Mass Wasting

Mass Wasting

1. Types

   • Slide -Slow or rapid movement of coherent block of rock or soil along a well defined slip-plane
     • Planar - Has a flat slip plane
     • Rotational - The slip plane is cured inwards (sometime referred to as slumps)
   • Creep - Slow transfer of soil downslope by winter freezing and expansion and summer thawing and contraction. This process forms small elongated ridges called terraces.
     • Signs of soil creep are: tilted gravestones, bent tree roots, bent telegraph poles, bent fence and tensional cracks in road and paths
   • Flow - An incohesive slurry of rock, soil and water. Usually occurs in sediment and weakly cemented rock. They are triggered by a build up of water pressures. Examples are: Lahars, mudflows or mudslides
   • Fall
     • Toppling - Widening of vertical fractures causes the fall
     • Wedge Failure - Slide and Fall of wedges of rock over daylighting fractures
     • Slab failure - Collapse of rock due to undercutting

2. Factors that contribute to mass wasting

   • Reduction in slope strength
     • Fractures, joints and bedding planes: Acts as slip planes and enable rocks to become loose
     • Increase in Groundwater pressure: Lubricate slip planes and push material upwards
     • Melting of ice that initially binded rocks
     • Vibrations from earthquakes, trains and cars
     • Saturation: softens soil and reduces cohesive strength
     • Organic effect: burrowing animals and decay of binding tree roots loosen sediment and reduce binding
   • Increase in gravitational force
     • Precipitation: The additional weight of now saturated sediment or snow can cause mass wasting
     • Weight of buildings, developments and other infastructure
     • Gradient of the slope, Steeper slopes have a higher graviatational force

3. Monitoring

   • Measurements of creep or strain - Will increase prior to major slide
     • Strain meters
     • Micro-Seismic monitor
     • Borehole distortion
   • Groundwater Pressure
     • Measured using piezometers installed in boreholes
   • Hazard Mapping - Areas of active landslides can be mapped in order to reconstruct development near unstable slopes
   • Surveying (Detecting surface changes caused by underneath movement)
     • Leaning structures
     • Build up of soil and debris
     • Change in slope shape
     • Formation of tension cracks

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