Mass Wasting

• Regolith

  • unconsolidated material
• Mass wasting

  • Downslope movement of rock/regolith (unconsolidated material) under the influence of gravity
  • happens when slopes becomes unstable
• Slope stability is dependent on:

  • balance of forces affecting mass/material
  • properties of mass/material
• Horizontal slope - stable

  
• Gentle slope

  
• Steep slope

  
• Angle of repose

  • dependent on size of sediment
  • Steepest angle at which sediment deposit or soil is stable and will not move downslope
• In general, bigger sediments have steeper angle of repose than smaller sediments
• Dry sand

  • friction holds grains together

  
• Wet sand

  • small amounts of water creates surface tension between grains

  
• Water saturated sand

  • water pushes grains apart

  
• Daylighting

  • When planar discontinuities (beds, fractures) are tilted in the same direction as the slope, increases likelihood of sliding downslope
• Triggering events

  1. rainfall - adds water to sediments --> push apart grains
  2. earthquakes - shake sediments --> grains move and fall
  3. oversteepening of slopes
• Ways to classify the various types of mass movement:

  1. Nature of slide material/mass - rock, debris, earth
  2. Type of movement - fall, topple, slide
• Fall

  • Abrupt, downward movements of rock or earth, or both, that detach from steep slopes or cliffs
  • Free-fall - falling without resistance
• Types of slide:

  1. Translational slide
  2. Rotational slide
• Translational slide

  • Rapid movements along a plane of distinct weakness between the overlying slide material and more stable underlying material
• Translational slide

  
• Rotational slide

  
• Rotational Slump

  • Downward rotation of rock or regolith along a concave-upward surface
  • Leaves arcuate scars or depressions on the hill slop
• Flow

  • Rapidly moving mass-wasting events
  • Loose material mixed with abundant water
• Flow

  • Commonly separated into debris flow (coarse material) and earthflow (fine material) depending on the type of material involved and the amount of water
• Avalanche

  • Rapid to very rapid landslides
  • Related to collapse of slopes or volcanic edifices
• Soil creep

  • Extremely slow downslope movement of unconsolidated material
  • Takes years to have a noticeable effect on the slope
• Reasons for soil creep

  • Expansion and contraction of surface sediment
  • Pull of gravity
• Hard landslides mitigating measures

  • involve actively setting up slope reinforcements/modifying slopes to increase their stability or lessening the impact of landslides by “shielding”, “catching”, or rediverting landslide deposits
• Hard engineering measures: benching

  • fight off angle of repose

  
• Hard engineering measures: riprap

  • avoid erosion

  
• Hard engineering measure: gabion walls

  • hold back soil with heavier materials

  
• hard engineering measure: shotcrete

  • band-aid solution

  
• hard engineering measure: retaining wall

  • similar to gabion walls

  
• Soft measures

  • include creating hazard maps, educating the community on landslide hazards and having Early Warning Systems
• Early warning system

  • monitoring systems designed to predict events that precede landslides in order to issue a hazard warning