Why does the pressure of the pore water increase when a load is applied to saturated soil?

The pressure of the pore water in saturated soil increases when a load is applied primarily because water is treated as incompressible in geotechnical engineering. When a load is applied to saturated soil, the soil grains are compressed, causing the voids between them to decrease. Since water cannot be easily compressed, this reduction in space effectively leads to an increase in pore water pressure.

As the soil compresses under the applied load, the pore water cannot escape instantaneously, so the pressure within the voids rises. This phenomenon is fundamental in understanding effective stress, which is crucial for analyzing soil behavior under various loading conditions. The idea that water is incompressible helps engineers predict how pore pressures will respond to external stresses, which is critical for stability analyses and the design of structures that interact with soil.

The other choices do not appropriately address the behavior of pore water under load conditions. While compressibility might seem relevant, in the context of saturated soils, it is the incompressibility of water that dictates the response. Thermal expansion is not typically a factor in immediate loading scenarios in soils, and gravitational effects do not account for the increase in pore water pressure due to loads applied over short time frames.