Mass Wasting

Definition: Mass wasting is the downslope movement of rock, regolith, and soil, under the influence of gravity. Also called mass movement.

Angle of repose
Refers to loose, unconsolidated materials (gravel, sand, soil, etc.). The angle of repose is the angle at which loose material sits at rest. Typically 25 - 40 degrees.

If a slope is articficially over-steepened (by man - such as in road cuts and excavations), the material will become unstable and begin to move downslope.

Water is important; it adds weight, which leads to instability. It also lubricates loose material, and starts it moving.

What about water pressure? Water is in the pore spaces of the materials (i.e., between the grains). If pressure is applied (weight of the material?), water tends to move because it is incompressible. This is termed "pore pressure".

In addition, some types of materials shrink and swell as they are wetted and dried. For example, a type of clay called montmorillonite clay expands when wet and shrinks when dry. It causes soil instability and can lead to mass wasting or downslope movement of soil.

Types of mass wasting:

1. Rapid movement
   1. Rock fall
   2. Slump
   3. Rockslide or debris slide
   4. Debris flow or mudflow
   5. Earthflow
2. Slow movement
   1. Creep
   2. Solifluction

Rock Falls

The free fall of detached pieces of material of any size; may fall directly downward or bounce and roll. May occur as result of freeze-thaw, or the loosening action of plant roots. Causes the formation of talus slopes. Signs along highways warn of ralling rock in mountainous areas where the road has been cut into the hillside.

A roadside reminder in the Appalachian Mountains of Tennessee.

Talus slope, Lost River, West Virginia.

Slumps

Slumps involve a mass of soil or other material sliding along a curved, rotational surface. (Shaped like a spoon.) Causes the formation of a small, crescent-shaped cliff or "scarp" at the upslope end. (Sometimes more than one scarp.) At the bottom (or toe) of the slump, earthflow, or flow of soil occurs.

Slumps are sometimes seen along interstate highways where the graded soil on the sides of the road is a little too steep.

Slumps, Cartersville, Georgia

Slump, Route I-675 south of Atlanta, Georgia

Series of small slumps on a hillside near Lost River, West Virginia

Slumps in suburban settings

These images, taken in the Seminary Ridge area of Alexandria, Virginia show evidence of slumping in a housing development built on unstable soil (probably montmorillonite clay or fill dirt) and overly steep slopes.

1. The first image shows terracing in a back yard in an attempt to stop the downslope movement of soil from above.
   
2. and C. The second and third images show the backs of houses on the hill above. In the second image, note that slumping has moved the soil downslope, leaving a scarp or cliff at the base of the foundation of the house. Railroad ties (in a retaining wall approximately 1 story high) have been added in an attempt to stabilize the soil and protect the house from further slumping.
   
3. The third image, slumping has also produced a scarp at the back of the house. Note where a deck had been attached (dark line just below the sliding glass doors) prior to slumping. The deck was destroyed by slumping.
   
4. The fourth image shows what happens in the backyard of a house below the slump. The toe of the slump is visible on the right, and has tipped up a stone backyard barbecue grill.

Rockslides

Also called debris slides or "landslides". Occurs when blocks of rock, or masses of unconsolidated material slide down a slope. These are among the most destructive of mass movements. May be triggered by rain or melting snow, or earthquakes.

Landslide in the Great Smoky Mountains National Park, Tennessee, near Clingman's Dome. First image is from 1976, and second image is in the same spot in 1980. Note the increase in the size of the landslide over four years.

Debris flows or mudflows

Commonly occur in volcanic areas, where they are called lahars. Mudflows generally follow established drainage patterns (valleys).

Earthflows

Form in humid areas on hillsides following heavy rain or melting snow, in fine-grained materials (clay and silt). Also occurs at the toe of slumps. Rate of movement varies (less than 1 mm per day to several meters per day), but may be long-lived (days to years). Includes the liquifaction associated with earthquakes.

Creep

A SLOW downhill movement of soil and regolith. Creep results in tree trunks that are curved at the base, tilted utility poles, fence posts, and tombstones, and causes retaining walls to be broken or overturned.

Bent tree trunk illustrating creep. National Zoo, Rock Creek Park, Washington, D.C.

Photo of bent tree trunks illlustrating creep. Rocky Mountain National Park, Colorado.

Solifluction

Occurs in areas underlain by permafrost. Occurs in the "active" surface layer that thaws in summer.

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