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### 12. Mass Wasting

Classifications (Definitions, processes and controlling factors)

Examples (Appling knowledge of processes)

Prevention of Mass Wasting (limiting and eliminating)

Definitions

Flow, Slide, Fall

Controlling Factors

Gravity

Water

Shear strength

Examples

Mt. St. Helens

Yosemite Rock fall/slide

Gros Ventre Slide

St. Francis Dam, Calif.

Be able to distinguish a flow from a slide.

How do controlling factors increase or reduce tendency of mass wasting?

How did the controlling factors lead to these events?

How would you use your knowledge of the controlling factors to reduce risk?

Terms and Concepts to LearnMaterial

Velocity

Creep

Debris

Imperceptibly Slow

Earth Flow

Debris

Slope and Material Dependent <5 km/hr

Mudflow

Saturated Debris

Avalanche

Debris or Rock

Very Fast 100 km/hr

Rotational Slide

Debris

Slow-mod. (short)

Rock Slide

Bedrock

Fast

Debris Fall

Debris

Fast

Rockfall

Bedrock

Fast

Classification of Mass WastingFlow

Type of Movement

Slide

Fall

Debris (soil) both slides and flows

Rotational Sliding

Zone of Depletion

Surface of Rupture

Flow

Zone of Accumulation

Surface of Separation

Earth Flow andRotational SlideZones of Depletion

Crown

Main/Minor Scarps

Head Transverse Cracks

Longitudinal Fault Zone

Zone of Accumulation

Transverse Ridges and cracks

Radial cracks

Foot/Tip/Toe

Crown

Slump/Earth- FlowLeft Flank

Right Flank

Head

Foot

Kehew, Fig. 12-22

Bedrock may slide and/or fall

Weathering reduces bedrock strength

Chemical

Mechanical

Eventually gravity wins

Earthquake triggered

Rock Slide and FallThe result of

Mechanical weathering

Rock falls and slides

Crushing and abrasion (more mechanical weathering)

Rock Avalanches

Slopes of rock fragments may let go and careen downhill as a very fast flow

Talus SlopesE.g. Unavoidable Rock Fall

- The Old Man of the Mountain, Cannon Mtn. NH

Mt. St. Helens

- Landslide triggers eruption
- Reduced shear strength from earthquakes and bulging
- Increased shear force as bulge grows and slopes steepen
- Eruption causes Mudflows

Gros Vantre Slide

- Sandstone and debris on Impermeable shale
- Saturation of sandstone and lubrication of shale
- Both reduced shear strength (added to shear force)
- Shear force overcomes shear strength
- Sandstone and debris slide

Shear Force vs. Shear Strength

- Driving Forces

i.e., Shear Force

- Fs = W sin q

Fs: Shear force

W: Weight

q : Dip of slope

Shear Force vs. Shear Strength

- Resisting Forces
- Friction and Cohesion of Soil or Rock
- Ff = m (W cos q)

m: coefficient of static friction

Shear Force vs. Shear Strength

- Resisting Forces

i.e., Shear Strength

- Friction and Cohesion of Soil or Rock
- S = C + se tan f

S: Strength

C: Cohesion

se: Effective Stress

f : Angle of internal friction

Effective Stress and Strength

sT

sT

Before RainfallAfter Rainfall

- se = sT – P se = sT – (P+DP)

sT: Total Stress

P: Fluid pressure of ground water (or soil water)

se: Effective stress (stress supported by the soil skeleton)

Note: fluid pressure is negative (less than atmospheric) if unsaturated and becomes positive when saturated

P

P+DP

Stability Analysis for Rotational Slides

- S.F. = SLR / WX

S: Shear Strength

L: Langth of potential surface of rupture

R: Moment Arm

W: Weight of Block

X: Length of gravity moment arm

X

R

C.G.

S

L

Use Knowledge of Mass Wasting to Avoid Risks

- Be able to recognize geologically unstable situations

Understanding Mass Wasting

Development causes:

- Increased shear force
- Steepened slope
- Added weight
- Decreased shear strength
- Devegetation
- Reworking of fill
- Saturation of soil

Reduce Risks

Some solutions include:

- Increase shear strength
- Re-compact soils
- Re-vegetate soil slopes
- Construct retaining wall with anchors
- Prevent Saturation
- Prohibit over-irrigation
- Install surface drains
- Install subsurface drains

Reduce Risks

- Increase shear strength with iron rods and anchors
- Remove risk

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