Shear Strength
The resisting force of a slope is that which opposes the driving force (gravity). This resistance is a result of the shear strength of the material on the slope, which represents the relationship between the internal friction (friction between particles) and the cohesiveness (ability of particles to stick together) of the material. Cohesion and internal friction are thus very important factors in slope failure. For example, solid rock has greater shear strength than large pieces of unconsolidated debris, because it is cohesive, and is thus more stable. Internal friction is also key to consider, because a reduction in the internal friction of material on a slope decreases shear strength, and can result in mass movement. A common way for this to happen is water saturation as a result of heavy rainfall or a rapid melting event.