Dilatancy (granular material)

When stressed, a lever motion occurs between neighboring grains, which produces a bulk expansion of the material.

On the other hand, when a granular material starts in a very loose state it may continuously compact instead of dilating under shear.

Its effect can be seen when the wet sand around the foot of a person walking on beach appears to dry up.

The phenomenon of dilatancy can be observed in a drained simple shear test on a sample of dense sand.

In general, the denser the soil, the greater the amount of volume expansion under shear.

Poorly / uniformly graded silt with trace sand to sandy that is non-plastic can be associated with challenges during construction, even when they are hard.

However, as shearing of the soil occurs in the active wedge due to gravity forces, strength is lost and the rate of failure accelerates.

This can be exacerbated by hydrostatic forces developing at the location(s) where water (drains to and) collects in tension cracks in or near the back of the active wedge.

As soon as the volume change results in a state of maximum void, the mobilised dilatancy angle,