Dilatancy, creep and relaxation of brittle rocks measured with the 8000 kN multipurpose triaxial apparatus

Abstract

This paper presents a preliminary report on a systematic study of the dilatancy, creep and relaxation behaviour of rocks in the brittle field. A comprehensive description is given of the new 8000 kN servocontrolled multipurpose triaxial apparatus. Four series of tests conducted at a constant strain rate of 5 × 10 −5 s −1 on 16 samples of Fangshan granite and Jinan gabbro have been performed at various confining pressures from 100 to 400 MPa and temperatures from 25 to 250°C. A new method is introduced to determine the dilatancy parameters n, D ∗ and C on the basis of constitutive equations for creep dilatancy presented earlier. The tests show an increase in dilatancy with increasing temperature and suppression of dilatancy with increasing confining pressure. In order to study the time-dependent deformations in the linear and dilatancy stages more closely, creep and relaxation tests have been conducted. Two creep tests on Fangshan granite have been carried out using stepwise loading at 100 and 200 MPa confining pressures and a temperature of 7 and 200°C, respectively. In this manner the dependence of creep and dilatancy on deviatoric stress and time can be obtained from one sample. Finally, two relaxation tests on granite at confining pressures of 100 and 200 MPa and temperatures of 7 and 200°C respectively have been performed using stepwise strain increase with time. The threshold values ƒ ∗ at the onset of dilatancy for creep and relaxation have been determined from the stress-strain relationships. An analysis of the time-dependent processes of dilatancy is presented in connection with structural changes due to crack growth, coalescence and generation.