Mass Wasting

• Mass wasting


  Downslope movement of rock, regolith (unconsolidated material), and soil under the influence of gravity
• Some mass-wasting processes act very slowly; others occur very suddenly, often with disastrous results
• Slope


  • On a flat surface, the force of gravity acts downward
  • On a slope, the force of gravity can be resolved into two components: (a) one acting perpendicular to the slope and (b) another acting tangential to the slope
• g_p


  Perpendicular component of gravity that helps hold the object in place on the slope
• g_t


  Tangential component of gravity that causes a shear stress parallel to the slope and pulls the object in the down-slope direction
• On a steep slope, the shear stress or tangential component of gravity, g_t, increases, and the perpendicular component of gravity, g_p, decreases
• Slope

  Mass wasting occurs when a slope is too steep to remain stable with existing material and conditions
• Water

  • Dry materials - Angle of repose increases with grain size (30 - 37 degrees), controlled by frictional contact between grains
  • Slightly wet - Very high angle of repose, surface tension between water and solid grains holds grains in place
  • Materials saturated with water - small angle of repose - material flows like a fluid; water gets between the grains and removes grain-to-grain removal
• Soil cover


  • Soil contain high proportion of clay minerals
  • Some clay minerals expand when they become wet as water enters the crystal structure
  • When clays dry out, loss of water causes clays to shrink/compact
• Geologic features that promote mass wasting


  • Type of rock
  • Presence of joints or fractures
  • Presence of bedding planes
  • If the joints or bedding planes dip in the same direction as the slope
• Triggering events for mass wasting


  • Ground shaking (earthquakes, volcanic eruptions, minor shocks)
  • Excessive rainfall
  • Minor shocks like heavy trucks rambling/man-made explosions
• Excessive rainfall

  • Heavy rains can saturate regolith, reducing grain to grain contact and reducing the angle of repose
  • Mudslides - occur during/after a heavy downpour and become a major disaster
• The Philippines experiences an average of 20 tropical cyclones per year, which can trigger mass wasting events
• Types of mass wasting processes


  • Rate of movement - rapid/slow (m/s to 100km/hr; 1mm/yr or day)
  • Type of movement - falling, sliding, or flowing
  • Types of material involved - rock, soil, or debris
  • Specific: Slump (rotational slide), Rock fall, Debris flow, Mud flow (earthflow), Creep
• Slump


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


  • Piece of rock on a steep slope becomes dislodged and falls down the slope
  • May be a single piece of rock or a mass of rocks
• Debris flow


  • Rock or debris slide down the pre-existing surface (e.g. bedding plane, foliation surface, or joint surface)
  • Differ from slumps because there is no rotation of the sliding rock mass
• Mud flow (Earthflow)


  • Highly fluid, high-velocity mixture of sediment and water (consistency similar to wet concrete)
  • Move at velocities >1 km/hr
  • Volcanic mudflows are known as lahars
• Creep


  • Requires years of gradual movement (a few inches to several feet per year) to have a pronounced effect on a slope
  • Due to the expansion and contraction of surface sediment, and the pull of gravity
• Mitigating measures for mass wasting

  • Hazard maps
  • Hard engineering measures (e.g. construction of features to stabilize slopes)
  • Soft measures (e.g. monitoring)
• Presence of vegetation is sometimes not sufficient to prevent rainfall-induced landslides - proper vegetative anchoring ("deep roots") is needed
• Engineering measures to mitigate mass wasting


  • Chicken wire
  • Concrete cover
  • Retaining wall
  • Coconet
  • Weep holes
  • Benching
  • Rock barriers
• Areas to avoid due to landslide risk


  • On existing landslides, old or recent
  • On or at the base or top of slopes
  • In or at the base of minor drainage hollows
  • At the base or top of a steep cut slope
• Mass Wasting Processes