Mass Movement

Question 1

Define mass wasting. When does it occur, and what is the energy that drives it?

Answer 1

Downslope movement of any rock/sediment due to gravity.

Occurs when slopes fail.

Gravity.

Question 2

List the two driving forces for mass wasting and the two resisting forces.

Answer 2

Driving forces: Gravity, weight of material and water.

Resisting forces: Oppose downward motion, shear strength of slope material.

Question 3

Slopes fail when what occurs? What two principles determine slope stability?

Answer 3

Slope too steep and unstable for materials and conditions.

Slope angle; strength of material.

Question 4

Material strength is controlled by what three factors?

Answer 4

Type of material.

Consolidation of material.

Presence of planes of weakness.

Question 5

Material strength is highest in _____.

Answer 5

Bedrock.

Question 6

Planes of weakness are more hazardous in what circumstances?

Answer 6

When inclined >15°or parallel to slope.

Question 7

Mass will slide if resisting forces < driving forces. What is another way of wording this?

Answer 7

Frictional resistance + Cohesiveness < Driving force.

Question 8

When the _____ acting within a block exceeds the strength of the material, slope failure will occur.

Answer 8

Shear stress.

Question 9

Road cuts are problematic for mass wasting in what two ways?

Answer 9

Steeping slope; adding weight.

Question 10

Refers to the height of a hill above the land below.

Answer 10

Topographic relief.

Question 11

What is the angle of repose?

Answer 11

Maximum angle a slope of unconsolidated material can remain stationary without failure.

Question 12

Characteristic weather typical of a region over decades.

Answer 12

Climate.

Question 13

What are the resisting and driving forces of water content?

Answer 13

Resisting: cohesiveness of water.

Driving: addition of mass, reduces effect of normal stress by raising pore water pressure.

Question 14

Vegetation provides three resisting forces. List them, and provide one driving force.

Answer 14

Protective cover, reduces impact of rain.

Roots strengthen and add cohesion to slope materials.

Remove water from substrate, reducing effect of pore water pressure.

Driving force: adds weight.

Question 15

Driving and resisting forces are affected by what two things?

Answer 15

Seasons (changes in water content, vegetation).

Weathering of rocks (decrease in strength of rock).

Question 16

List five triggers for mass wasting.

Answer 16

Increase in water content (saturation).

Decrease in water content (loss of cohesiveness).

Removal of vegetation.

Steepening of slopes.

Earthquakes.

Question 17

Regarding classification, what three elements are involved in movement?

Answer 17

Falling: free fall of Earth material from cliff.

Sliding: downslope movement of coherent block of Earth material.

Flow: downslope movement of unconsolidated material, particles move and mix within mass (fast or slow).

Question 18

Define creep. What is it driven by?

Answer 18

Slow downslope movement (flow) of regolith or soil.

Freeze-thaw or wet-dry cycles that cause expansion and contraction.

Question 19

Define solifluction. What is it characteristic of?

Answer 19

Movement of summer thawed soil above the permafrost.

Tundra region.

Question 20

What is earthflow?

Answer 20

Slow flow of water-saturated fine-grained material.

Question 21

What is mudflow?

Answer 21

Rapid flow of water, soil or regolith.

Question 22

What is debris flow?

Answer 22

Mudflows with coarser materials, picks up boulders and rocks.

Question 23

What are lahars?

Answer 23

Mudflow, mixture of volcanic ash from currently erupting or previously erupted pyroclastic cloud, water from snow and ice that melts in volcano’s heat or from heavy rain.

Question 24

What are quick clays?

Answer 24

Clay with little or no strength if disturbed, liquefies easily, even at low slope angles.

Question 25

List six factors influencing quick clay behaviour.

Answer 25

High water content (up to 80% water).

Weakest if clay flakes have orientation parallel to direction of potential shear failure.

Weathering removes cement.

Removal of support downslope.

Vibrations.

Special care with marine clays where cations in pore water hold clay grains together

Question 26

One consequence of slow slides are slumps. What are they?

Answer 26

Semi-coherent rock or soil blocks move downslope as a unit, slump block moves down a failure surface.

Question 27

What are fast slides?

Answer 27

Bedrock or regolith detaches from slope, sliding rapidly downhill on failure surface.

Question 28

What are talus?

Answer 28

Blocks that fan out at base of cliff.

Question 29

What are eight triggers for mass falling?

Answer 29

Earthquakes.

Increase in structures.

Increase in soil moisture.

Devegetation.

Undercutting of slopes.

Frost heave.

Chemical dissolution.

Bioturbation

Question 30

What are five mitigations for mass wasting?

Answer 30

Afforestation.

Terracing.

Slope stabilization (retaining walls, bolts, cages).

Land use regulations.

Drainage / cementing rocks to prevent infiltration.