In soil mechanics, what does the term 'dilatancy' refer to?

Dilatancy in soil mechanics refers specifically to the phenomenon where the volume of a soil mass increases when it is subjected to shear stress. This behavior is typically observed in dense, fine-grained soils, such as sands and silts, especially when they are subjected to loading or stress.

When stress is applied, the rearrangement of soil particles can lead to the temporary formation of more voids, which results in an increase in volume. This is particularly significant during conditions of rapid shearing or loading when the soil may initially expand instead of contracting. This property plays an important role in understanding the behavior of soils under different loading conditions, especially in the context of stability and failure analyses.

The other options do not accurately define dilatancy: a decrease in pore water pressure relates more to drainage conditions and how water is expelled from soil pores, a measure of soil stability is typically linked to factors like friction angle and cohesion rather than volume change, and a type of fault movement pertains to geological structures rather than soil behavior under stress.