Week 9

Question 1

What is a landslide

Answer 1

• a slide of land
• the movement of a mass of rock, debris, or earth and/or mud down a slope
• rocks can slide or roll downhill: rockfall
• may occur when large volume of material moves downslope quickly: rotational/translational landslides
• may be wet: debris flows
• may occur gradually: creep

Question 2

Rotational slides

Answer 2

• move downward and outward above curved slip surface, with movement rotational about an axis parallel to slope
• head moves downward and rotates backward
• toe moves upward on top of landscape
• move short distance

Question 3

Translational slides

Answer 3

• move on planar slip surface such as fault, joint, clay rich layer
• move as long as on downward-inclined surface, and driving mass exists
• Underlying material fails so overlying material slides
• different behaviors:
  1. remain coherent as block
  2. deform and disintegrate to form debris slide

Question 4

Forces on a slope

Answer 4

1. driving force: gravity pulls down on material
   - external forces contribute, e.g earthquakes, eruptions
2. Resisting force: holds material in place
   - strength of material and amount of friction
   - Factors contributing to slope failure: slope steepness, material weight, moisture content

Question 5

Slope and load

Answer 5

• relationship between slope angle and load is a key factor in slope failure
• steeper slope - greater driving force - greater likelihood of slope failure
• angle of repose: steepest angle at which any loose material is stable
• depends on angularity and size of grains and moisture content

Question 6

Driving vs Resisting force

Answer 6

• frictional resistance depends on:
  1. slope angle
  2. Load - a critical mass
  3. cohesion

Question 7

Source of weakness for planar internal surfaces

Answer 7

• layers in sedimentary rock
• fractures in any kind of rock
• contact between rocks of different strength
• faults, or slip surfaces of old landslides

Question 8

Cohesion adding to frictional resistance

Answer 8

• important force for holding soil grains together
• due to surface tension of water ( or other glue material) between loose grains
• cohesion is overcome when driving force is large enough

Question 9

role of water

Answer 9

• small amounts of moisture can stick together, increasing angle of repose
• effective frictional strength of rocks decreased by water
• decrease cohesion

Question 10

Slope changes due to external factors

Answer 10

1. stream or wave action
2. loading upper slope by construction
3. removing vegetation

Question 11

Impact of rockfall

Answer 11

• very fast: 1 m/s to 100 m/s
• short runout
• patchy impact

Question 12

Impact of debris flow

Answer 12

• fast: 1 cm/s to 10 m/s
• long runout
• impact through burial, boulders and dynamic forces
• level of impact related to sediment concentration, debris content, velocity and depth

Question 13

Impact of Rot./ tran landslide

Answer 13

• fast: 1 cm/s to 10 m/s
• long runout
• complete impact through burial, boulders, and dynamic forces
• can transform into debris avalanche

Question 14

Impact of Creep

Answer 14

• slow: 0.3mm/yr to 3.1cm/yr
• short runout
• damage to brittle structures through ground movement

Question 15

Mitigation

Answer 15

• a lot of monitoring comes from the mining industry
• remote sensing
• adding materials to base of slope to reinforce weak layers (heavy boulders)
• removing materials from top of slope to reduce weight
• building suspension roads, instead of on the slope
• planting vegetation