giant mass movements.
  50. 50. MASS MOVEMENTS SUBSIDENCE SLOW Delta compaction, New Orleans Oil withdrawal, Houston Groundwater withdrawal, Mexico City CATASTROPHIC Limestone sinkholes
  51. 51. MASS MOVEMENTS SUBSIDENCE Slow: Mississippi Delta, New Orleans Left) Loosely packed sand and clay have high porosities near the surface. When buried they expel the water and contract over a period of time. Right) As much as 100 ft of subsidence of the Mississippi Delta around New Orleans over the past 20,000 years. Has been faster recently up to 3m in the past 50 years making city very prone to damage from hurricanes.
  52. 52. MASS MOVEMENTS SUBSIDENCE Catastrophic: Sink Holes View of sinkhole formed by the collapse of a limestone cavern on May 31 1981 in winter park Florida. Sinkhole is 100 m across by 30 m deep. Most of southern and eastern US lies on limestone rocks created from the deposition under seawater of soft shells made of calcium carbonate. Uplifted above seawater they are prone to dissolution from circulating acidic fresh ground water. Creates unstable caverns underground.
  53. 53. Use Knowledge of Mass Wasting to Avoid Risks • Be able to recognize geologically unstable situations
  54. 54. Understanding Mass Wasting Development causes: • Increased shear force – Steepened slope – Added weight • Decreased shear strength – Devegetation – Reworking of fill – Saturation of soil
  55. 55. Reduce Risks Some solutions include: • Increase shear strength – Re-compact soils – Re-vegetate soil slopes – Construct retaining wall with anchors • Prevent Saturation – Prohibit over-irrigation – Install surface drains – Install subsurface drains
  56. 56. • Increase shear strength with iron rods and anchors • Remove risk Reduce Risks