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A2 Mass Movement (slope engineering (SLOPE MODIFICATION- lowering the…
A2 Mass Movement
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
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FABRICS- mesh structures made of metal draped over a slope to increase strength and prevent rockfall
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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
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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
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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)
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