Strength characteristics and shear acoustic anisotropy of rock core subjected to true triaxial compression

Abstract

Results are presented from an initial experimental programme aimed towards evaluating the capabilities of a new true triaxial cell, designed to apply independent and unequal principal stresses to the curved surfaces of cylindrical core plugs. A series of discrete failure tests on dry specimens from two sandstone lithologies exhibiting different deformation, strength and poroperm characteristics, were conducted under azimuthal stress anisotropy (σ2>σ3) with σ1 being applied axially. The true triaxial cell consistently orientates induced brittle shear fractures so that they strike parallel to the direction of σ2, and slip against the direction of least confinement, σ3. Both peak (fracture) and residual (friction) strengths are shown to be strongly dependent on the magnitude of the applied σ2, as well as on that of σ3. Results from multi-failure state testing using the conventional “triaxial” compression configuration are contrasted with discrete failure tests conducted in the true triaxial cell, by means of the familiar von Mises and extended 3-D Griffith criteria. Digitised records of shear-waves obtained at 40, 60 and 80% of peak failure strength during true triaxial testing, show clear evidence of progressively increasing stress-induced “splitting” or birefringence between the arrival of the faster S1(∥σ2) and the slower S2(∥σ3) shear-wave. Microseismic data and macroscopic observations from discrete failure tests performed within the true triaxial cell, are thus supportive of a brittle deformation mechanism involving stress-induced dilatant microcracks extending parallel to σ2 and opening against σ3, progressively coalescing with increasing σ1 to form a pervasive fault also oriented by the applied 3-D stress field.