Mass Wasting Landslides

1
Mass Wasting(Landslides)
2
Mass Wasting Overview

• Mass wasting f(x)
• Gravity
• Material strength
• Planar weakness
• Only common Earth surface process also active on
  Moon, Mars
• Classic threshold behavior

3
Shear Stress
?

• Driving force resisting force
• Driving shear stress
• Mass times
• Gravity times
• Sine of slope angle
• t ?Vg sin(a)
• Calculated over a unitarea of 1 cm2
• t ?gh sin(a)
• Shear Stress per unit area

h
4
Shear Strength
?

• Driving force resisting force
• Resisting Strength Friction
• S c s(tan?) Coulomb Equation
• Strength cohesion effective normal stress
  times angle of internal friction
• Cohesion varies with material and condition
• Quartzite, clay, wet sand, dry sand
• Effective normal stress varies with saturation
• Dry, moist(?), saturated
• ? varies with material from 90 (quartzite) to
  33 (granular materials) to lt10 (clay).

5
Rock Mass Strength

• Regression against slope angle
• Intact rock strength
• Weathering
• Joint spacing
• Joint orientation
• Joint Widths
• Joint continuity
• Groundwater outflow

20
10
30
20
7
7
6
6
Factor of Safety

• FS Resisting/Driving Strength/Stress
• If strength exceeds stress (FSgtgt1), the slope is
  stable.
• If strength stress (FSgt1), the slope is
  metastable.
• If stress minutely exceeds strength (FS1), the
  slope is unstable and failure occurs.

7
Factors that Change FS

• Direct
• ?
• Cohesion
• Inverse
• Thickness
• Slope
• Ht. of saturation

8
Factors leading to Landslides

• Increase Driving force
• Increase a how?
• Increase h how?
• Increase g how?
• Increase ? how?
• Decrease Resisting force
• Decrease c how?
• Decrease s how?
• Decrease tan? how?

Undercutting
Cut/fill
Earthquake
Saturation
Jointing
Saturation
Weathering
9
Causes vs. Triggers

• Causes long-term factors
• Weak materials
• Joint orientations
• River incision
• Triggers short-term factors
• Rainfall/snowmelt
• Construction cut/fill
• Earthquakes
• Hazard assessment study of causes

10
Landslide Classification

• After Varnes (1958, 1978)
• Material
• Rock
• Debris
• Earth
• Mode
• Fall
• Topple
• Slide
• Slump
• Flow
• Creep

11
Numerical Analysis of Slides

• t ?gh sin(a)
• Example Slab avalanches
• Measurements?
• Slope angle
• Thickness
• Shearstrength
• Density
• Problems?
• Where/when
• Changing conditions

12
Numerical Analysis of Slumps

• Planar cross-section
• Torque
• Driving / opposing
• Driving / resisting

13
Slump-flows

• Initial failure
• Flow transport
• Lobate terminus
• Secondary failures
• Steep scarp
• Loaded toe
• Hummocky topo
• Undrained depressions
• Springs

14
La Conchita Slump

• Typical urban landslide
• Preexisting slide masses
• Development to the limit
• Failure / lawsuit / not guilty
• 9 houses and property values ?

15
La Conchita Evolution
http//www.geog.ucsb.edu/jeff/projects/la_conchit
a/photo_interp/page.html
16
Slumgullion Earthflow

• San Juan Mtns, CO
• Volcanics
• Dams Lake Fork of the Gunnison

17
Slumgullion Flow Activity

• Resurveyed constantly

18
Vaiont Rockslide
http//www.kingston.ac.uk/ku00323/landslid/vaiont
.htm

• Dam completed 1960
• Highest thin arch
• Left bank considered unstable
• 10/9/63 failure
• 30 m/sec
• 2500 lives lost
• Dam survived

http//www.land-man.net/vajont/vajont.html
19
Portuguese Bend, CA
20
Thresholds (Debris Flows)
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Yungay, Peru

• 1963 and 1970
• EQ-triggered M7.8
• Rock and ice avalanches
• Towns built on old debris
• 20,000 casualties

22
Story Hills
scarp
23
Story Hills
Closed depression
24
Story Hills
Lobate toe
25
Story Hills
DF
Drunken forest
26
Creep
27
Terracettes
28
Little Hat Mountain, CA
29
Jess Valley, CA
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Final Thoughts

• Episodic behavior hard to study
• Local extent hard to fund