Lecture 10+11: Mass Movements

Question 1

driving force, resisting force and mass movement

Answer 1

• when driving force > resisting force, mass movements occur
• the driving force is the component of gravity parallel to the slope
• resisting force is proportional to inertia and friction and counteracts the driving force

Question 2

conditions decreasing stability (external)

Answer 2

• adding mass high on a slope (external role of water: rain erodes material)
• steepening of slope (less steep = more stable)
• removing support at the base of the slope (less support = less stable)
• removal of lateral support decreases stability (example of a rotational slide; more unstable)

Question 3

decreasing stability through presence of weak material

Answer 3

• lubricants like clay
• material that deforms practically under stress and loses strength
• crumbly poor consolidated material

Question 4

friction decreasing stability

Answer 4

• controlling factors: surface roughness of the surfaces; presence of lubricants
• friction coefficient

Question 5

fractures in rock decreasing stability

Answer 5

• rock fractures naturally in parallel planes of weakness
• more stable fractures = perpendicular to the slope
• less stable fractures = parallel to the slope
• regional scale; local scale; hand sample scale; microscopic scale

Question 6

internal roles of water on internal stability

Answer 6

• adding weight to porous earth materials
• dissolving cement binding minerals together
• liquefaction: low pore pressure - strong binding force; high pore pressure - weak binding force
• congelifraction: disintegration of a rock due to several cycles of water freezing and melting

Question 7

geochemistry of the crust

Answer 7

• structures of silicon and oxygen
• basic building block is the silicon-oxygen tetrahedron molecule
• four oxygen ions surrounding a much smaller silicon ion

Question 8

Silicates and clay

Answer 8

• silicates: group of minerals containing Si and O combine with other elements
• Clay: general term which describe a variety of complex sheet silicates
• structures is a stack of think sheets
• swelling clays: group of clays which tend to swell when exposed to water, results in loss of strength

Question 9

sensitive clays

Answer 9

• clays that can change internal structure almost instantaneously
• stable conditions can become unstable without warning

Question 10

flocculated structure

Answer 10

• stiff and strong
• salt bind the clay sheets and slit together

Question 11

dispersed structure

Answer 11

• soft and weak behaves almost like a fluid

Question 12

three types of rocks and their structure

Answer 12

• igneous rocks form by cooling and solidification from a molten liquid called magma
• sedimentary rocks form by the erosion and compaction of rock fragments or by precipitation
• metamorphic rocks form by the alteration of existing rocks by head and or pressure

Question 13

triggers of mass movements

Answer 13

• natural causes: earthquakes; volcanoes; erosion; water
• human causes: construction; tunneling

Question 14

falls

Answer 14

• rapid free fall mass movement
• fragment size form grain to large blocks

Question 15

slides (planar surface and curved surfaces)

Answer 15

• mass movement involving motion along a failure surface
• material remain as a block
• Planar surface = translational slide
• curved surface = rotational slide

Question 16

translational slides

Answer 16

• slide in which earth material move parallel to planer failure surface
• if material is unconsolidated: debris or earth slide
• if material is bedrock: rock slide

Question 17

lateral spreads

Answer 17

• special case of translational slide in which movement of earth material results from liquefaction of subjacent material

Question 18

flow

Answer 18

• mass movement involving continuous internal deformation of the moving material
• main difference between slides and flows: little deformation within the moving material
• flow: material thoroughly deformed during mass movement
• slow - creep
• fast - rock, debris or earth flow

Question 19

solifluction

Answer 19

• type of flow observed in northern regions, linked to the melting of the top layer of permafrost
• water cannot infiltrate through permanently frozen, impermeable layer below
• soil becomes saturated and weak and flows down gentle slopes

Question 20

subsidence

Answer 20

• slow subsidence: gradual sinking of the land surface
• rapid subsidence: surface collapse of underground cavities
• man made: abandoned tunnels
• natural: sinkholes

Question 21

sinkholes

Answer 21

• occur typically in sedimentary rocks that can dissolve in water
• like limestone (common in florida)
• urban sinkholes: broken water mains can cause sinkholes in ottawa

Question 22

mitigation

Answer 22

• decreasing slope angle
• benching of the slope
• rock scaling
• reinforce the hazard
• support the hazard
• contain the hazard
• protect against hazard