1. Landslides

2. Learning Objectives
Understand basic slope processes and the causes of slope failure
Understand the role of driving and resisting forces on slopes and how these are related toslope stability
Understand how slope angle and topography, vegetation, water, and time affect both slope processes and the incidence of landslides
Understand how human use of the land has resulted in landslides
Know methods of identification, prevention, warning, and correction of landslides
Understand processes related to land subsidence

3. Mass Wasting
Definition: mass wasting refers to a downslope movement of rock or soil as a more or less coherent mass.
Comprehensive term – all inclusive term for any downslope movement of earth materials

4. Slopes Weak and Soft Rock Form slope with 3 segements
Convex upper part
Straight central segment
Concave lower part
Hard Rock
Form free face with talus slope at base

5. Important types of mass wasting
Slide – downslope movement of coherent block of earth material
Slump – is sliding along a curved slip plane producing slump blocks
Fall – rocks fall from vertical face
Flow – Downslope movement of unconsolidated material in which particles move about and mix within the mass
Subsidence is the sinking of of a mass of earth material below the level of surrounding material

6. Landslides are commonly complex combinations of slding and flowage
Upper slump block
Lower flow

7. Forces on Slopes
The stability of a slope expresses the relationship between resisting forces and driving forces

8. Driving forces – forces which move earth materials downslope
Downslope component of weight of material including vegetation, fill material, or buildings
Resisting forces – forces which oppose movement
Resisting forces include strength of material

9. Potential Slip Planes
Geologic surfaces of weakness in the slope material – bedding, foliation, fractures

10. Safety Factor The ratio of resisting forces to the driving forces
RF/DF
SF > 1 Slope is stable
SF < 1 Slope is unstable

11. Factors Affecting Slope Stability
Type of earth material
Slope Angle and Topography
Climate
Vegetation
Water
Time

12. Rotational Slides
Sliding occurs along a curved slip plane

13. Translational Slides
Sliding occurs on a planar surface or on a slip plane

14. Slump (a type of slide) Indicators: Scarp
“Hummocky” terrain on and below (earthflow)

15. Slump
scarp

18. Debris Flow
Debris flows are the downslope flow of relatively coarse material
> 50% of particles in a debris flow are coarser than sand
Movement may be very slow or very fast, depending on topographic conditions
Mudflows, debris avalanches, and debris flows
Small to moderate magnitude events, occasional large magnitude events

20. Debris Flow

21. Rockslide Rock moves because there’s nothing holding it back!
Generally requires a pre-existing low-friction surface...

22. Rockslide
like a clay layer, once it’s wet...

23. “Earthquake Lake”, MT
28 deaths in 1959, triggered by earthquake

24. Role of Earth Materials
Slopes formed by weak rocks such as shale or have thick soil deposits typically fail by rotational slides
Slopes formed by hard rocks typically fail by translational slides
Soil slips occur above bedrock and fail by translational slides

25. Role of Slope and Topography
Hillslope angle is a measure of the steepness of a slope = slope gradient
Steeper slope = increased driving forces
Steep slopes associated with rockfalls
Subarid to arid environments

26. Role of Vegetation
In subhumid to humid environments, vegetation is thick and abundant
Landslide activity includes deep complex landslides, earthflows, and soil creep.
Vegetation influences slope stability by:
Providing a cover that cushions the impact of rain falling on slopes and retards erosion on surface
Vegetation has root systems that tend to provide an apparent cohesion which increases resistance to landsliding
Vegetation adds weight to the slope increasing the driving forces

27. Role of Water Water can affect slope stability by:
Shallow soil slips can develop during rainstorms when slopes become saturated
Slumps or translational slides can develop months or years after slope is saturated
Water can erode the base or toe of a slope decreasing slope stability

28. Role of Climate
Climate influences the amount and timing of water in the form of water or snow
Influences type and amount of vegetation

29. Role of Time
Physical and chemical weathering can weaken slope materials decreasing resisting forces

30. Earthflow basically a very viscous (thick) debris flow slow-moving
faster in wetter weather

31. Earthflow

32. Creep
very slow
result of freezing and thawing

33. Creep

34. sandstone
shale

35. Creep
from D. Schwert, NDSU

36. Triggers for rapid Mass Wasting
Rain
Oversteepening
cutting at foot of slope
piling on head of slope
Deforesting / Devegetating
Earthquakes