Module 4 - Landslides, Slope Failure, and Other Mass Wasting Processes

By Anni Watkins and Scott Hughes

Major Topics In Module 4:

Reading:  Chapters 6, p. 131-160 in textbook, Keller, 2000

Review of topics covered in this module. PDF notes page format, or larger file size version in PDF format.


- Heavy rains saturate the Portland, Oregon area. Slick, debris-filled mud slides down steep slopes blocking the scenic Columbia River Gorge Highway.

- Warm rains rapidly melts snowpack. Snowmelt-induced debris flow threatens lives and damages property.

- A major earthquake hits the West Coast. It triggers numerous landslides in a 200-mile radius.


Each of these events represents some type of slope failure. Slope failure, also referred to as mass wasting, is the downslope movement of rock debris and soil in response to gravitational stresses. Three major types of mass wasting are classified by the type of downslope movement. The types of movement, falls, slides, and flows, will be covered in this module. In addition, another type of ground failure, subsidence, will be covered.

View a paper on the Slumgullion landslide in Colorado.



Material is constantly moving downslope in response to gravity. Movement can be very, very slow, barely perceptible over many years. . . . . Or, movement can be devastatingly rapid, apparent within minutes. Whether or not slope movement occurs depends on slope steepness and slope stability.

slope figure

Slope Profile

Some slopes are gently rounded, while others are extremely steep. Profiles of naturally-eroded slopes are primarily dependent on climate and rock type.

Resistant rock and rock from semi-arid regions have similar profiles because in both cases chemical weathering is slow. Erosion may be slow or fast depending on the amount of material weathered (loosened) or freely available (uncovered) for transport. Typically, the crest of the slope is slightly convex to angular, the cliff (freeface) is nearly vertical, and a debris slope is present. The debris slope has an angle of repose of 30° -35° , which is the maximum angle at which loose material is stable.

Resistant Rock -- Semi-arid Region

resistant rock

Non-resistant rock and rock in semi-humid regions also have similar profiles. Climatic conditions and softer rock types result in material that weather rapidly and erode easily. The crest of the slope is convex, while the base of the slope is concave. This type of slope contains a thick soil cover.

Non-resistant Rock -- Semi-humid Region

non-resistant rock

Slope Stability (a lesson in physics)

When is a slope not stable? Slope stability is based on the interplay between two types of forces, driving forces and resisting forces. Driving forces promote downslope movement of material, whereas resisting forces deter movement. So, when driving forces overcome resisting forces, the slope is unstable and results in mass wasting.

Driving Forces

The basic concept of these two types of forces is quite simple. You experience the interplay between driving forces and resisting forces each time you drive down a steep slope. The driving forces are gravity and the acceleration of the vehicle (if you step on the accelerator), the resisting force is the brake on the vehicle.

The main driving force in most land movements is gravity. The main resisting force is the material's shear strength.

Does gravity act alone? NO!! Slope angle, climate, slope material, and water contribute to the effect of gravity. Mass movement occurs much more frequently on steep slopes than on shallow slopes.

Water plays a key role in producing slope failure. In the form of rivers and wave action, water erodes the base of slopes, removing support, which increases driving forces. Water can also increase the driving force by loading, i.e., adding to the total mass that is subjected to the force of gravity.

Chemical weathering (interaction of water with surface rock and soil) slowly weakens slope material (primarily rock), reducing its shear strength, therefore reducing resisting forces.

An increase in water also contributes to driving forces that result in slope failure. The weight (load) on the slope increases when water fills previously empty pore spaces and fractures. The shear strength of the slope material is decreased by increasing the pore water pressure (pressure that develops in pore spaces due to the increased amount of water).


Resisting forces act oppositely of driving forces. The resistance to downslope movement is dependent on the shear strength of the slope material. And shear strength is a function of cohesion (ability of particles to attract and hold each other together) and internal friction (friction between grains within a material).

Water contributes to resisting forces when sediment pores are partially filled with water. The thin film of water acts as a binder, making the particles cohesive (remember surface tension of water? Module 2-part 2).

The ratio of resisting forces to driving forces is the safety factor (SF):

SF = Resisting Forces

      Driving Forces

If SF > 1 then SAFE

If SF < 1 then UNSAFE

NOTE: Usually a safety factor of ~10 is used in design to accommodate slight variances in materials and construction practices.

Factors of Slope Stability: - Slope stability is therefore a function of . . . . .

Each of these factors may play a significant role in controlling driving or resisting forces.

