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 ( σ 2 > σ 3 ) with σ 1 being applied axially. The true triaxial cell consistently orientates induced brittle shear fractures so that they strike parallel to the direction of σ 2 , and slip against the direction of least confinement, σ 3 . Both peak (fracture) and residual (friction) strengths are shown to be strongly dependent on the magnitude of the applied σ 2 , as well as on that of σ 3 . 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(∥ σ 2 ) and the slower S2(∥ σ 3 ) 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 σ 2 and opening against σ 3 , progressively coalescing with increasing σ 1 to form a pervasive fault also oriented by the applied 3-D stress field.