Soil slump
Illustrations of the mass movement known as soil slump.

 
This diagram illustrates that at the base, or toe, of a slump, sometimes an earthflow may occur as well. In either case, a building foundation cannot survive this kind of movement.


As indicated by the photos below, the Thompson Lane soil slump didn't develop overnight. Rather, the original grade (i.e. slope) on the overpass soil face was too steep. In addition, the plants that grew on the face were shallow-rooted grasses that added weight, but provided little holding power. The overpass was mowed regularly which also discouraged growth of deeper rooting plants. Finally, due to the weight of the mowing machines, ruts developed on the soil face, providing excellent access for water to flow into the soil mass, adding weight and decreasing internal friction.
 

This photo illustrates a very typical site -- minor slumping of the soil mass. Just allowing a low slope area to lose vegetative cover can often result in this amount of movement (i.e. slight.) Road maintenance crews tried repeatedly to repair this recurrent damage over a 5 or 6 year period until finally, in 1999, the slope totally failed. 
As you can see in the middle and bottom photos, a large detachment scar developed at the top of the slump, and the toe deformed by earthflow -- just as the cartoon above illustrates. 
 
Today, this site has been regraded, is mowed less frequently, and deeper rooted plants have been allowed to grow. Just goes to show, you shouldn't fool with Mother Nature.
Photo 1 (top) - 1996; Photo 2 (mid) - 1999; Photo 3 (bottom) - 1999