Subsidence, landslides and other geohazards Flashcards
subsidence due to mining
Subsidence due to longwall mining of coal. Many mines not logged + voids have been left underground. Rocks above mined seems often fractured= subsidence that spreads out
how is tilt calculated
change in elevation between 2 point/ distance between 2 points
how long does it take for subsidence to complete after longwall extraction?
4 months
sinkhole
hollows or holes in the ground caused by collapse of a surface layer, usually by dissolution of limestone
how do faults and salt extraction effect subsidence?
faults extend duration of hazard and can reactivate subsidence
salt extraction leaves voids= subsidence
what causes crown holes?
Human activity e.g. gypsum mining. Mining voids enlarged by heavy rain percolating down into cavity causing it to fill= sides of cavity go into solution and increase the size until roof collapses.
How do sinkholes form?
Natural dissolution of rock. Mild acidity of rainwater attacks chalk+ limestone. As rock dissolves surface soil seeps down forming a depression. Excess water causes cave to collapse.
what can be done to help prevent the effects of subsidence? x2
reinforced foundations
satellites imaging or ground survey to check for cave systems
name examples of mass movement x5
landslips, slides, slumps, flows and fails
How are landslips classified? x2
velocity they move
what type of material is moved
Competent rocks and mass movement
Form steeper slopes. Areas of weakness: joints, faults, bedding planes. Weak point fails= rock fall= talus slope at base of cliff. Transitional slide also occur
translational slide
rocks moves as an entire entity down the slope- as it gains speed it breaks up forming a more fluid like landslide
incompetent rocks and mass movement
More isotropic and form lower angle slopes. Fail= resulting slump occurs along a spoon-shaped surface=toe of debris
What triggers mass movement?
Mass of rock on slope help by frictional forces. If mass or slope angle increased= increase risk of movement. If friction decreased by adding water you can trigger movement. Heavy rain, earthquakes, lowering water table
How can human actions destabilise a slope? x3
removing vegetation, adding weight to slope and removing the base of the slope
talus
the debris accumulating at the foot of a slope due to erosion of the rock face above
isotropic clay
having equal properties in all directions
tsunamite
a tsunami deposit which is a sedimentary unit deposited onshore during the inundation phase
slope modification to increase slope stability x4
regrading sides of cutting to make lower angle slops
ledges on slope 5m wide 10m high
vegetation
geotextiles
drainage
rock and gravel drains can control the water and stop slope slipping
name 3 types of slope supports
concrete retaining walls
gabion- wire mesh boxes filled with rocks
shotcrete- sprayed liquid concrete
ground anchors x2
rock bolts and anchors are used to pin loose rock to sound rock
sheet metal piling
