Geology Exam 3 Lecture 19

Front 1

Mass Movement

Back 1

Caused by gravity

down slope movement of rock, regolith (includes sediment, soil, and debris) and ice/snow due to gravity

Front 2

What affects slope stability:

Back 2

• Weathering weakens rocks, lowering slope stability
• Rocks held together by chemical bonds are stronger than fractured rock surfaces held together by friction
• Failure surface
- wet clay
- wet unconsolidated sand
- surface parallel joints
- weak bedding planes (shales)
- foliation planes
• Removing material from the base or adding material to the top can cause instability
• Additions could be:
- Water
- Fill material
- Infrastructure (roads and buildings)
• Removal could be due to:
- Erosion
- Construction (road cuts, quarrying)

Front 3

What could trigger a mass wasting event:

Back 3

• Precipitation = adding water = weight to the top of the slope
• Precipitation, volcanic eruptions, earthquakes, changes in land use, overloading slope, undercutting slope = all put weigh/addingt on top of slope or taking away from the base of a slope which = change in the slope/triggers a slope failure
• Vibrations break bonds/ decrease friction → decreasing the resistance force
• Liquefaction – quick clay - when shaken and acts like liquid, the whole thing will just slide because the layers of clay are just pretending to be a solid

Front 4

Slope strength- water:

Back 4

Slope strength- water:
• A little water increases cohesion between grains
• A lot of water decreases cohesion

Front 5

Slope Strength – Orientation of bedding:

Back 5

• If layers are on top of each other and tilted downwards and there is a weak layer within the layers = more likely to fail
• If layers are on top of each other but not tilted and there is a weak layer within the layers = chance it will fail but less likely than when tilted

Front 6

Slope strength- Vegetation:

Back 6

• Plants add strength to the slope
- roots
- uptake water

Front 7

What distinguishes the 5 different types of slope failure discussed in class

Back 7

• Creep – to – slump – to – flow – to – slide- to – fall = slowest to fastest

CSFSF
Courtney Scott Fucks Sears Face

Front 8

Creep

Back 8

• Gradual downslope movement of regolith due to seasonal freezing and melting (slow ~ 1 cm/yr)
• Water is freezing, ground expands, water melts and then goes back into earth due to gravity – moving down the slope (slowly) each time it freezes then thaws= small movement downwards

Front 9

Slump

Back 9

• Mass of regolith detaches and slips semi-coherently downslope
• Really slow and can happen over a period of days or weeks
• Plane of weakness (jointing, weathering) and everything on top of it slides down mountains (trees can still grow but tilted)

Front 10

Debris Flows and Mudflows (flow)

Back 10

• Water mixes with regolith to create a slurry
• travel up to ~ 35 mph
- just mud → mudflow
- mixed with rock → debris flow
- volcanic ash → lahar
• Some sort of sediment/mud/debris that is mixed with water
• Faster than slumps – like a fast moving river
• A flow includes some type of water
• Can do a lot of damage

Front 11

Landslides (slide)

Back 11

• Detachment of bedrock or regolith on a failure surface roughly parallel to the slope surface
- fast (~300 km/hr)
• Large rain storm or source of water = unstable
• Anything else you can think of where rocks slide down a mountain
• Can go really fast because they can travel on a layer of air

Front 12

Rock and Debris Fall (fall)

Back 12

• Rocks are falling vertically – straight down from a cliff
- very fast (300 km/hr)
• Generates huge blasts of air because as rocks are falling air they push the air away from under them and push dust away so there is usually a lot of dust
• Talus- Fallen rocks at base of the cliff

Front 13

What factors contributed to the landslides at Portuguese Bend, CA:

Back 13

Homeowners in the exclusive Portuguese Bend neighborhood near Long Beach, CA have filed a class action suit in state court against the County of Los Angeles. This suit seeks compensation for damages to 160 homes affected by a landslide encompassing an area of roughly 270 acres. The homeowners allege that this landslide was caused by road construction along Crenshaw Boulevard, a county highway that traverses the northern portion of the slide area. The lawsuit also alleges fraud and negligence on the part of the developers for participating with the county in road construction in a geologically unstable area.

Front 14

Angle of repose

Back 14

the angle of the slope when granular debris tends to pile up and create the steepest slope it can without collapsing

Fine sand - 35 degees

Front 15

Avalanche

Back 15

turbulent clouds of debris mixed with air that rush down steep hill slopes at high velocity

Front 16

Failure surface

Back 16

the surface on which the moving mass (a slump) slips

Front 17

Head scarp

Back 17

the distinct, curving step at the upslope edge of a slump, where the regolith detached

Front 18

Turbidity current

Back 18

Sediments avalanche underwater, as the turbidity current slows, sediment settles out in sequence from coarse to fine, creating graded beds

Front 19

Undercutting

Back 19

excavation results in the formation of an overhang