Which type of settlement involves a volume change from water expulsion?

Primary consolidation settlement is the correct choice because it specifically refers to the process where soil undergoes volume change due to the expulsion of water from its pores under the influence of excess loading or stress. When a load is applied to saturated soil, the soil begins to consolidate as water is forced out from the voids within the soil structure. This process occurs over time as the soil skeleton compresses and the pore water is expelled, leading to a reduction in volume.

In this context, immediate settlement is characterized by the rapid adjustment of the soil skeleton to applied loads, primarily reflecting elastic deformations rather than changes in pore water pressure. Secondary consolidation, on the other hand, occurs after primary consolidation has taken place and involves ongoing deformation due to changes in soil structure, typically attributed to creep mechanisms rather than the expulsion of water. Elastic settlement, similarly, accounts for the instantaneous response of soil upon loading and does not involve water expulsion or volume change over time. Therefore, primary consolidation settlement is uniquely focused on the hydraulic process linked with water movement and volume change in saturated soils under load.