1. Mass Wasting and Hillslopes
Gravity overcomes Friction
Steep slope
Gd > F
W = mg
F0 = Gd F
Boulder
moves F
downslope
Moderate
slope
Gd= F Gd Gp Gd W Gp
Boulder
on verge
of moving W F
Gentle
slope
Gd< F Gd Gp W
Boulder is
stable

2. Sliding Threshold when gravity component = friction component, both parallel to slope
Shear Forces are parallel to 2 touching surfaces.
If the slab is about to move, then the
downhill force = resisting force pointing uphill
Downhill force = mass x gravity x sine of dip
F0 = mg sin (dip) (1) a is the same as the dip
F0 = mg sin(α)
dip mg h α

3. Your book uses mg = weight "w"
Downhill force = mass x gravity x sine of dip
F0 = w sin (dip) (1) a is the same as the dip
Shear Force = F0 = w sin(α)
dip
Aside: Bloom confuses shear force with shear stress.
Stress = Force / unit area
Stress units are, e.g. Newtons/m2
or pounds force/ inch2 aka psi
That said, we will skip the issue by staying with Forces w h α

4. Role of water for slabs Friction Force is proportional to Normal Force
It is the amount of Force needed to lift the surfaces apart
Increased water pressure between the surfaces lifts the upper slab, and it will slip at a lower dip angle.
Proportionality constant c
dip mg N h α

5. Friction Coefficient c?
Ff uphill = N x constant “c”
Notice N = mg cos a
When it slips, F0 = Ff = N x constant
Then
F0 = mg sine a = mg cos a x c
so c = sine a / cos a

6. Example Suppose the rock slips at a = 30o sine 30o = 0.5
Cosine 30o = 0.866
c = Sine 30o /cosine 30o
c = 0.5/0.866 = 0.577
dip mg N h α

7. Water's role for slabs: Before Fall

8. Water's role for slabs: After Fall
Of course, in our
area, winter freezing
causes frost wedging,
breaks loose any
remaining bonds

9. Classification of slope movements
Slides
Flows
Slumps
Falls
(note rotation)

10. Slow mass movement indicators
Example: Soil Slump

11. Soil Creep CD
Scarp
Lobe DF

12. Signs of Soil Creep Vertical features exposed in new roadcut
Vertical features (if available) curved near surface
Vertical features
exposed in new roadcut

13. Creep Typical Features
“Drunken forest”

14. Solifluction Soil saturated with water, soggy mass flows downhill
When soil moisture cannot flow deeper, trapped in soil

15. Gelifluction: Freezing lifts particles, thaw drops them further downhill

16. Gelifluction: Thaw

17. Rapid Mass Movement Flows: mixture moves downslope as a viscous fluid
Slumps: move downslope along a concave slip surface
Slides: move downslope along preexisting plane of weakness as a single, intact mass
Falls: rock drops from steep slope

18. Rapid Mass Movement

19. Flows
Mixture moves downslope as a viscous fluid
Flows with a high water content are less viscous, faster and more dangerous
Debris avalanches- rain- regolith detaches 200 kilometers per hour
Lahars
Liquefaction- Quick Sand due earthquake - increased pore water pressure - grains separate - liquefies instantaneously
Mudflow swift slurry- heavy rains
Earthflows dry masses of clayey regolith
1-2 meters per hour

20. Debris Avalanche Yungay Avalanche May 31, 1970 Ancash Earthquake
Town in Peru
Earthquake dislodged
Slab ice => landslide
25000 killed
Source: Lloyd S. Cluff

22. Lahar

23. Liquefaction - Quick Clay or Sand
Asphalt Parking Lot
Caused by Earthquakes
Sediment not compacted is like “pick-up-sticks
Seismic waves increase fluid pressure, force grains apart, structures above resting on water, they sink in.

24. Mudflow in Sarno, Italy, 1998

25. Slumgullion Earthflow
Earthflows dry masses of clayey regolith
1-2 meters per hour
San Juan Mtns, CO
Volcanics
Dams Lake Fork of the Gunnison

26. Slides Slumps: special case, weakness is curved Mudslides Rock Slides
Slides: move downslope along preexisting plane of weakness as a single, intact mass
Slumps: special case, weakness is curved
Mudslides
Rock Slides
Avalanche and Debris Slides

27. Slump
Slumping with visible Scarps in Dorset, England These are rotational

28. Little Hat Mountain Slump, CA
scarp
Toe,
no veg.

29. La Conchita Slump Typical urban landslide, after heavy rains
Preexisting slide masses
Development to the edge of existing
Lawsuits
9 houses destroyed
Property values down

30. Snow Avalanche
slump scar

31. Turtle Mountain Debris Slide

32. East limb limestones at steep angle
Locals mining coal seam
under thrust fault
April 1903

33. Falls: Rockfall Frost heave, Yosemite NP. Glacier Point climbing area.
162,000-ton granite slab.
160 mph speed.
Killed several people.

34. Angle of Repose
For loose materials, the angle of repose dictates the maximum steepness a material can be arranged before it will move downslope
Bloom claims: p 189 lower right to 190 “The angle of the talus is a function of fragment size and angularity ….”
Rockfall Talus Slope

35. An Example
These talus cones illustrate the characteristic steep slopes. Talus, due to its large grain size, has a steep angle of repose.
Talus cones from Glacier National Park in Canada.

36. Angle of Repose depends on particle size and shape?
Is this right? Should we believe this? Do an experiment.
What is your null hypothesis?

37. Slope Stability
Slope characteristics such as composition, vegetation, and water content also influence slope stability.
Haiti is plagued by slides after many trees were cut down.

38. Natural Triggers Natural triggers such as:
torrential rainstorms 1967 central Brazil
Earthquakes 1812 New Madrid, Missouri
volcanic eruptions 1980 Mount St. Helens
produce damaging mass movements

39. Human Triggers excessive irrigation clear-cutting of steep slopes
slope oversteepening or overloading
mining practices
can also cause mass movement.