Dilatancy, compaction, and failure mode in Solnhofen limestone

Failure mode is intimately related to porosity change, and whether deformation occurs in conjunction with dilatation or compaction has important implications on fluid transport processes. Laboratory studies on the inelastic and failure behavior of carbonate rocks have focused on the very porous and compact end-members. In this study, experiments were conducted on the Solnhofen limestone of intermediate porosity to investigate the interplay of dilatancy and shear compaction in controlling the brittle-ductile transition. Hydrostatic and triaxial compression experiments were conducted on nominally dry samples at confining pressures up to 435 MPa. Two conclusions can be drawn from our new data. First, shear-enhanced compaction can be appreciable in a relatively compact rock. The compactive yield behavior of Solnhofen limestone samples (with initial porosities as low as 3%) is phenomenologically similar to that of carbonate rocks, sandstone, and granular materials with porosities up to 40%. Second, compactive cataclastic flow is commonly observed to be a transient phenomenon, in that the failure mode evolves with increasing strain to dilatant cataclastic flow and ultimately shear localization. It is therefore inappropriate to view stress-induced compaction and dilatancy as mutually exclusive processes, especially when large strains are involved as in many geological settings. Several theoretical models were employed to interpret the micromechanics of the brittle-ductile transition. The laboratory data on the onset of shear-enhanced compaction are in reasonable agreement with Curran and Carroll's [1979] plastic pore collapse model. In the transitional regime, the Stroh [1957] model for microcrack nucleation due to dislocation pileup can be used to analyze the transition from shear-enhanced compaction to dilatant cataclastic flow. In the brittle faulting regime the wing crack model provides a consistent description of the effect of grain size on the onset of dilatancy and brittle faulting.