1. Slope Processes

3. Mass Wasting
Landslide and other ground failures posting substantial damage and loss of life
In the United States, average 25 to 50 deaths and up to about 100 to 150 if collapses of trenches and other excavations are included; damage more than $3.5 billion
Landslide and subsidence: Naturally occurred and affected by human activities

4. Mass Wasting Mass wasting
Comprehensive term for any type of downslope movement
For convenience, definitions of landslide here includes all forms of mass-wasting movements
Subsidence is another type of mass wasting except earth materials move downward in a vertical manner as opposed to along a slope

5. Slope Processes Slopes are the most common landform on Earth
Although they appear stable, they are dynamic, evolving systems
Slopes are not uniform in shape but composed of segments that are straight or curved
Slopes that are straight typically form vertical cliffs
Gentler slopes typically have three segments
Upper convex slope
Lower concave slope
Straight slope (separating the two above)

8. Slope Processes
Vertical slopes are more common with hard rock in an arid environment with little vegetation
Convex and concave slopes are more common on softer rock with a humid climate and thick soils and vegetation
Local conditions largely determine what type of slope is present
Materials on slopes are constantly moving
Imperceptibly slow
Incredibly fast
Slope processes help explain why valleys are typically much larger than the streams they contain

9. Types of Mass Wasting
Earth materials may move in the course of several situations
Rock fall
Creep
Earth flow
Mud flow
Debris flow
Slump

10. Rock Fall
Free fall of earth materials from a vertical cliff face

11. Creep
Very slow flowage of rock or soil

12. Earthflow Rapid flowage When soil partially liquefies and runs out
Typically forms a bowl-shape depression at the source area

13. Earth Flow

14. Mud flow
Rapid flowage
A mixture of rock, soil, and organic matter that mixes with air and water
More than 50% fine material
A debris flow is similar except it has less than 50% fine material

15. Debris Avalanche A rapid to very rapid debris flow
Can cause catastrophic damage and loss of life

16. Debris Avalanche

17. Lateral Spread Occur on nearly flat surfaces
Involves liquefaction of fine particles by earthquake vibrations
Start suddenly then become larger in a slow, progressive manner

18. Subsidence May occur on slopes or flat ground
Involves the sinking of earth materials below the level of the surrounding surface

19. Slump
Almost vertical movement along a surface which translates into a lobe-shaped flow toward the bottom

20. Slump

21. Types of Slope Processes
Important variables that are used to classify different slope processes include
Type of movement
Slope material
Amount of water present
Rate of movement

22. Types of Mass Wasting

23. Forces on Slopes
The stability of a slope is expressed as the relationship between driving forces and resisting forces
Most common driving force is the weight of material
Including anything on the slope (vegetation, buildings, etc.)
The most common resisting force is the strength of the earth material

24. Slope Stability
Slope stability is computed by using a factor of safety (FS)
Ratio of resisting forces to driving forces
Driving and resisting forces are not permanent and can change over time
These forces are determined by the interrelationships of the following
Type of earth material
Slope angle
Climate
Vegetation
Water
Time

25. Earth Materials
The material of the slope affects the type and frequency of movement
Rotational slides (slumps) follow a curved surface
Translational slides follow a planar surface
These planar surfaces can potentially include
Fractures in rock layers
Bedding planes
Weak clay layers
Foliation planes in metamorphic rock

26. Earth Materials
The type of material composing a slope may influence the type of slope processes that occurs
Weak rock (shale) will likely see creep and possibly earthflows
Resistant rock (sandstone, granite)is typically much more sturdy and secure

27. Slope Angle
Slope angle greatly affects the relative magnitude of the driving forces on slopes
Driving forces increase on steeper slopes
Steep slopes are associated with rockfalls and debris avalanches
Gentle slopes are associated with creep

28. Climate
Can be defined as the characteristic weather at a region over an extended time frame
Climate influences the amount and timing of water that may infiltrate a slope
Also influences the amount of vegetation that may be present

