A new true triaxial cell for testing mechanical properties of rock, and its use to determine rock strength and deformability of Westerly granite

A new true triaxial cell has been designed, fabricated, calibrated, and suc cessfully tested. Its main feature is very high loading capability in all t hree orthogonal directions, enabling the testing to failure of hard crystal line rocks subjected to large least and intermediate principal stresses. Al l three principal stresses applied to rectangular prismatic specimens, 19 x 19 x 38 mm in size, are servo controlled. The cell was used to conduct an extensive series of tests in Westerly granite. A new true triaxial strength criterion for the ruck was obtained that takes into account the effect of the intermediate principal stress. This turns out to be so significant that it raises serious questions about the suitability of criteria such as thos e named after Mohr, Coulomb, Griffith, and others. Measurements of strain i n all three principal directions revealed that the onset of dilatancy relat ive to the major principal stress at failure rises substantially as the int ermediate principal stress increases. The true triaxial tests also demonstr ate that for the same least horizontal stress the main fracture dip angle i n Westerly granite increases as a function of the intermediate principal st ress, suggesting a strengthening effect. Limited thin section and SEM study shows that microcrack propagation, crack localization, and main fracture c haracteristics are basically similar to those observed in common triaxial t ests. (C) 2000 Elsevier Science Ltd. All rights reserved.