Dilatancy: Soil Expansion Under Shear

The question asks us to identify the term that describes the expansion of soil when it undergoes shear deformation while maintaining a constant confining pressure. This is a fundamental concept in soil mechanics related to how soil behaves under stress.

Explaining Soil Dilatancy

Dilatancy refers to the tendency of granular soils, particularly dense ones, to increase in volume when subjected to shear stress. When these soils are sheared, the particles rearrange themselves. In dense soils, this rearrangement often involves particles climbing over one another, which creates more void space between them. This increase in void volume leads to an overall expansion of the soil mass. This phenomenon occurs typically when the soil is dense enough that particles cannot easily slide past each other without lifting. The condition of constant pressure is specified because the volume change is measured under this constraint.

Imagine a tightly packed box of marbles. If you try to slide layers past each other (shear), some marbles might have to lift up to allow movement, creating gaps. This is similar to what happens in dilatant soil.

Why Other Options Are Incorrect

Let's look at why the other options don't fit the description:

• Apparent cohesion: This term describes a cohesion that seems to exist due to capillary forces or suction, especially in unsaturated soils. It's not related to the physical expansion of soil particles during shear.

• True cohesion: This refers to the intrinsic binding forces between soil particles (like electrostatic forces in clays) that hold them together, independent of pore water tension. While cohesion affects shear strength, it doesn't directly describe volume expansion due to shear rearrangement.

• Consistency: This term is used primarily for fine-grained soils (clays and silts) and relates to their different states based on water content, such as liquid, plastic, and solid states. It describes the soil's stiffness or fluidity, not its expansion under shear stress.

Therefore, the expansion of soil due to shear at a constant pressure is specifically termed dilatancy.