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 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 Outline
I. Introduction to landslides
A. Landslides cause substantial damage and loss of life
2. total damage is about $3.5 billion per year in the United States
B. Landslides and other types of ground failure are natural phenomena
C. Mass wasting
1. comprehensive term for downslope movement of Earth materials
3. subsidence: vertical deformation
II. Slope processes and types of landslides
A. Slope processes
1. slopes are dynamic evolving systems
2. free face
3. other slope segments in weaker rocks
a. gentler
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Slope Processes, Landslides,
and Subsidence
Chapter 10 – Instructor’s Manual
e. straight
B. Types of landslides
2. sliding
4. flowage
5. subsidence
C. Landslides are commonly complex combinations of sliding and flowage
D. Important variables in classifying slope movements
2. slope material type
4. rate of movement
III. Slope stability
A. Forces on slopes
1. driving versus resisting forces
3. factor of safety: ratio of resisting forces to driving forces (see A Closer Look: Calculating a
Factor of Safety for a Simple Landslide)
4. driving and resisting forces determined by several variables
a. type of Earth materials
B. The role of Earth material type
2. translational slides: fractures, bedding planes, weak clay layers, foliation planes (see A Closer
Look: Translation Slides Along Bedding Planes)
4. creep: soil or rock near surface
5. earthflows or mudflows: saturated materials
C. The role of slope and topography
1. slope greatly affects magnitude of driving forces on slopes
3. debris flows: downslope flow of relatively coarse materials
D. The role of climate
2. influences abundance and type of vegetation
E. The role of vegetation
2. root systems add cohesion
4. in special cases, vegetation can increase landslide probability
F. The role of water
2. slumps or translational slides can develop months or years after water infiltrates deeply
4. quick clay: spontaneous liquefaction of clay-rich sediment
G. The role of time
2. chemical weathering acts in presence of water
3. see Case History: Vaiont Dam
IV. Human use and landslides
A. Timber harvesting
2. in some cases, logging does not increase landslide occurrence
4. logging road construction can also lead to landslides
B. Urbanization
2. Los Angeles/Southern California: hillside development
4. Allegheny County, Pennsylvania: modification of sensitive slopes
V. Minimizing the landslide hazard
A. Identifying potential landslides
1. examining geologic conditions in the field
3. landslide inventory
5. grading codes
B. Preventing landslides
2. grading
4. landslide prevention can be expensive, but benefit-to-cost ratio is 10–2000:1.
C. Warning of impending landslides
1. warning systems do not prevent landslides
3. monitoring hazardous areas
D. Correcting landslides
2. drainage program to reduce water pressure, increasing resisting forces
VI. Snow avalanches
A. Snow avalanche
1. definition: rapid downslope movement of snow and ice
VII. Subsidence
A. Withdrawal of fluids
2. if fluid is removed, support is reduced
3. Central Valley, California: thousands of square kilometers have subsided as result of
B. Sinkholes
2. natural or artificial water table fluctuations
4. Lehigh Valley, Pennsylvania
C. Salt deposits
2. Lake Peigneur, Louisiana: collapse of salt mine due to flooding caused by well drilling
D. Coal mining
2. most common where underground mining is close to surface
3. examples of coal mining related subsidence: Ohio, 1995; Scranton, Wilkes-Barre, and
Pittsburgh, Pennsylvania
VIII. Perception of the landslide hazard
Answers to Review Questions and Critical Thinking Questions
Review Questions
1. A landslide is the downslope movement of a relatively coherent mass of Earth materials. The term
is also used for mass movements in general.
3. The safety factor is a representation of slope stability, defined as the ratio of resisting to driving
forces.
5. The slope angle affects the magnitude of driving and resisting forces.
7. Spontaneous liquefaction occurs when disturbed clay loses shear strength and flows. It can be
triggered by earthquakes or by river erosion of the toe of a slope.
9. Lessons learned from the Vaiont Dam disaster were that geology must be examined for landslide
10. Urbanization influences slope stability through removal of vegetation, cutting of slopes, and
building of roads.
12. High fluid pressure supports overlying rock. Removal of water or oil reduces that support and
Critical Thinking Questions
1. The plan for estimating landslide risk should include examination of rock types, vegetation,
2. The plan of action to promote citizen awareness should include compilation of local conditions as