Mass Movement Hazards

• Rock Fall(Avalanches):

  • occur on steep slopes, and blocks of rock are broken away along weaknesses like joints/bedding planes
  • under normal conditions, only a small number of blocks fall at one time but large falls may be triggered by an earthquake
• Landslides:

  • occur along one or more failure surfaces; a rotational slide moves downwards and outwards along a curved slip surface (horizontal distance moved relatively short due to curved movement + damage severe, but not widespread)
  • translational slides can cover larger areas as rock mass moves down/out and continues as far force/slope allow (if material becomes deformed, the result is a debris slide)
• Debris Flow:

  • moving material behaves like a fluid; material ranges in size from huge boulders (debris flow) to clay grains (mudflow)
  • loose unconsolidated sediment is susceptible after heavy rain or earthquake shaking
• Angle of Slope (increase shear strength):

  • an increase in slope angle (especially at slope base) results from natural erosional processes or human activity (eg. road building at cliff-base); steeper slope = greater effectiveness of gravity, making collaspe more likely
• Lithology:

  • granite/limestone will hold a vertical slope without collaspsing + sand/clays are only stable if slope is at a low angle
  • loose/unconsolidated rocks more prone to collaspe + weaker rocks on top of stronger rocks prone to sliding over underlying strata
• Weathering (decrease shear strength):

  • weathering processes break down rock minerals into new clay and other minerals which expand when water is present
  • as rock weathering continues, the proportion of these clay minerals will increase in some rock types
• Load:

  • (decrease shear strength); overlying material removed by mass movement so material below under less weight + expansion occurs opening up cracks increasing porosity
  • (increase shear strength); additional weight on slope from increase water content (sedimentary rocks, porosity 10-30%) + if filled with water, weight increase greatly
• Groundwater Regime (decrease shear strength):

  • burrowing animals/development of soil pipes by throughflow of water weakens slope material; increase in groundwater from heavy rainfall make areas vulnerable to landslides
• Rainfall (decrease shear strength):

  • increasing water content of slope materials; reduces internal cohesion/friction + water trapped in rock/sediment pores, under pressure from material above
  • more compression/higher water content = higher pore-water pressure (can lift up sediment and start movement)
• Ground Vibration (increase shear strength):

  • shocks/vibrations from an earthquake or heavy machinery; vibrations from earthquakes/volcanoes frequently trigger landslides (cause greatest damage where silts/clays undergo liquefaction)
• Vegetation Cover (decrease shear strength):

  • removal of vegetation result of wildfires/human activity (overgrazing, building and deforestation) causes looser surface material due to loss of roots; slope vulnerable as trees intercept rainfall and bind soil/rock particles
• Slope Monitoring:

  • Ground Levelling/Surveying - surveyors used a Theodolite to measure distances and angles to monitor slopes.
  • Micro-Seismic Events - slopes are monitored for minor earthquakes with magnitudes 1-3 using an array of geophones linked to a central computer.
  • Electronic Distance Measurement (EDM) - involves a laser beam measuring the distance between fixed points on a slope.
• Slope Monitoring:

  • Borehole Distortion - water level in boreholes constantly monitored + porewater pressure measured using piezometer + extensometer measures variation in movement of unconsolidated material at depth.
  • GPS - aerial photographs/satellite imagery can be viewed at regular intervals and changes over time identified + satellites used to measure distance of a fixed point from the satellite.
• Slope Monitoring:

  • Ground Deformation (creepmeters, strainmeters, tiltmeters) - tiltmeters are anchored to slope and measure change in slope angle + creep/strainmeters fixed to bedrock across faults and joints/cleavages; strainmeters record changes in stress across weakness + creepmeters measure distance moved.
  • Groundwater Pressures - porewater pressure measured using piezometer.
• Drainage Control (Slope Management):

  • drainage increases shear strength of materials by reducing porewater pressure + drains improve water movement out of rock preventing saturation/decreasing risk of landslide event
  • also lowers water table so rocks at height that are saturated with water have time to dry out/strengthen (slope more stable)
• Retaining Structures (walls, gabions, rocks bolts/anchors and wire fences):

  • material deposited at slope foot reduces the shear stress + retaining walls are used to stabilise upper slope (eg. steel-mesh curtain)
  • toe of slope stabilised by a retaining wall + upper slope may have rock anchors/mesh curtains + shotcrete used to reduce infiltration into slope
  • toe stablised by gabions/earth fill + rocks bolts used to stabilise slope by securing joints/cleavages
• Regrading Slopes/Terracing (Slope Management):

  • terracing (benches) regrades the slope to produce more stable angles + other slopes are regraded to reduce slope angle making a mass movement event less likely