1. Mass Wasting
Nancy A. Van Wagoner
Acadia University

2. Mass Wasting Defined
The down slope movement of material under the force of gravity

3. Agenda Factors controlling mass wasting
Factors that can change slope stability
Types of mass wasting
Examples
Prediction and mitigation

4. Overview mass wasting occurs throughout the world
The total global property damage from landslides in a single year equals that caused by earthquakes in 20 years.

5. Factors that control mass wasting
Steepness of the slope
Orientation of the rock layers
Strength and cohesion of materials
Pore water
Factors acting to change slope stability

6. Steepness of the slope figure attached factor of safety example
Show overhead

7. Steepness of Slope f d n W
Resisting Force = R = shear strength = internal resistance to movement =
f x n
Factor of Safety = R/d = F.S.
Most building codes require F.S.>1.5
Problem: calculate d for 1000 kg block for slope angles of 60 degrees and 30 degrees n
Angle of the slope
f = friction
n = normal force (component of W perpendicular to slope)
d = driving force (component of W parallel to slope)
W = weight of the block = mass x gravity) W

8. Orientation of rock layers (figure 10-20)
dip slope vs rocks dip perpendicular to the slope
Show overhead
DIP SLOPE = Rock layers are dipping or inclined in the same direction as the slope.
VERY UNSTABLE
Dip of rocks is perpendicular to the slope = better, more stable

9. Strength and cohesion of materials
strength = ability of material to resist deformation
cohesion = ability of particles to stick together
examples
unweathered granite vs poorly indurated sedimentary rock

10. Pore water
angle of repose = maximum slope or steepness at which loose material remains stable
The angle of repose for dry sand is about 35 degrees
Damp sand achieves slopes up to 90 degrees
Wet sand (saturated) has little strength, and almost no slope

11. Pore water (continued)
why the difference in the slopes?
water is a polar molecule, able to attract grains of sand by surface tension and hold them together if the pore water pressure is less than zero
If the pore water pressure exceeds zero, the pressure exerted by the water, floats the particles away from each other

12. Factors acting to change slope stability (Triggering Events)
Change in the abundance of pore water
Earthquakes
Slope modification and undercutting
Volcanic eruptions

13. Change in abundance of pore water
increase pore water pressure
decrease cohesion of particles
increases the weight of the slope
ways of changing the amount of pore water
rain
housing development
water lawn
build swimming pool
septic field
change in groundwater level

14. Water (continued)
Liquefaction - the transformation of material to a liquid-like mass
results from a increase in water content
may be associated with ground shaking
Expansive clays (shrink-swell soils)

15. Earthquakes and other shocks
can lead to liquefaction
earthquakes frequently generate landslides
Grand Banks under water slide (turbidity current)
Peru 1970
Grand Banks, Newfoundland:
1929
Earthquake generated a turbidity current (submarine landslide)
The turbidity current broke underwater telephone cables
Based on the succession of breaks, determined turbidity current was:
at least 150 km wide
traveled at least 470 km from source
max vel = 93 km/hr
Peru, 1970
Earthquake triggered debris avalanche that roared 3.5 km down the side of Mt. Huascaran.
speed = up to 400 km/hr
destroyed towns of Yungay and Ranrahica
Killed 20,000 people
Eyewitness account from a geophysicist: from Murck, et al., 1996
(attached)

16. Slope modification and undercutting
road construction
natural processes
streams
waves

17. Volcanic eruptions deposit unconsolidated debris rapidly
may be associated with melting of glacier and/or rain

18. Types of Mass Wasting Can occur slowly or rapidly
imperceptible (cm/yr) to rapid (400 km/hr)
3 main types (figure 10-23)
Flow
Slide
Fall

19. Flows - 3 types Unconsolidated material moves as a viscous fluid
creep (slow)
solifluction (slow)
debris flows, earthflows, mudflows (fast)

20. Creep (figure 10-24 to 10-27) about 1 cm/yr
results from alternate expansion and contraction of surface materials due to
freezing and thawing
wetting and drying
may notice bent rock layers, tree trunks curved at the base, tilted fence posts or grave stones

21. Solifluction (fig. 10-29) important in frigid zones
high latitude or high elevation when ground has a layer of permafrost
summer or spring
upper layers of permafrost may melt saturating the upper surface
water can’t percolate downward because of ice below
earth flows slowly downward on icy layers below, even on gentle slopes

22. Debris flow, Mudflow, Earth Flow, Lahar
fluid motion of water saturated debris
occur everywhere including
semi-arid environments
volcanoes
viscous, able to float cars
fast, up to 100 km/hr

23. Semi-arid flows (continued)
infrequent/high precipitation rainstorms
loose debris
little vegetation
easily saturated by rain
acquires consistency of concrete,
moves down slope

24. volcanic mudflows (Lahars)
instant deposition of ash
usually associated with rain and/or melting ice caps
example: Nevado del Ruiz

25. Landslides - two types
slump
rock slide, or rock avalanche

26. Slump
downward slipping of a mass of rock or unconsolidated material, moving as a unit, along a curved surface
see figure 10-23, note
slump block
surface of fracture
slump scarp

27. Slump example Portuguese Bend-Abalone Cove Landslide
Southern California, coastal community
area = 10’s of square miles
very fancy homes, million $ range

28. Portuguese Bend geology
shale, dipping seaward overlain by poorly consolidated Portuguese Tuff
area of natural sliding, undercut and oversteepened by waves

29. Portuguese Bend urbanization
accelerated slumping
over steepened slopes-terraces, road cuts
added weight and water to slope
swimming pools
irrigation
sewage
Result = ground slumps along plane of weakness
150 homes destroyed
Remediation = pumping excess groundwater
Figure B2.1, p.165
add also overhead
drawing

30. Rock slide, rock avalanche- blocks of bedrock break loose and move down slope
fastest and most destructive type of mass movement
common conditions
steep slopes
dip slope
common triggers
earthquake
excessive rain and/or melting snow
lubricates and adds weight to the slope

31. Rock slide, avalanche examples
Gros Ventre landslide, Wyoming (fig )
Frank Alberta (p. 268)

32. Gros Ventre conditions result slope = 15-20 degrees Spring 1925
river cut through sandstone, removing toe of the slope
Spring 1925
heavy rains and melting snow
increase weight, increase pulling force
increase pore water pressure, decrease cohesion
lubricate sandstone/slay contact
result
38 million cubic metres of debris gave way
descended a vertical distance of 600 m
rose up 100 m on the opposite side to come to rest in the river bed, damming the river
show figure 10-31

33. Frank Alberta
show diagram