Subsidence, landslides and other geohazards

Question 1

subsidence due to mining

Answer 1

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

Question 2

how is tilt calculated

Answer 2

change in elevation between 2 point/ distance between 2 points

Question 3

how long does it take for subsidence to complete after longwall extraction?

Answer 3

4 months

Question 4

sinkhole

Answer 4

hollows or holes in the ground caused by collapse of a surface layer, usually by dissolution of limestone

Question 5

how do faults and salt extraction effect subsidence?

Answer 5

faults extend duration of hazard and can reactivate subsidence
salt extraction leaves voids= subsidence

Question 6

what causes crown holes?

Answer 6

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.

Question 7

How do sinkholes form?

Answer 7

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.

Question 8

what can be done to help prevent the effects of subsidence? x2

Answer 8

reinforced foundations

satellites imaging or ground survey to check for cave systems

Question 9

name examples of mass movement x5

Answer 9

landslips, slides, slumps, flows and fails

Question 10

How are landslips classified? x2

Answer 10

velocity they move

what type of material is moved

Question 11

Competent rocks and mass movement

Answer 11

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

Question 12

translational slide

Answer 12

rocks moves as an entire entity down the slope- as it gains speed it breaks up forming a more fluid like landslide

Question 13

incompetent rocks and mass movement

Answer 13

More isotropic and form lower angle slopes. Fail= resulting slump occurs along a spoon-shaped surface=toe of debris

Question 14

What triggers mass movement?

Answer 14

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

Question 15

How can human actions destabilise a slope? x3

Answer 15

removing vegetation, adding weight to slope and removing the base of the slope

Question 16

talus

Answer 16

the debris accumulating at the foot of a slope due to erosion of the rock face above

Question 17

isotropic clay

Answer 17

having equal properties in all directions

Question 18

tsunamite

Answer 18

a tsunami deposit which is a sedimentary unit deposited onshore during the inundation phase

Question 19

slope modification to increase slope stability x4

Answer 19

regrading sides of cutting to make lower angle slops
ledges on slope 5m wide 10m high
vegetation
geotextiles

Question 20

drainage

Answer 20

rock and gravel drains can control the water and stop slope slipping

Question 21

name 3 types of slope supports

Answer 21

concrete retaining walls
gabion- wire mesh boxes filled with rocks
shotcrete- sprayed liquid concrete

Question 22

ground anchors x2

Answer 22

rock bolts and anchors are used to pin loose rock to sound rock
sheet metal piling