Mass Wasting

Most slopes in nature are on materials that are not loose collections of grains. They occur on bedrock or on soils which are bound together by organic material, etc. Yet many of the principles used to explain mass wasting in aggregates still apply. Instead of mass wasting taking place as an avalanche, however, it results from a portion of the slope breaking off and sliding down the hill. We usually call these events landslides, or avalanches, if they are large and damaging, or slumps if they are smaller.

If the gravitational forces acting on a mass of material are greater than its strength, a fracture will develop, separating the mass from the rest of the slope. Usually this fracture will be nearly vertical near the top of the break, curving to a much lower angle near the bottom of the break. Such events can be triggered by an increase in the driving forces (for example, the weight of the slope), a decrease in the strength of the material, or both.

When people build on slopes they often add to the gravitational forces by constructing very heavy things, such as houses and swimming pools. A period of heavy rain can add a lot of weight to a slope, too. Rain may also change the strength of the material making up the slope. As we have seen, saturated soil with no surface tension is much weaker than moist soil. But there is another effect that occurs even in solid rock.

Even very solid rocks contain pores, and many of these pores are interconnected. It is through such pores that water and oil move toward wells. Below the water table, all the pores are filled with water with no surface tension to eliminate. So it might seem that rock down there would not be affected by rainfall at the surface. As the rains come, however, the water table rises, and the additional water increases the pressure in the fluids in the pores below. This increase in pore pressure pushes adjacent rock surfaces apart, reducing the friction between them, which lowers the strength of the rock and makes it easier for fractures to develop. Elevated pore pressures are implicated in many dramatic mass wasting events.

When southern California gets heavy rains, the television news cameras record homes moving down slope in a landslide or mud flow to become rubble at the bottom. Such dramatic examples of mass wasting are impressive, but represent only a fraction of what is all around us everyday.