Characterization of Rockmass Properties and Excavation Damage Zone (EDZ) Using A Synthetic Rock Mass (SRM) Approach

Abstract

A detailed understanding of strength and fracturing behaviour of the rockmass is essential for coherent design of underground structures such as deep geological repositories (DGRs) for long-term nuclear waste storage. The development of advanced numerical approaches, such as Synthetic Rock Mass (SRM), provides an effective tool to simulate the failure process of the rock at various scales from laboratory scale samples to excavation scale problems. As an alternative to empirical methods and physical testing, the SRM technique can also be employed to estimate the deformability and strength properties of jointed rockmasses under a wide range of scales and boundary conditions. In this study, a two-dimensional code based on the discrete element synthetic rock mass model is used to simulate the behaviour of low-porosity rocks under hydro-mechanical loading. A methodology is developed to calibrate the SRM models to the response of laboratory-scale intact rock. Using numerical simulation, the role of connectivity of induced micro-cracks on increasing the permeability of a laboratory-tested sample is investigated. The capability of the model to simulate the effect of pore fluid pressure on weakening the strength of the rock is examined. Next, the calibrated model is applied to simulate the formation of the excavation damage zone (EDZ) around unlined underground tunnels in crystalline granitic and laminated sedimentary rocks. The understanding gained from simulating damage development around case history tunnels leads to recommendations for calibrating the micro-parameters of the model for tunnel-scale applications. In addition, an SRM model coupling the discrete fracture network (DFN) model and discrete element grain-based model (GBM) is used to characterize the strength and deformability of highly interlocked jointed rockmasses under confined and unconfined conditions. The scale dependency and variability of the rockmass properties are investigated using the concept of Representative Element Volume (REV). The results of numerical modelling are then compared with the predicted values obtained from empirical methods. It is demonstrated that the SRM technique can be successfully applied to simulate the observed shape and extent of the damage zone around underground excavations. The estimated strength based on the SRM modelling results is in agreement with the unconfined strength predicted by the empirical Hoek-Brown criterion.