Pore Pressure

Definition

Pore pressure (u) within a soil is the net pressure of the fluids within the voids that exist between the soil particles. Pore pressure is most commonly associated with pore water, which has a pore-water pressure (u_w). Pressures of other fluids such as pore-air pressure (u_a) and other non-soluble materials may be considered for specialized analyses.

The simplest spatial distribution of u_w is the result of hydrostatic pressure, where the pressure at a given point is the result depth below the phreatic surface, and thus u_w increases with increasing depth. Where the phreatic surface or water table is the height of water within observation wells that correspond to a u_w equal to zero. The hydrostatic pressure can be calculated from the unit weight of water (γ_w), and the depth (d) below the phreatic surface (Terzaghi et al. 1996).

$$ {u}_w={\gamma}_wd $$

The surface tension of the water interacts with the fine interconnected voids within soil and results in a capillary rise of the...