Learning Objectives

1. Mass wasting is movement of bedrock, rock debris or soil downslope as a mass in response to gravity.

2. Mass wasting is classified on the basis of rate of movement, type of material, and nature movement. Rates vary from slow (< 1 cm/year) to rapid. Types of material include bedrock, unconsolidated debris, and soil. Movements include flow (movement as viscous fluid), slide (movement as coherent mass along defined surface), and fall (free-fall).

3. Slides may be either translational (plane parallel to slope) or rotational (also called slump) (movement along a curved surface).

4. Factors promoting mass wasting include: steep slopes, high local relief, thick debris above bedrock, planes of weakness parallel to hillside, freeze and thaw, saturation of debris with water, long periods of drought with episodes of heavy precipitation, and sparse vegetation (Table 9.2).

5. Gravity is the driving force for mass wasting. Shear force is parallel to slope. Shear strength is resistance to movement or deformation. If shear force is greater than shear strength, mass wasting occurs.

6. Water is a critical factor in mass wasting. Small amounts of water actually inhibit mass wasting because surface tension increases shear strength. As the water content increases, the rate of movement increases, for example a change from creep to mudflow.

7. Creep is very slow, continuous downslope movement of soil or unconsolidated debris promoted by water in the soil and cycles of freeze and thaw where shear forces are slightly greater than shear strength.

8. Debris flows are mass wasting taking place as a moving mass, including earthflows, mudflows and debris avalanches. Earthflow is slow or rapid downslope movement of water saturated debris as a viscous fluid. Rotational sliding (movement along a curved surface) (previously called slumping) is commonly associated with earthflow. Solifluction is a variety of earthflow that occurs above permafrost in colder climates. Mudflow is a flowing mixture of debris and water usually moving in a channel. Debris avalanche is a very rapidly moving mass of debris, air and water.

9. Rockfalls are blocks of bedrock that fall freely or bounce down a cliff. A rockslide is a mass of bedrock moving along an inclined surface; a rock avalanche is a larger version of the same process. Debris slides and debris falls are the same process involving movement of a coherent mass of debris rather than bedrock.

10. Landslides can be prevented by recognizing potential problems and proper engineering. Avoiding oversteepening of slopes and undercutting slopes as well as providing adequate drainage for excess water are useful measures.