Landslides: Causes

Cause vs Trigger

  • causes are factors(often long-term) leading the instability of a given slope
  • reduce the shear strength (or increase shear stress) of a slope
  • they don’t initiate movement
  • triggers are factors (usually short events) that translate instability into motion
    • like earthquakes or heavy rainfall
  • there can be many causes, but only one trigger

External Causes

  • external causes are factors that act on the outside of the slope that affect its stability
  • examples:

  

  1. slope angle

      1. you must have slope to have movement 2. steeper slopes = more potential for movement

  1. undercutting

      1. the lower part of the slope is removed 2. removes material supporting the slope 3. caused by roads, rivers, buildings, etc.

  1. overloading

      1. adding weight 2. caused by buildings, roads, other landslides, large tress, etc.

  1. vegetation

      1. roots bind loose material 2. removal of vegetation can make slopes unstable

  1. climate

      1. if average temperature and rainfall is high

         1. more water 2. increased weathering of rocks (more fractures, more soil, etc) 2. if average temperature is lower than 32 degrees F… see internal causes

Internal Causes

  • internal causes are factors that act on the inside of the slope that affect its stability
  • some examples:

  

  1. water content

      1. in all slopes, water:

         1. adds weight (overloading) 2. decreases friction and thus shear strength 3. increases weathering (decreasing cohesion of material) 4. acts as a medium for flow 2. in sediment (rocks, sand, mud), water:

         1. can help or hinder cohesion 2. depends on amount of water 3. in solid rock:

         1. water reduces shear strength along panes of weakness (fractures) 2. frost wedging/ free-thaw

  1. inherently weak materials

      1. some materials are very weak 2. fail at relatively and/or very low angles of repose 3. ex: quick clay

  1. bad geologic structures

      1. unfortunate layer of fracture orientation 2. structures angled in an unstable direction or layered precariously