Modeling added compressibility of porosity and the thermomechanical response of wet porous rock with application to Mt. Helen Tuff

Tlus paper is concerted with modehng the response of a porous brittle solid whose pores may be dry or partially filled with fluid A form for the Helmholtz free energy is proposed which incorporates known Mle-Grtineisen constitutive equaUons for the nonporous sohd and for the fired, and whtch uses an Einstein formulation with variable specific heat. In addiUon, a functional form for porosity is postulated which depends on two material constants that control the added elastic compressibility of porosity observed m porous rock. The irreversible process of pore crushing is modeled using compaction and dllataon surfaces to determine a scalar internal variable that measures unloaded porosaty Restrictaons on constatutwe assumptions for the composate of porous solid and fired are obtained which ensure thermodynamac consistency Examples show that although the added compressabihty of porosity as determined by fitting data for dry Mt Helen tuff, the predicted responses of saturated and partially saturated tuff agree well wath experimental data