Biot’s Coefficient


Pore fluid pressures will reduce the effective stress supported by the rock mineral frame. This effect has been well known since the publication of Terzaghi and Peck and has been documented by numerous investigators. The most common form for the effective stress law is

σe=σa-nPp (1)

where σeis the effective stress, σa the applied stress on the rock surface, Pp, and the pore pressure. The effective stress coefficient n is also called Biot’s poroelastic term.

n=1-Kd/K0 (2)

where Kdis the dry rock bulk modulus and Ko the mineral bulk modulus.

Hydraulic Fracturing TestUltrasonic Test