where S, is specific storage [L-1], and t is time [T]. Specific storage is a measure of the

volume of formation fluid per unit volume of media per unit head of a saturated

formation that is stored or expelled from storage due to compressibility of the matrix and

change in fluid pressure. Specific storage is defined as Ss=pfg(a+nl) where a is matrix

compressibility [M-1 L T2], and fl is the compressibility of the formation fluid [M-1 L T2].

Matrix compressibility (a) is defined by a = [1/(V)](dV /doe) where V is the bulk

volume [L3] and oe is the effective stress [ML-1T-2]. The second term on the right hand

side is a source term representing changes to fluid volume or pressure, such as due to

loading, per time per unit volume of media [T-1].

 During loading, the stress added from the sediment load is partitioned between the

pore fluid and the matrix. The loading efficiency, y, denotes the fraction of the stress

added to the pore fluid pressure and, assuming the sediment grains are incompressible

relative to the matrix, is defined as follows (after Neuzil, 1986)

 y= a/(a+nf ) (6)

For highly compressible sediments, the loading efficiency is near 1.

 The increase in pore pressure caused by loading drives fluid flow. When initial

conditions, boundary conditions and the controlling hydrogeologic parameters k and a

are known, Equation 5 can be solved for hydraulic head at any point at any time, and

thus, pore fluid velocity relative to the sediment framework can be calculated using

Equation 4a.

 The pore pressure is related to effective stress (Ge) by


o O P