A comparative study of UDEC simulations of an unsupported rock tunnel

Spatial configuration of discontinuities often defines how rock masses behave during excavation. This study investigates how different representations of this configuration influence the predictions by UDEC simulations of the mechanical behaviour of a fractured rock mass hosting an unsupported tunnel. The first set of representations is assembled by the default joint generator command JSET of UDEC, while the second set of discrete fracture network (DFN) representations is realized by a new code developed using UDEC's built-in programming language. The first set typically contains nearly uniform joint systems producing rock blocks with similar sizes and shapes. The simulations with this representation predict relatively thin and uniform failure zones around the excavation boundaries. On the contrary, the DFN's stochastic joint systems with blocks of various sizes and shapes tend to develop much thicker and localized failure zones around the excavations. The vertical, horizontal and total displacements of UDEC-DFN models are generally greater than those of UDEC-JSET models. It is suggested that numerical simulations of excavations in fractured rock masses be performed using UDEC-DFN models to realize possible range of excavation conditions and to be prepared for them by formulating alternative designs.