Mass movements

• Mass Movements

  • Hazard - natural event that has the possibility to cause loss of life/ economic damage
  • Natural disaster - when the natural event DOES cause loss of life or economic damage
• Mass Movements

  • A mass movement is the largescale downslope movement of soil, sediment and rock
• Rockfall - rapid freefall of rock from a steep cliff face. Made worse by freeze thaw.
• Debris and mud flows - occurs on steep slopes. Rapid sudden movement that occurs after period of heavy rain - not enough vegetation to hold soil in place. Common in tropical settings
• Translational slide - a slide downslope movement of material occurring along a distinctive surface of weakness, e.g a bedding plane
• Rotational slide/landslip - occasional, rapid movements of a mass of earth or rock sliding along a concave plane. Can occur after long periods of heavy rain where water lubricates rock making it liable to slide. Also caused by undercutting of a steep slope by river or sea erosion.
• Soil creep - very slow movement occurring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods. Soil slowly moves downslope.
• Soilfluction - slow downhill flow of soil in arctic conditions. Results from complete waterlogging of the sediment
• Mechanisms of mass movements are linked to:

  • Angle of slope - shear stress vs shear strength
  • Lithology/material - incompetent vs competent
  • Weathering
  • Load - increases shear stress
  • Groundwater/rainfall - water can act as a lubricant to accelerate movement by reducing friction between blocks/ particles
  • Ground vibration
  • Vegetation - holds soil in place slowing influx of water
  • Human activity
• • Shrinking and swelling of the ground is one of the most damaging geohazards in Britain today
  • Shrink-swell is the volume change that occurs as a result of changes in the moisture content of clay rich soils. Swelling pressures can cause heave, or lifting of structures, whilst shrinkage can cause settlement or subsidence
• • Kaolinite group - common clay mineral. It is known as a 1:1 type clay mineral, which means it comprises one tetrahedral sheet linked to one octahedral sheet. Strong bonding means that the lattice is fixed and no other cations or water can fit between sheets. Low ion absorption.
  • Smectite group - forms a 2:1 type structure. Octahedral aluminium layer sandwiched between two tetrahedral silica layers. Units weakly bonded meaning water can enter the clay by moving into the spaces between units meaning it can expand up to 1500%
• Mitigating against shrinking and swelling clays

  • Engineering - reinforced raft foundation helps prevent damaged to smaller structures. Can artificially lower the water table or have an effective drainage system
  • Chemical treatment with lime or calcium oxide reduces expansion potential to around 100%. Bacteria can be used to reduce iron decreasing swelling tendencies.
• Sinkholes

  • Sinkoles - consequence of natural dissolution of bedrock - associated with chemical breakdown of carbonates e.g. limestone. Joints and bedding planes are enlarged allowing more water in and more dissolution to take place forming cave systems.
  • Crownholes - result of human activity e.g. voids enlarged from gypsum mining.

  • Mitigating sinkhole hazards
  • Avoid karst terrain for major structures - a topography formed for the dissolution of soluble rocks
  • Drainage of water