SLOPE STABLITY
Slope stability is important when a judgement is needed about the structures being built on rock. As a precaution, miners have to be sure that their slopes are stable, otherwise they risk losing money and lives. If the downard force horizontal with the slope is greater than the frictional force, the slope will fail.
Imagine that there is a natural joint in rock being mined and during a storm it is filled with water. Now there is an added force from the water called pore pressure and the normal force is decreased making the slope more likely to fail. The following diagram represents this scenario with 'W' as the gravtitational force downard of the upper rock, 'p' as the pore pressure from the water, and 'A' as the area that the water affects.
(courtesy of Rajive Ganguli, Ph.D. Associate professor of Mining Engineering, UAF)