9.2 hazards resulting from mass movements

Question 1

nature and causes

Answer 1

• classification: speed, water, movement
• important factors: gravity, slope, angle, pore pressure
• safety factor: relative strength or resistance of slope compared with force trying to move it

Question 2

human impact

Answer 2

• increasing slope angle
• extra weight on slope
• removing vegetation
• exposing joints and bedding planes

Question 3

management

Answer 3

• building restraining structures
• excavating and filling steep slopes
• draining slopes
• watershed management

Question 4

factors contributing to increased shear stress

Answer 4

human
- removal of lateral support by undercutting or slope steepening
- removal of underlying support
- loading of slope
- lateral pressure
- transient stresses
natural
- erosion, wave action, faulting, previous falls and slides
- undercutting by water, subsurface solution, loss of strength by exposure of sediments
- water weight, vegetation, debris
- water in cracks, freezing, swelling, pressure release
- earthquakes, windy trees

Question 5

factors contributing to decreased shear strength

Answer 5

• weathering effects
• changes in pore-water
• changes of structure
• organic effects: burrowing of animals, decaying roots
• disintegration of granular rocks, hydration, solution…
• saturation, softening of material pressure
• creation of fissures in clays, remoulding sands and clays

Question 6

avalanches

Answer 6

• mass movements of snow and ice
• newly fallen snow may fall off older snow while in spring thawed snow moves
• often triggered by skiing
• frequently on slopes over 22 esp north facing where lack of sun inhibits stabilisation of new snow
• avg speed 40-60km/h
• snow held by interlocking crystals and cohesion by electrostatic bonding of snow crystals:strength
• change in overlying pressure, compaction by freshly fallen snow, T changes, movement of meltwater

Question 7

prediction and hazard mapping

Answer 7

• careful analysis, planning with appropriate stabilisation techniques, impacts can be reduced or eliminated
• assessment: based partly on past events, to evaluate magnitude and frequency, mapping and testing of soil and rock properties (determines susceptibility to destabilising processes): land use planners
• prediction and early warning: intense rain, monitored high risk areas
  engineering schemes:
• rock bolts and retaining walls
• drainage pipes
• slope regrading
• retaining devices
• grading of slope
• diversion walls
• gabions
• eliminating/restricting human activity on areas likely to slip : land susceptible to mild failures may be suitable for some forms like recreational parks

Question 8

tsunami

Answer 8

• South Asian Tsunami on 26 december 2004
• up to 280 000 people killed in 30 countries