Natural factors of Mass Movement

Climate
1. Role of water
  1. Slope saturation
    1. Weaken a slope by increasing shear stress and decreasing shear strength
      1. Increases shear strength
        Water fills up pore spaces
        Capillary attraction
        Results from surface tension
        Surface tension: the force that holds the unconsolidated wet materials as a cohesive mass
        Increasing shear strength
      2. Increases shear stress
        When soil becomes saturated
        Pore spaces between particles are completely filled up
        Increases pore water pressure
        Eliminates grain-to-grain frictional contact
        Increases shear stress
        Flow like fluid
  2. Act as a lubricant
    1. Eliminates surface tension
      1. Allows sediments to move over one another
        Increases shear stresss
  3. Rapid change in water table level
    1. Occurs where slope is adjacent to a body of water
      1. Water table level falls
        Groundwater level cannot dissipate quick enough
        Results in higher than normal artificial water table
        Saturated with water thus increases weight of slope
        Increases shear stress & slope becomes unstable
        Leads to slope failure --> Mass movement occurs
  4. Stimulates vegetation growth
    1. Roots of plants help to hold soil together
      1. Increases shear strength
  5. Encourage weathering
    1. Stimulates regolith formation
      1. Soil sediments become unconsolidated and weak
        Increases shear stress
        Leads to slope instability
        Slope failure (Mass movement)
Vegetation
1. Maintains slope stability
  1. Holds soil particles together and intercepts rainfall (interception storage)
    1. Increases shear strength
Geological Structure
1. Rock characteristics
  1. Rock's porosity and permeability
    1. Dependent on presence of joints and cracks and bedding planes in rock
  2. Granite, shale (impermeable) : More porous, less permeable
    1. Steepness of granite slope weakened by weathering
      1. Formation of regolith + weight of water in pore spaces
        Regolith: Unconsolidated, unstable weathered material
        Increases shear stress
        Slope more unstable, increased chances of slope failure (mass movement)
    2. More prone to mass movement due to porosity
      1. Retain more water, increase weight
  3. Sandstone (permeable): Less porous, more permeable
    1. Less prone to mass movement due to permeability
      1. Less water retained, not as much weight
2. Steepness of slope
  1. Shear stress: Force that causes mass material to move downslope under influence of gravity
    1. When angle of slope > angle of repose
      1. Shear strength > Shear stress
        Slope becomes unstable leading to slope failure (mass movement)
        EXAMPLE: Undercutting of slope either by human activities or water bodies
        Undercutting slope removes toe of slope
        Increases gradient of slope
        Therefore angle of slope > angle of repose, leading to slope instability
        EXAMPLE: Previous slope failure sites
        Closely spaced joints leads to faster weathering
        More unconsolidated sediments
        Increases shear stress > shear strength
        Slope failure
        Annotations:
        Sedimentary rock --> Upslope dipping strata and downslope dipping strata Upslope dipping strata --> High resisting strength because bedding plane is not parallel to hillside Downslope dipping strata --> Less resisting strength because bedding plane is perpendicular to hillside. Therefore, detached masses of rock slide over more easily and readily
  2. Shear strength: Force that maintains mass material on slope hence maintaining slope stability
    1. When slope angle < angle of repose
      1. Shear stress = shear strength
        Slope remains stable, thus no slope failure
Character of Regolith
1. Refer to granite/shale and sandstone >>>
Tectonic Activity
1. Seismic Activity
  1. Intense ground/seismic shaking & Increase pore water pressure
    1. Liquefaction occurs more easily
      1. Water-saturated unconsolidated soil sediment (fluid-like)
        Decreases grain-to-grain frictional contact --> Increasing pore water pressure
        Increase in shear stress
        Slope more unstable, leading to slope failure (mass movement)
2. Volcanic Activity
  1. Volcanic eruption causes pyroclastic material and debris
    1. Pyroclastic material and debris mixes with water from torrential rainfall, forming unconsolidated material of mud and debris
      1. Causes lahar (volcanic mudflow)
        Fluid, easily flows downslope
        Increase shear stress > shear strength
        Slope more unstable, leading to slope failure (mass movement)

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Natural factors of Mass Movement

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	Created by Emma Lam over 10 years ago