Transcript 12. Mass Wasting

12. Mass Wasting
A. Classifications (Definitions,
processes and controlling factors)
B. Examples (Appling knowledge of
processes)
C. Prevention of Mass Wasting (limiting
and eliminating)
Terms and Concepts to Learn
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Definitions
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Flow, Slide, Fall
Controlling Factors
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Gravity
Water
Shear strength
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Examples
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Mt. St. Helens
Yosemite Rock fall/slide
Gros Ventre Slide
St. Francis Dam, Calif.
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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?
Classification of
Mass Wasting
Flow
Fall Slide
Type of Movement
Classification of Mass Wasting
Material
Velocity
Classification
Debris
Imperceptibly Slow
Creep
Debris
Slope and Material
Saturated Debris Dependent <5 km/hr
Earth Flow
Mudflow
Debris or Rock
Very Fast 100 km/hr
Avalanche
Debris
Slow-mod. (short)
Rotational Slide
Bedrock
Fast
Rock Slide
Debris
Fast
Debris Fall
Bedrock
Fast
Rockfall
Creep
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Imperceptibly slow flow
Expansion - contraction
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Heating – Cooling
Freeze – Thaw
Fig. 9.6
Earth Flow and
Rotational Slide
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Debris (soil) both
slides and flows
Rotational Sliding
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Zone of Depletion
Surface of Rupture
Flow
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Zone of
Accumulation
Surface of
Separation
Zones of Depletion
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Crown
Main/Minor Scarps
Head Transverse Cracks
Longitudinal Fault Zone
Left Flank
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Right Flank
Slump/Earth- Flow
Crown
Head
Foot
Zone of Accumulation
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Transverse Ridges and
cracks
Radial cracks
Foot/Tip/Toe
Kehew, Fig. 12-22
Rock Slide and Fall
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Bedrock may slide
and/or fall
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Weathering reduces
bedrock strength
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Chemical
Mechanical
Eventually gravity
wins
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Earthquake triggered
Talus Slopes
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The result of
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Mechanical
weathering
Rock falls and slides
Crushing and
abrasion (more
mechanical
weathering)
Rock Avalanches
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Slopes of rock
fragments may let go
and careen downhill
as a very fast flow
E.g. Unavoidable Rock Fall
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The Old
Man of the
Mountain,
Cannon
Mtn. NH
Mt. St.
Helens
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Landslide triggers
eruption
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Reduced shear
strength from
earthquakes and
bulging
Increased shear
force as bulge
grows and slopes
steepen
Eruption causes
Mudflows
Gros Vantre Slide
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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
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Driving Forces
i.e., Shear Force
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Fs = W sin q
Fs: Shear force
W: Weight
q : Dip of slope
Shear Force vs.
Shear Strength
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Resisting Forces
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Friction and Cohesion
of Soil or Rock
Ff = m (W cos q)
m: coefficient of static
friction
Shear Force vs.
Shear Strength
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Resisting Forces
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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
Safety Factor
S resisting forces
S.F.=
S Driving Forces
If S.F.>1 then failure
occurs
Effective Stress and Strength
sT
sT
P
P+DP
After Rainfall
se = sT – (P+DP)
Before Rainfall
 se = sT – P
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
Stability Analysis for
Rotational Slides
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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
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Be able to recognize
geologically unstable
situations
Understanding Mass Wasting
Development causes:
 Increased shear force
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Steepened slope
Added weight
Decreased shear
strength
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Devegetation
Reworking of fill
Saturation of soil
Reduce Risks
Some solutions include:
 Increase shear strength
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Re-compact soils
Re-vegetate soil slopes
Construct retaining wall with
anchors
Prevent Saturation
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Prohibit over-irrigation
Install surface drains
Install subsurface drains
Reduce Risks
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Increase shear
strength with iron
rods and
anchors
Remove risk