Nowadays, the construction and development of small spacing tunnels is one of the hot issues in transportation area. Numerical simulations on the failure mechanism of small spacing tunnel are carried out by using PFC2d, considering the factors such as tunnel spacing, buried depth and surrounding rock strength, and the whole failure process of small spacing tunnels is successfully simulated in the calculations. Some conclusions can be drawn from the calculations. Firstly, when the tunnel spacing is small, the surrounding rock above each tunnel collapse separately at first, while the collapse develops to a certain degree, the middle rock pillar collapse, cracks appear and enlarge above the two tunnels, and finally the surrounding rock under the cracks drops and so a combinated collapse arch formed. Secondly, the failure mode of small spacing tunnel is different under different tunnel spacing. The smaller the spacing is, the easier and faster the combinated collapse arch formed, and the more serious the collapse degree is. When the tunnel spacing is big enough, the two tunnels collapse separately. The bigger the tunnel spacing is, the smaller the interaction of the two tunnels is, and the more slightly the collapse degree is. Thirdly, the failure mode of small spacing tunnel is different under different tunnel buried depth and surrounding rock strength. When the buried depth and surrounding rock strength are small, combinated collapse arch cannot be formed above the two tunnels, and finally the collapse develops to the ground surface. Finally, for small spacing tunnels, the stabilization of the middle rock pillar is much more important.
The construction and development of small spacing tunnels is one of the hot issues in transportation area, because it has an important practical significance in improving transportation efficiency, protecting and properly using the nature environment, as well as promoting economic development. Studies of small spacing tunnels focus on blast shock controlling and monitoring[1, 2], construction method  and discussion of appropriate spacing . Few studies were carried out on the failure mechanism of small spacing tunnels. In this paper, the influences of tunnel spacing, buried depth and surrounding rock strength on small spacing tunnel failure mechanism are analyzed by using PFC2d.