A breakage–damage framework for porous granular rocks in surface-reactive environments

High-porosity granular rocks are moisture-sensitive solids, in that their strength and deformability are modulated by the relative humidity of their environment. Here, a novel continuum breakage–damage framework is developed to characterize and simulate the inelasticity of cemented granular materials subjected to changes of relative humidity. A microstructural model is proposed to describe the evolution of the solid–fluid interfaces emerging from grain breakage and cement disintegration. The proposed thermodynamic framework links the microstructural model to the energy dissipation and macroscopic rate-dependence of sandstones. The performance of the model is assessed against experimental data for sandstones subjected to loading under both dry and wet conditions. It is shown that the proposed model can accurately predict the yielding and stress–strain response of sandstones by capturing the moisture-weakening effects. The simulations of the model indicate that the increase of moisture lowers the yield stress and reduces the brittleness of the post-yielding response of variably saturated cemented granular materials. It is shown that the rate of damage and breakage growth control the distortion of the yield surface. When inelasticity is dominated by damage, cement bonds are disintegrated and the yield surface shrinks, thus resulting into augmented brittleness. By contrast, and in agreement with experimental evidence, the response of lightly cemented granular solids is found to be dominated by the breakage of the skeletal grains. As a result, changes in relative humidity are predicted to be accompanied by hardening behavior.