4.1 Landslides/Mass Movements

Question 1

What triggers landslides

Answer 1

Earthquakes
Water or heavy rainfall
Volcanoes -> pyroclastic flow 
Increasing slope angle
Loss of vegetation

Question 2

How does increasing the slope cause a landslide

Answer 2

The steeper the angle the more unstable the slope is
Different sediments have different critical angles at which they start to fail
Eg sand is approx 30C - so all sand dunes have same steepest slope angle

Question 3

What is the importance of vegetation

Answer 3

Vegetation makes slope more stable because:
Roots bind loose unconsolidated material together
Roots absorb water and lower fluid pressure within slope
Add load to the slope so increase friction

Question 4

What is the importance of water/pore pressure

Answer 4

Water alone does not make slope less stable

Water must be high pressure to push grains apart and reduce friction

Question 5

What is the importance of load

Answer 5

A greater load on a slope can be more stable because it would take higher water pressure to push grains apart and reduce friction

Question 6

What is lithology

Answer 6

Rocks dipping in the same direction as the slope (day lighting)
Shale is weak when it is wet - porous but impermeable causing water pressure to build between 2 beds and increase risk of landslides
Sandstone is porous and permeable- again causes water pressure to build between 2 beds

Question 7

How are landslides managed?

Answer 7

1. Retaining walls - reinforces toe of slope
2. Rock step - catches rocks from higher slopes and reduces steepness of slope
3. Rock bolts- hold bedding planes and fractures together for extra strength

Question 8

How are landslides monitored?

Answer 8

1. Piezometers and water dip meters measure water pressure within rock
2. A)Electro optic distance photogrammetry - used with earthquakes and volcanoes - EDM (Electronic Distance Measurement)
   B) Tiltmeters and rock pressure cells behind walls - used with e/qs and vs
3. Drill hole inclinometer and extensometers are bore hole distortion meters - senses change in borehole shape due to movement
4. Precise levelling and surveying - records any change in the land
5. Surface extensometers Tell-Tales -> creep meter to monitor small scale ground movement
6. Rock noise monitoring - micro seismic monitoring to detect any small fractures in the rock

Question 9

What are gabbions?

Answer 9

Wire baskets filled with rocks to support unstable slopes

They are cheap and permeable