| Two visco-elasto-plastic constitutive models for intact rocks and rock joints have been developed to predict stand-up time of excavations in rock. Each of the models includes parameters for rock or joint softening with plastic deformation (or plastic work in the case of rock joints) and with viscous deformation (or viscous work in the case of rock joints) to account for the degradation in the rock strength that occurs in real situations. An explicit finite difference computational scheme and the computer program ('VEPAC') have also been developed to predict the time-dependent response of a rock mass. To avoid mesh sensitivity in the numerical analysis, a non-local continuum model proposed in the literature has been adopted. The program 'UDEC', which is a distinct element code, has been modified to use the proposed model for rock joints.; These computer programs have been used to investigate the type of time dependent behavior that may develop around excavations. Several important issues such as size effect, joint spacing and the role of in-situ stress on stand-up time are addressed. A comparison is also made between the results of the proposed model and the empirical stand-up time rules. Results are encouraging, and provide good insight into the factors which may control stand-up time in practice. However, in order to encompass the elasto-plastic and time-dependent strength degradation features considered essential to adequately represent real situations, the model requires 17 physical constants. Many of these parameters have not been measured experimentally and, in some cases, it is not clear how they could be measured (e.g in-situ values of joint properties). For the model to provide a more useful basis for stand-up time calculation, further study of possible inter-relationships between the physical constants and sensitivity of excavation behavior to the constants is needed. |