29. Vegetation Role of vegetation is complex
Vegetation is significant for many reasons
Provides cover that cushions the impact of falling rain
Facilitates the infiltration of water into the subsurface
Slows erosion of soil
Root systems provide cohesion for earth materials which increases resistance to movement (similar to iron bars in concrete)
Adds weight to a surface

31. 1969 debris flow Two lives were lost in this event

32. Water
Water is almost always involved in slope processes, either directly or indirectly
Water can affect slope stability in three ways
Landslides can develop when soil become saturated during rainstorms
Landslides and slumps can occur months or years after water has infiltrated into a soil
Water can erode the base of a slope, increasing the likelihood of a slope failure
This includes rivers eroding a stream bank or waves eroding a coastal cliff

33. Slope failure due to river erosion

37. Time The forces on slopes often change over time
Driving and resisting forces may change seasonally
Chemical weathering, which reduces the strength of rock, takes place over time

38. Humans and Landslides
The effect of humans on slope failures ranges from insignificant to very significant
People need to learn where, when, and why slope failures occur to minimize building in hazardous areas
Human activity has been shown to increase the likelihood of slope failures, including
Timber harvesting
Urbanization

39. Minimizing Hazards Minimizing the effects of landslides requires
Identifying areas where landslides are likely to occur
Designing slopes or engineering structures to prevent landslides
Warning people of impending slides
Controlling slides after they start moving
The most preferential and least expensive option is to avoid development on sites where slides are likely to occur

40. Identifying Potential Landslides
Identifying areas with a high potential for landslides is the first step in developing a plan to avoid landslides hazards
Sites at risk for landslides can be identified by local geologic conditions and aerial photographs to recognize past slides
Once identified, a landslides inventory should be prepared
The inventory is a map that shows locations in terms of their relative activity

42. Preventing Landslides
Preventing landslides is difficult but can be done with common sense and good engineering practices
People should avoid overloading the top of slopes, cutting into sensitive slopes, placing fills on slopes, and changing water conditions on slopes
Common engineering practices include
Drainage control
Removal of unstable slope material
Construction of retaining walls

43. Drainage Control
Drainage controls are usually effective in minimizing slope failures
The objective is to divert water to keep it from running across or infiltrating into a soil
The amount of water infiltration can also be controlled by covering the slope with an impermeable layer such as soil-cement, asphalt, or plastic

44. Drainage Controls

45. Grading
Carefully planned grading can increase the stability of a slope
In many cases, material from the upper part of the slope is removed and placed near the base of the slope
In some cases, the slope may be cut into benches or steps with drainage controls to divert water

46. Grading

47. Slope Supports
Retaining walls constructed from concrete, stone-filled wire baskets, or concrete, steel, or wooden piles can provide support at the base of a slope
They are typically anchored well below the base of the slope, backfilled with permeable gravel, and provided with drainage controls

48. Slope Supports

52. Subsidence
The very slow to rapid sinking or settling of earth materials
Subsidence can be caused by several factors
Withdrawal of fluids
Chemical weathering
Mining

53. Fluid Withdrawal
The withdrawal of fluids from the subsurface can cause incidents of subsidence
Fluids beneath the surface have a high fluid pressure that supports overlying materials
If the fluid is removed, the support is also removed and subsidence can occur
Example: Overpumping of groundwater in central California for agricultural purposes

56. Chemical Weathering
The dissolution of limestone or salt by groundwater can cause overlying earth materials to subside
One of the most common features that forms by this process are sinkholes
Sinkholes occur as limestone is dissolved by groundwater, enlarging voids within the rock
The overlying rock then collapses into the subsurface

58. Mining
Coal mining has caused a number of issues with ground subsidence
The subsidence is most common where underground mining is close to the surface
Usually, 50% of the coal is left behind as pillars that support the overlying rock
Over time these pillars can weaken, producing subsidence
More than 8,000 square kilometers of land has subsided due to mining and subsidence can continue long after mining operations cease