Application of DEM in design of big tunnels in rocks at relatively shallow depth

In this Paper, failure mechanism is initially investigated for four unsupported tunnels with various spans and cover depths using Itasca's Universal Distinct Element Code (UDEC). Then, two cases with a large cross section (4-lane tunnel) but with different cover ratios (0.25 and 0.5) are analysed, respectively with passive and active supports using both UDEC and Finite Element (FE) program Phase(2). The results show: (1) tunnel failure mode is dependent on tunnel span and cover as well as geological conditions; and (2) for tunnels in fair/good rocks at relatively shallow depths, Discrete Element Method (DEM) (e.g. UDEC) is more reasonable than FE method (e.g. Phase(2)) to capture failure mechanism and evaluate the performance of tunnel support.