The application of critical state soil mechanics to the mechanical behaviour of porous sandstones

The mechanical properties of three cohesive sandstones of different porosities (φ) and average grain diameters (R) have been investigated. These were Tennessee sandstone (φ=0.07), Darley Dale sandstone (φ=0.12), and Penrith sandstone (φ=0.25). Unconfined uniaxial compression, constant displacement rate triaxial, and hydrostatic experiments were conducted. Yield stress data produced approximately circular envelopes that decreased in size with increasing porosity or grain size when plotted in the differential stress versus effective mean stress (Q−P) space. Normalization of these data with respect to the hydrostatic grain crushing pressure (P^*) resulted in a unique yield envelope for sandstone. Extending these data into the Q–P–φR space allows the principles of critical state soil mechanics to be applied. The critical state line for porous sandstone (the crestal line of the yield surface) appears to correspond to the transition from dilatant behaviour with localized faulting at low effective mean pressures (P/P^*<0.5), to pervasive cataclastic flow at high effective mean pressures (P/P^*>0.5). Post-yield, deformation progresses towards the critical state as observed by constant volume deformation. The critical state model developed for soil mechanics can be applied to make generalizations about the deformation of cohesive, porous sandstones. The expected behaviour of any porous sand appears to be predictable to a useful degree from a knowledge of P^*, which can be estimated from the simple parameters of porosity and mean grain size.Sensitivity to the presence of water, attributed to sub-critical crack growth, was observed in hydrostatic and uniaxial compression tests in all rock types tested. Considerable strength and elastic anisotropy was also observed.