Litho: Mass Movement Flashcards
Define shear strength
Define shear stress
Shear strength: resistance which slope offers against mass movement
Shear stress: the force trying to enact movement
What is the criteria for a stable slope? (Hint: safety factor: strength/stress)
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Name the 5 factors affecting mass movement
Gravity/slope angle, nature of slope, geology, water content, triggering mechanisms
How does nature of slope affect shear strength?
Sand and gravels: strength generated by friction between particles and can be reduced by pore pressure
Silt and clay: cohesive forces. Too much moisture can undermine cohesive force
Rock slopes: greatest internal strength to resist mass movement. Crystallisation create very strong chemical and physical bonds. Strength may be reduced by joints and faults.
Define mass movement
Downslope movement of weathered rock materials. Dependent on shear stress and shear strength of slope.
How does geology affect shear strength?
Bedding planes or joints in the slope can weaken the slope. Presence of daylighted beds which deep at a gentler angle than the slope of the hill are ideal zones of slippage since there is no resisting load holding them back other than friction.
How does water content contribute to mass movement?
- Saturate the soul and reduce cohesion between clay and silts. Creates pore pressure as water forces the particles apart.
- Reduces friction between soil particles, allowing them to slip over each other easily
- Increases weight of sediment, increasing shear stress
What triggering mechanisms can contribute to mass movement?
Earthquakes and heavy precipitation
What is the speed of soil creep?
Slow but widespread and highly effective type of mass movement
How does soil creep occur?
- Actions of plants and animals where growth of tree roots etc dislodges particles unto the initial disturbance
- Soil heave from heating and cooling, wetting and drying of soil or formation of ground ice. Expansion causes the surface of slope to heave at right angles to the surface resulting in net downslope migration of particles with each cycle
What are evidences of soil creep?
Leaning old fences, poles and trees on slopes
How do slides occur?
Sudden and rapid. A triggering mechanism such as an earthquake or human actions is usually needed to cause a slide. The moving mass slides down an inclined plane. These slide planes can be lubricated during wet periods to increase the likelihood of mass movement.
How are rock slides different from landslides?
Rock slides affect hard rocks while landslides affect less strong rocks and unconsolidated regolith material such as sands and clays.
How do landslides occur?
1) Due to prolonged buildup of ground water, stress on the slope increases while friction along slide planes decreases so landslides occur.
2) when weak layers support heavier ones above (eg. Layers of shale beneath thick strata)
Define flows
Movement of a mass by internal deformation under its own weight. It takes place when moisture content is high
