sIntroduction
- 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.
Take a Virtual Field Trip to the Slumgullion landslide.
Links to Good Landslide and Slope Failure sites: | USGS Landslide News and Information - http://landslides.usgs.gov/html_files/landslides/hawaii99.html | | http://walrus.wr.usgs.gov/elnino/landslides-sfbay/photos.html contains a computer-simulated landslide | | Landslides at Hagerman Fossil Beds, Idaho - http://www.nps.gov/hafo/landslides.htm | | Yosemite landslide News - http://landslides.usgs.gov/html_files/landslides/Yosemite99.html |
SLOPES
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 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

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

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
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 . . . . . * material * strength of rock or soil * slope angle * climate * vegetation & * time
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: | 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 = NW sin A = D | |
TYPES OF MASS WASTING
Study the following table before reading this section. Return to the table to review. TYPE OF MOVEMENT | | MATERIAL INVOLVED | | | | Rock | Soil | 1. FALLS | | * rockfall | * soilfall | 2. SLIDES | rotational translational | * rock slump block * rock slide | * soil slump blocks debris slide | 3. FLOWS | slow
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fast | * rock creep | * soil creep * saturated & unconsolidated material * earthflow * mudflow (up to 30% water) * debris flow * debris avalanche | 4. COMPLEX | combination of two or more types of movement | | |
FALLS
Rockfall
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!
Cutbank
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.

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.