Topics for todayMass Wasting (Ch. 13 418-425)

1. Controlling factors– Soil strength – Hill slope gradient

2 Physical model

3 Natural cases– Rain storms– Timber harvesting– Roads

3. Coefficient of friction• Water in soils

4. Mechanisms of movement

5. Mass wasting types

6. Large landslides

Mass wasting

Mass wasting is the down slope movement of rock, regolith, and soil, under the influence of gravity.

Rock fall

Rock fall

Rock fall

Landslide

Landslide

Creep

Debris flow

Coefficient of friction

Angle of repose = angle of internal friction in dry, loose material

Cohesion

Water reduces cohesion, removes frictional contact between grains

Landslide

Mechanisms of movement

• Pure flow – Earth flows– Mudflows

Pure slide/fall – Rock falls– Avalanches– landslides

• Creep– Gradual movement - Freezing cycles

Dry mass wasting from Slow to fastCreep------->Rock fall

Creep

Soil Creep

Creep

Rock fall

Rockfall

Rockfall

Rock fall

Talus slope

Slow-medium velocity mass wasting from dry to wet

Earthflow

• 6 m/yr• Highway 149 • The earthflow

began 700 years ago.

• About 350 years ago, a second earthflow

Slumgullion Earthflow, CO

Solifluction

• Fasts mass wasting processes from dry to wet

Rockfall

Debris avalanche

Mud flow

Blackhawk rockslide, Ca

• It is a prehistoric landslide ~20,000 years old

• Source was 4000 ft height

• 5 miles long X 2 miles wide X 50 feet thick

• Velocity = 270 miles/hour

• Traveled time 8 sec

Huascaran Debris avalanche, Peru (1970)

• Huascaran (6768m)

• Trigger by an earthquake M=8

• Buried the city of Yungay, 18,000 people died

• Traveled 9 miles• 175-200 mph

Explanations

• Liquefaction-wet • Air cushioning • Acoustic fluidization (high

vibration)

Liquefaction

Lost of friction between grain

Air cushioning

• Air layer underneath the slide reduces friction and can travel farther.

Acoustic fluidization (high vibration)