What is the primary characteristic of saturated soil?

Saturated soil is defined as soil in which all the pore spaces are filled with water, meaning there is no air present in those voids. This characteristic distinguishes saturated soil from unsaturated soil, where voids contain both air and water. The absence of air voids in saturated soil has significant implications for the behavior of the soil, including its strength, permeability, and compressibility.

For example, the degree of saturation affects the soil's ability to conduct groundwater, and with all voids filled with water, the soil is fully saturated, which can influence drainage and stability conditions.

The other options focus on different properties: permeability relates to how easily fluids pass through the soil, not directly on saturation; organic matter refers to the components of soil affecting its fertility and composition but doesn't specifically define saturation; while compressibility deals with the soil's response to stress rather than the saturation condition itself. All these aspects are crucial to understanding soil mechanics but do not capture the defining characteristic of saturated soil as comprehensively as the lack of air voids.