Ground support design for rock tunnels or caverns often adopts precedents or empirical methods, which were mostly developed in 1960s and 1970s. These methods have undoubtedly contributed to completions of many projects; however, they are unique and closely related to local geology condition at the locations where the methods were developed. Discrete Fracture Network (DFN) model allows inclusion of discontinuities data from site-specific geotechnical investigation stage to be stochastically quantified and used explicitly as design input. Hence, this offers a more quantifiable, verifiable, and reproduceable method to assess rock mass quality & behaviour. Bias in rock mass characterisation and engineering judgement involved in design process therefore can be avoided. This paper demonstrates benefits gained from DFN approach in ground support design for large span rock caverns in urban area. The case study involves a few caverns constructed in jointed volcanic tuff at shallow depth and within a complex bifurcation scheme. Certainly, controlling ground settlement and maintaining ground stability are of importance in this situation. However, balance between maintaining public safety and ground support design should be achieved. The use of DFN and explicit modelling has allowed ground support to be optimized based on more realistic anticipated ground conditions. Coupled with 3D Distinct Element Method, the DFN approach also assisted in assessing pillar condition of the bifurcation area due to elevated stress more realistically rather than relying on conservative method. Overall, it contributed to successful and economical project delivery.
Rock mass stability in underground opening is controlled by several factors, including discontinuity conditions & spacing (which control rock mass rating), intact rock strength, and in situ stress. These conditions lead to different failure modes; i.e., stress-induced failure, structurally induced failure, or combination of both; as illustrated by Kaiser et al. (2000). Stress-induced failure typically occurs in situations where in situ stress is relatively high compared with rock strength. Often, this situation is encountered in deep mines. Structurally induced failure is governed by the presence of jointing as the weak element in rock mass where stress condition is relatively low compared with rock strength.
