Stability analysis of a large-span and deep tunnel

Hanjialing tunnel, the first large-span tunnel in the northern area of Liaoning province, China, is scheduled to be completed in2003. This tunnel is 460m in length, 21.242m in width and 15.52m in height. The rock formation of this tunnel is composed ofthick-bedded muddy sandstone, thin alternations of sandstone and shale. The sandstone and shale alternation is less weathered andis not severely stained. However, the stability of this large-span tunnel is affected by faults or joints located nearby. Hence, a betterunderstanding of the mechanics of influence, especially regarding the risk assessment of faults, is required. The influence of faults onthe stability of underground openings has been investigated using numerical methods.In this paper, we use rock failure process analysis (RFPA) developed by the Centre for Rockburst and Induced SeismicityResearch (CRISR) as the primary numerical tool. The heterogeneity of the material was taken into account in this numericalmethod. For a better understanding of the effect of faults on the stress distribution and displacement around excavations, acomparison is presented here; especially the displacement behaviour for different locations of a fault around the large-span openingis discussed. From the numerical results, we can conclude that, although the stress is concentrated during excavation, it does notcause the failure of the tunnel wall rock. Also, we can conclude that the tunnel wall rock fails partially when the load reaches 52MPa(Fig. 1); however, failure does not occur in the support structure, which validates that the support structure is reasonable. Thedeformation of the fault as a result of excavation is also briefly presented.The numerical results accord well with the field results. It is hoped that this case study will shed some light on future projects of asimilar nature, and provide a good reference benchmark for the design and construction of similar tunnel engineering projects.