A2 Mass Movement

toppling- failure of a rock face due to the widening of vertical fractures

slab failure- the collapse of a rock face due to under-cutting and removal of the supporting rock at the base of the cliff. This type of fall is common on coastal cliffs due to wave erosion

wedge failure- the slide and subsequent fall over steeply inclined fractures that are dipping towards the open face of a cliff. Triggered by heavy rain due to build up of groundwater pressure which reduces friction

Rock fall- debris accumulates at the base of the slope to form an expansive spread of angular rock fragments (talus)

Slide- slow or rapid movement of a coherent block or rock and soil along a defined slip plane

planar slides- where the slip plane is flat

rotational slides- where the slip plane is curved so movement slumps

Debris flow- cohesive mass of rock, soil and water which flows downhill as a saturated slurry. Also called mudflows or mudslides

Soil creep- slow transfer of soil downslope due to repeated annual cycle of winter expansion and summer contraction

factors that decrease slope strength

  • rock fractures e.g joints and fractures. Daylighting are more unstable
  • increase in groundwater pressure following heavy rainfall. Acts upwards so reduces frictional strength
  • melting of ice. More common due to global warming
  • earthquake vibrations
  • weathering which creates new fractures and softens rock
  • the softening of soil and rock material due to water saturation
  • organic effects, such as the decay of supporting tree roots and burrowing animals

factors that increase gravitational forces acting on the slope

  • increased weight of groundwater following heavy rain
  • weight of building
  • steep slope gradients (e.g engineering or volcano swelling)

MAM TOR LANDSLIDE

  • carboniferous sandstone on impermeable Edale shale
  • initial slope failure 3600 years ago
  • triggered by groundwater pressure
  • large slips in 1974 and 1977 on the A625
  • since initial failure of toe, the landslide has moved 320 metres

ABERFAN DISASTER

  • 15000 m2 slide killed 116 adults and 28 children
  • seven spoil tips of unconsolidated waste built up
  • unknown spring under tip 7
  • heavy rain added to weight
  • high gradient
  • vibrations from the train

monitoring

hazard mapping- areas of active landslides can be mapped in order to restrict development near unstable slopes

surveying- detect any surface changes

  • surface cracks
  • changes in shape of slope
  • build up of soil debris
  • leaning pole and bent trees

groundwater pressure- sensors installed within boreholes called piezometers

measurement of creep or strain- rates increase prior to a major landslide. Measured by

  • strain meter
  • borehole distortion monitors
  • micro seismic monitors

Mont de la saxe- minor movements and rock falls started in 2009. In springtime, snowmelt and heavy rain promote slope instability and accelerate slope movement. Monitoring:

  • EDM surveying
  • GPS receivers for continuous measures
  • borehole displacement sensors
  • borehole wire extensometres to measure ground stress
  • open pipe, borehole peizometres to monitor groundwater pressures

slope engineering

SLOPE MODIFICATION- lowering the gradient of the slope to reduce gravitational forces and increasing stability (steps)

GROUTING- impermeable cement that fills fractures and pores to increase rock strength and decrease permeability

ROCK BOLTS- steel rods of up to several metres cemented into the rock face which pin loose rock to the intact rock behind

ANCHORS- longer and wider than rock bolts

FABRICS- mesh structures made of metal draped over a slope to increase strength and prevent rockfall

RETAINING WALL- anchored concrete wall built at base to prevent sliding

GABIONS- mesh cages filled with rocks

DRAINS- channels or tunnels built to drain water and prevent build up of groundwater pressure

VEGETATION- plants fix soil into place and reduce water infiltration