STRENGTH CHARACTERISTICS AND SHEAR ACOUSTIC ANISOTROPY OF ROCK CORE SUBJECTED TO TRUE TRIAXIAL COMPRESSION

Results are presented from an initial experimental programme aimed tow ards evaluating the capabilities of a new true triaxial cell, designed to apply independent and unequal principal stresses to the curved sur faces of cylindrical core plugs. A series of discrete failure tests on dry specimens from two sandstone lithologies exhibiting different def ormation, strength and poroperm characteristics, were conducted under azimuthal stress anisotropy (sigma(2) > sigma(3)) with sigma(1) being applied axially. The true triaxial cell consistently orientates induce d brittle shear fractures so that they strike parallel to the directio n of sigma(2), and slip against the direction of least confinement, si gma(3). Both peak (fracture) and residual (friction) strengths are sho wn to be strongly dependent on the magnitude of the applied sigma(2), as well as on that of sigma(3). Results from multi-failure state testi ng using the conventional ''triaxial'' compression configuration are c ontrasted with discrete failure tests conducted in the true triaxial c ell, by means of the familiar von Mises and extended 3-D Griffith crit eria. Digitised records of shear-waves obtained at 40, 60 and 80% of p eak failure strength during true triaxial testing, show clear evidence of progressively increasing stress-induced ''splitting'' or birefring ence between the arrival of the faster S1(parallel to sigma(2)) and th e slower S2(parallel to sigma(3)) shear-wave. Microseismic data and ma croscopic observations from discrete failure tests performed within th e true triaxial cell, are thus supportive of a brittle deformation mec hanism involving stress-induced dilatant microcracks extending paralle l to sigma(2) and opening against sigma(3), progressively coalescing w ith increasing sigma(1) to form a pervasive fault also oriented by the applied 3-D stress field.