Natural slopes are generally in equilibrium. The degree of
steepness, amount of rainfall, soil thickness, angle of repose
of rock debris, and vegetation combine to produce a slope that
will tend to stay put until something changes. In many cases
something does change that weakens the slope. The changes can be
natural, such as a large rockfall sheet that overloads a slope,
a run of several unusually wet months or years that saturates
and weakens upper rock layers and soils, or stream erosion that
undercuts the base of a slope. Other changes can be induced by
humans. The excavation of roadcuts, quarries, or trenches may
remove supporting earth and rock from the base of a slope.
Conversely, piling up mine tailings, landfills, or earth
embankments for building sites or roads may overload a slope. In
addition, any of these changes may alter the natural runoff of
rain or snowmelt and streams.