What does consolidation in soil mechanics refer to?

Consolidation in soil mechanics specifically refers to the process where soil gradually decreases in volume due to the expulsion of water from its pores under applied load. When an external load, such as a structure, is placed on saturated soil, pore water pressure increases momentarily, and as water is expelled, the soil structure rearranges itself, leading to a decrease in the voids between soil particles. This decrease in the void fraction indicates that the soil has become more dense and stable.

Thus, the correct choice accurately reflects the essential concept of consolidation: as soil consolidates, it reduces its voids, thus decreasing the void fraction. This process is critical in understanding settlement behavior in geotechnical engineering, as it directly affects the stability and settlement of structures built on or above soil layers.