Study in your textbook how each factor controls the type of failure and the likelihood that failure will occur.

How does slope angle affect both driving and resisting forces?

Study the following relations between slope angle, weight of material, the component of weight in the direction of failure (driving force), and the normal component of weight (resisting force). You can easily see how SF changes dramatically:

slope angle

The total weight (W) of a mass that rests on a potential failure surface. A can be divided into two components, N and D. For a given W, N and D change dramatically with change in the slope angle A.


N = the vectoral component of weight that acts normal to the failure surface. Increase in N increases the frictional component, thereby increasing the resisting forces.


D = the vectoral component of weight that acts in the direction of failure.

W cos A = N

W sin A = D


Study the following table before reading this section. Return to the table to review.




  • rockfall
  • soilfall
rotational translational
  • rock slump block
  • rock slide
  • soil slump blocks debris slide
  • rock creep
  • soil creep
  • saturated & unconsolidated material
  • earthflow
  • mudflow (up to 30% water)
  • debris flow
  • debris avalanche
combination of two or more types of movement



Have you ever noticed warning signs posted along roads stating "beware of falling rocks"? These signs let you know that you are in a hazard zone, an area where rockfall is common! Rockfall (free fall of rock) is an extremely rapid process and occurs without warning. Rockfall is typically the result of frost wedging. Frost wedging is a process where water enters cracks in rocks, freezes, expands, and breaks the rock apart. Frost wedging results in a fan-shaped pile of rock fragments at the base of the slope. The rock fragments are called talus and the slope is referred to as a talus slope. The presence of a talus slope is a "warning sign" itself! Zion National Park rock fall


Do you remember studying cutbanks in module 3? Cutbanks are the result of stream erosion. A stream undercuts the outer bend, which results in the remaining overlying stream bank falling, dropping into the moving water. This is an example of soilfall.



Another example of soilfall is produced by ocean waves undercutting cliff faces. The end result is loss of support! This type of undercutting can also result in slumping.

SLIDESLandslide Bonners Ferry, Idaho

Slides, either in rock or soil, will have rotational or translational movement. The behavior of the slide depends mostly on the type of material and whether that material is:

(1) homogeneous (isotropic) material (similar properties in all directions), or (2) inhomogeneous (anisotropic) material with planes of weakness.



rotational movement

Caused by slope failure of homogeneous (isotropic) materials (similar properties in all directions).

translation movement

Caused by slope failure of inhomogeneous (anisotropic) material with planes of weakness.


Translational Movement

Imagine yourself sliding down a slide at a playground. This is similar to what occurs in translational rock- and land- slides. Slides move in contact with the underlying surface! The "sliding surface" is commonly a bedding plane, but may also be a fault or fracture surface.

translational rock
rotational slide

Rotational Movement

The sliding of material along a curved surface called a rotational slide or slump. A common cause of slumping is erosion at the base of a slope. For example, coastal storm waves erode cliff bases, removing supporting material (remember?). Slope failure occurs. The slump block rotates downward, producing a scarp (cliff) at the top of the slope.


Flows are the downslope movement of unconsolidated material in which the material behaves like a viscous fluid. Flows can be very slow or can be exceedingly fast.

Creep - a type of flow

Have you ever noticed a group of trees on a slope where the base of each tree bows outward in the downslope direction?

kneesHave you ever driven through an older, well-established neighborhood where houses are located "above" the sidewalks and streets? Where retaining walls try to hold the sloping lawns in place? Where some of the retaining walls are failing, bulging out over the sidewalks?

If you can answer yes to these questions then you have witnessed creep, the very slow flow process of soil movement!

Other types of flows are all quite similar. Generally the size of individual particles and the amount of water present is the distinguishing criteria. Frequently, the same event is referred to by more than one name. Confusing, huh?!! So, don't be surprised if you discover what one source called a mudslide another source referred to it as a debris flow! Generally, the following rules apply:


button Moderate-to-steep slopes
button Movement may be slow-to-rapid
button The earth material is saturated
button May begin as shallow soil slip (shallow slides in soil over rock that parallels the slope


button Occur on moderate-to-steep slopes
button Movement is generally rapid
button A slurry containing up to 30% water
button Primarily fine-grained material (smaller than sand-sized particles)
button May begin as shallow soil slip (shallow slides in soil over rock that parallels the slope)
button Typically flows down slopes or follows drainage channels

Debris flows

button Movement very slow to very rapid
button Consists of coarse material (more than 50% is sand-sized particles or larger [AS IN BOULDER-SIZED])
button Often begins as slumps or slides that change as the mass breaks up and mixes with water and air
button Often follows drainage systems downslope

Debris avalanches

button Occur on very steep slopes
button Movement a combination of fall, flow, and slide
button Material consists of a mixture of rock, soil, and organic debris (trees, shrubs, etc.)
button May have the form of head, chute, and debris fan (at base)

As you can see, most mass wasting involves more than one type of movement


Assignment 1:

Visit the U.S. Geological Survey Geological Hazards site and study the various types of landslides and their effects on human lives. Select FOUR different events from the Landslide Images directory and record and answer the falling questions:

List them by name and/or place and date of the event, if known.

