Abstract: In general, when dealing with issues on strength of clay, emphasis should be paid on the stress variation and the relative change in pore pressure. The unsaturated clay due to the existence of the solid, fluid and gas phases is much more complicated to deal with. In this paper applying the elastic deformation theory and theoretical gas law, and assuming the conversation of mass , a general pore pressure formula has been derived; as the result the Skempton formula’s coefficients A and B are obtained. On other hand, applying the mixture theory the Bishop formula’s coefficient X is obtained also. In parallel works, the experimental results have been presented to validate the academic theories. It shows that the capillary action has a little influence on pore coefficient.

Key words: pore pressure coefficient, Bishop formula, Skempton formula, poroelasticity, nonsaturated clay