Mass movement lesson 9


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Mass movement lesson 9

  2. 2. What?• Downslope movement of material under the direct influence of gravity• Most type of mass movements are aided by weathering and erosion• Major landslides have natural causes• Minor landslides are result of human activities• Slope stability depends on the relationship between shear strength and force of gravity
  3. 3. How are mass movements classified? Mass movements are classified by their dominant behaviour according to:i. The type of motion – rapid or slow?ii. The rate of motion – (flowing, sliding, falling, heave)iii. The type of material involve – rock, soil debris
  4. 4. i. Rate of movement• Rapid- visible movement- sudden movement- involve rock, soil, debris- dangerous
  5. 5. i. Rate of movement• Slow – moving at an imperceptible rate about 0.01cm per year- detectable only by the rate effects of their movement (e.g. tilted trees, cracked foundation)- responsible for downslope transport of great volume of weathered material
  6. 6. ii. Types of movement •Falling •Sliding •Flow •Heave
  7. 7. Falling• Rapid• Rocks of any size fall through the air• E.g. Rockfall
  8. 8. Sliding• Involve movement of material along the slideplane• Do not suffer internal disruption as material moves in• One block at the same rate at every level• Slow to rapid movement• E.g. landslide, slumps, rotational slip
  9. 9. Landslide
  10. 10. Landslide
  11. 11. Rotational Slip
  12. 12. Flow• Material flows as a viscous fluid• Involves internal disruption of material• The surface layer contains smaller rounded particles move fastest while the rate of movement decreases with depth as particles become larger and more angular• E.g. Earthflow, mudflow, solifluction
  13. 13. Earthflow
  14. 14. Solifluction•
  15. 15. Heave• A slow form of mass movement• Involve the rising of particles at right angle to the slope followed by vertical setting• E.g soil creep
  16. 16. Factors influencing slope failure? • Slope angle • Weathering and climate • Water content • Vegetation • Slope dip • Trigger mechanisms
  17. 17. Slope angle• The steeper the slope , the less stable it isi. Natural processes:• Undercutting by stream or wave action• This process removes the slope’s base, increases the slope angle  increases the gravitational force acting parallel to the slope• Wave action, especially during storms, often results in mass movement along the shores or large lake
  18. 18. ii. Human processes• Grading the slope too steeply or cutting into its side• Cause stress in the rock or soil until it is no longer strong enough to remain at the steeper angle and mass movement.• Excavation for road cuts and hillside building sites are major cause of slope failure
  19. 19. Weathering and climate• Occur in loose or poorly consolidated slope material• As soon as rock is exposed at Earth’s surface, weathering begin to disintegrate and decompose and reducing its shear strength and increasing its susceptibility to mass wasting• Common in areas of deep weathering
  20. 20. Water content• Two ways:1. Large amount of water from melting snow or heavy rainfall greatly increase the likelihood of slope failure.2. Water percolating in a slope material helps decrease friction between grains  loss of cohesion
  21. 21. Vegetation• Absorbing the rain decreasing water saturation of slope’s material leads to loss of shear strength• Root system also helps stabilise a slope by binding soil particles together
  22. 22. Overloading• Result of human activities: dumping, filling or piling up of materials• A material’s load is carried by its grain to grain contact• With the friction between the grains maintaining a slope• The additional weight create by overloading increases the water pressure within the material  decreases its shear strength  weakening the slope material
  23. 23. Slope dip• Rock dip in the same direction as the slope – Water can percolates along the various planes and decrease the cohesiveness and friction. – E.g. clay becomes slippery when wet.
  24. 24. Slope dip
  25. 25. Horizontal dip with joints that are cut parallel to the slope• If the rocks are horizontal or dip in a direction opposite that of the slope, joints may dip in the same direction as the slope.• Water migrating through them weathers the rocks and expands these opening until the weight of the overlying rock causes collapse and mass wasting occur
  26. 26. Trigger Mechanisms• Rapid mass movements – triggered by a force that temporary slope equilibrium• For examples: – strong vibrations from earthquakes, – Excessive amounts of water from snowmelt – Heavy rainstorm