Record the general features of each one.

Write a very short synopsis of the problem that was encountered during the failure and answer the question: How or why did it fail?

The Human Impact of Landslides

Landslides are natural occurring phenomena. Landslides, or slope failure, occur whether people are there or not! But, human land-use does have a major impact on slope processes. The combination of uncontrollable natural conditions (earthquakes, heavy rainstorms, etc.) and artificially altered landforms can result in disastrous slope failures, and does.

Yellowstone Landslide - summer 2004

To see more pictures and read about the landslides at Yellowstone this past summer click the picture.

yellowstone slide

Excavation into a slope (for a road or construction site) creates a flat area at the base of a slope. However, it also oversteepens the slope. Removing the basal support can result in slope failure.

How does a roadcut affect driving and resisting forces?


Harvesting timber can also have an impact. Removing slope-supporting material (trees and brush), as well as creating roads, affects the landscape. If the surficial (and sometimes subsurface) geology is unstable, mass wasting often occurs. BUT, if clear-cutting (harvesting all trees in an area) and road building occur on geologically stable land, timber harvesting has considerably less impact.

Urbanization also has an effect on slope stability. Grading hillsides (cutting benches for building homes on) greatly increases landslide potential. Construction of homes on unstable slopes has similar effects. Changing the slope face, the additional weight (homes and fill material), plus the added water (homeowners' sprinkler systems and septic tanks) make a formerly stable slope unstable. Add a heavy rainy season and you have lots of landslides!


Minimizing landslide hazards requires three steps: 1) identification of landslide potential areas, 2) prevention of landslides, and 3) corrective measures when a landslide occurs.


Identification is accomplished by 1) studying aerial photographs to determine sites of previous landslides or slope failures, and 2) field investigations of potentially unstable slopes. Potential mass-wasting areas can be identified by steep slopes, bedding planes inclined toward valley floors, hummocky topography (irregular, lumpy-looking surface) covered by younger trees, water seeps, and areas where landslides have previously occurred. The information is then used to generate a hazard map depicting the various landslide-prone areas.

Areas in the western U.S that are prone to slope failure:

hazards map


Controlling drainage and reducing the slope angle reduces landslide potential. Concrete interceptor drains can be constructed to contain runoff and prevent infiltration. Steep slopes can be graded into gentler slopes. A series of "stair-steps" can be created on very steep slopes.

Engineering methods can be used to help prevent slope failure. Retaining walls, rock bolts, and "shotcrete" (coating of concrete-rock mixture on slope surface and crevices to prevent water entry) are used to inhibit slope failure. Wire cables and wire fences minimize the danger of rockfall.


Correction of some landslides is possible. This is accomplished by installing a drainage system, which reduces water pressure in the slope, thereby preventing further movement.

Assignment 2:

Locate in your local area or on the web an example of how a slope is being stabilized (or needs to be stabilized). Write only a sentence or two that describes what you have found. (If you can include a picture of the slope for fun that would be great!)



A depression is the result of subsidence. By definition, subsidence is the very slow to rapid sinking or settling of the land surface.

karst landscape

Subsidence can be the result of natural causes. Some types of carbonate rock underlies topography containing numerous natural depressions, known as sinkholes. The topography is known as karst topography. Limestone and dolomite, both carbonate rocks, are soluble and susceptible to chemical weathering. Chemical weathering produces void spaces (very very small to hugh caverns). Sinkholes result when enough "support" has been removed from the carbonate layer. The surface then collapses into the void space, producing a sinkhole.


Assignment 3:

What is the major internal cause of slope failure? What three conditions are external causes of slope failure?

Terms to Look Up and Know:

* Pore water pressure
* Saturation
* Progressive wettinge
* Rapid Drawdown
* Liquefaction
* Quick clay
* Seeping
* Avalanche
* Hummocky terrain
* Oversteepening by erosion

End of Module #4

Continue to Module #5

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