Ch08 - Slope Processes, Landslides, and Subsidence

Learning Objectives

After reading and studying this chapter, students should
* 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 to slope stability
* Understand how slope angle, 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

Chapter 08 Summary
This chapter consists of a thorough review of landslide occurrence, processes, and mitigation. The chapter begins with an introduction to mass wasting and the types of landslides. The subsequent sections focus on driving and resisting forces and specific factors related to slope stability, including Earth material type, topography, climate, vegetation, water, and time. The chapter then addresses the influence of human activities, especially timber harvesting and urbanization, on landslides, and on methods of reducing landslide hazards. The chapter closes with sections concerning snow avalanches, subsidence, and perception of landslide hazards.

Chapter 08 Outline
I. Introduction to landslides
A. Landslides cause substantial damage and loss of life
1. approximately 25 people per year killed in U.S.
2. total damage exceeds $1 billion per year in U.S.
B. Mass wasting
1. comprehensive term for downslope movement of Earth materials
2. “landslides” used for all mass movements for sake of convenience
3. subsidence: vertical motion
II. Slope processes and types of landslides
A. Slope processes
1. slopes are dynamic evolving systems
2. strong rock: free faces and talus slopes
3. weaker rocks: convex upper slope, straight slope, lower concave slope
B. Types of landslides
1. sliding
2. slumping
3. falling
4. flowage
5. subsidence
C. Landslides are commonly complex combinations of sliding and flowage
III. Slope stability
A. Forces on slopes
1. driving versus resisting forces
2. slip planes: geologic surfaces of weakness
3. safety factor: ratio of resisting forces to driving forces
4. stability determined by several variables
B. The role of Earth material type
1. rotational slides: soil or weak rock
2. translational slides: fractures, bedding planes, weak clay layers, foliation planes
3. soil slips: soil over rock
4. creep: soil or rock near surface
5. earthflows: saturated materials
C. The role of slope and topography
1. slope greatly affects magnitude of driving forces on slopes
2. debris avalanches: steep slopes
3. debris flows
D. The role of climate
1. influences the amount and timing of water
2. influences abundance of vegetation
E. The role of vegetation
1. provides cover that slows raindrops, promoting infiltration and inhibiting grain-by-grain erosion
2. root systems add cohesion
3. adds weight
F. The role of water
1. landslides can develop during storms when slopes become saturated
2. slumps or translational slides can develop months or years after water infiltrates deeply
3. water can erode base or toe of slope
4. quick clay: spontaneous liquefaction of clay-rich sediment
G. The role of time
1. forces change seasonally as the moisture content or water table position alters
2. chemical weathering acts in presence of water
IV. Human use and landslides
A. Timber harvesting
1. landslides responsible for most of erosion in coastal Pacific Northwest
2. logging on unstable slopes can increase landslide erosion by several times
3. logging road construction can also lead to landslides
B. Urbanization
1. Rio de Janeiro: logging and urban development on steep slopes
2. southern California: hillside development
3. Hamilton County, Ohio: landslides in glacial deposits
4. Allegheny County, Pennsylvania: modification of sensitive slopes
V. Minimizing the landslide hazard
A. Identifying potential landslides
1. examining geologic conditions in the field
2. examining aerial photographs
3. landslide inventory
4. slope stability or landslide hazard map
5. grading codes
B. Preventing landslides
1. drainage control
2. grading
3. slope supports
4. landslide prevention can be expensive, but benefit-cost ratio is 10–2000:1.
C. Warning of impending landslides
1. warning systems do not prevent landslides
2. can provide time to evacuate people and possessions, stop trains, reroute traffic
D. Correcting landslides
1. attack process that started the slide
2. drainage program to reduce water pressure
VI. Snow avalanches
A. Snow avalanche
1. definition: rapid downslope movement of snow and ice
2. thousands occur every year in western U.S.
3. tend to move down chutes
VII. Subsidence
A. Withdrawal of fluids
1. fluids in Earth materials have fluid pressure that supports overlying material
2. if water is removed, support is reduced
3. central valley, California: thousands of square km have subsided as result of overpumping of groundwater
B. Sinkholes
1. removal of subterranean Earth materials by natural processes
2. natural or artificial water table fluctuations
3. Winter Park, Florida
4. Lehigh Valley, Pennsylvania
C. Salt deposits
1. solution mining leaves cavities in rock
2. Lake Peigneur, Louisiana: collapse of salt mine due to flooding caused by well drilling
D. Coal mining
1. produces serious subsidence problems in western and eastern U.S.
2. most common where underground mining is close to surface
VIII. Perception of the landslide hazard
A. Common reaction is to assume landslides will not happen at one’s location
1. landslide hazard maps will not prevent all people from moving into hazardous areas
2. infrequency of large slides tends to reduce awareness
B. What you can do to minimize landslide hazards