A study of the coupling between mechanical loading and flow properties in tuffaceous rock

Coupled mechanical/flow tests were performed on a number of samples of Apache Leap tuff. Both the axial and lateral permeabilities were measured while samples were subjected to uniaxial loading. Measurements were made in the linear portion of the stress-strain curve, during unloading, in the strain-hardening region near the peak stress, and in the strain-softening region after macroscopic fractures had formed. The axial and lateral permeability tests show very interesting relationships between mechanical loading and hydraulic conductivity. The axial permeability (permeability in the direction of uniaxial loading) decreases dramatically with mechanical loading in the linear-elastic portion of the stress-strain curve, due to the axial closing of cracks and pores. On unloading the permeability returns to near the original level. In the strain-hardening region, the axial permeability increases due to the formation of microcracks, and in the strain-softening region, the axial permeability of post-failure specimens is 30 times higher than that of the original unstressed samples. The lateral permeability (permeability perpendicular to the direction of uniaxial loading) shows some significant differences with the axial flow results. In the linear-elastic portion of the stress-strain curve, the lateral permeability increases due to the propping open of cracks and pores perpendicular to the direction of maximum compression. Also, the lateral permeability is noticeably higher than the axial permeability in the strain-hardening and strain softening regions.