All soils are compressible so deformation will occur whenever stress is applied to soils. Soil
minerals and water are both incompressible. Therefore, when saturated soils are loaded, the load
first acts on the pore water causing pore water pressures that are in excess of the hydrostatic
pressures. The excess pore water pressures are largest near the application of load and decrease
with distance from the loading. The variations in excess pore water pressure cause total head
gradients in the soil which, according to Darcy’s Law, will induce water to flow from locations
of high total head to low total head. The excess pore water pressures dissipate as water flows
from the soil and, to compensate for the applied stress, the stress is transferred to the soil
minerals resulting in higher effective soil stress. The flow of water from the soil also causes
reductions in the soil volume and settlements at the ground surface. Fine-grained soils have very
low permeability so they can require substantial periods of time before the excess pore water
pressures fully dissipate. This process of time-dependent settlement is referred to as
consolidation. Terzaghi’s theory for one-dimensional consolidation provided the means to
calculate the total amount of consolidation settlement and the consolidation settlement rate. In
practice, engineers obtain representative soil samples, conduct consolidation tests and use
Terzaghi’s consolidation theory to predict the total settlement and time rate of settlement for
embankments and foundations.