What are the hazards associated with mass movement and slope failure? Flashcards
Causes of mass movements and slope failure
- Reduction in shear strength
- Increase in shear strength
- Slope system and angle no longer in equilibrium so needs to change angle (dynamic equilibrium)
What are the factors holding slopes in place?
- Reduction in shear strength
- Increase in shear strength
- Slope system and angle no longer in equilibrium so needs to change angle (dynamic equilibrium)
What are ways of classifying mass movements?
- By speed or rate of movement
- By type of movement
- By type of material
- By water content
What are slides?
- Slides commonly occur where there is a combination of weak rocks, steep slopes and active undercutting
- Slides are often caused by a change in the water content of a slope or by very cold conditions
- As the mass moves along the slip plane it tends to retain its shape and structure until it impacts at the bottom of a slope
- Slides range from small-scale slides that might be seen close to roads in hilly areas, to large-scale movements
What are slumps?
- Slumps occur on weak rocks, notably clay, and have a rotational movement along a curved slip plane
- Clay absorbs water, becomes saturated and thereby unstable
- Frequently slumps occur when the base of a cliff has been undercut and weakened by erosion, as a result reducing its strength
- The clay then flows along a slip plane
- Human activity can also intensify the condition by causing increased pressure on the rocks, for example by water entering the soil from leasing pipes and drains and, on the coast, by the building of groynes starving the beach of sediment and making it easier for waves to erode the cliff
What are flows?
- By contrast, flows are more continuous, less jerky and are more likely to contort the mass into a new form
- Fine grained materials, such as deeply weathered clays, becomes saturated with water, lose their cohesion and flow downhill as a very fluid mass
- The flow is fastest at the surface and slows down at depth
What is soil creep?
Soil creep is a very slow movement, occurring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods. When wet, soil particles increase in size and weight, and expand at right angles. When the soil dries out, it contracts vertically. As a result, the soil slowly moves downslope.
What is a mudflow?
Mudflow occurs on steep slopes over 10°. It’s a rapid sudden movement, which occurs after periods of heavy rain. When there is not enough vegetation to hold the soil in place, saturated soil flows over impermeable sub soil, causing great devastation and endangering lives.
What is rockfall?
Rockfall is the rapid, free-fall of rock from a steep cliff face. Rock fragments fall from the face of the cliff because of the action of gravity. This is made worse by freeze-thaw action loosening the rock. Bare, well-jointed rock is very vulnerable to rockfall - water enters the joint, freezes and expands, cracking the rock. A scree slope of fallen rock is formed at the bottom of the cliff.
How can climate increase mass movement?
Wet, lots of weathering, extremes of temperature - reduction in shear strength
How can slope angle increase mass movement?
Steepness of slope - reduction in shear strength
How can drainage increase mass movement?
Wet areas are lubricated - reduction in shear strength
How can rock type increase mass movement?
Geology e.g clay, structure, beds, porosity and tilt of rocks - reduction in shear strength
How can vegetation increase mass movement?
Type and percentage of cover - reduction in shear strength
How can animals increase mass movement?
Burrowing animals, walking on slopes - reduction in shear strength
