Influences of stress dilation on shape of failure surface for shallow tunnels

摘要

For shallow tunnels of single-lane railway and four-lane road, numerical simulation using finite differential code was conducted. The mechanical behavior of loose rock masses was studied considering the influences of shear dilation on the shape of the failure surface for the shallow tunnels, and the break angles using numerical simulation was compared with those using the Rankine's analytical solutions and design code. From the comparisons, it is found that the difference between the break angle by numerical simulation and the design code is small when the shear dilation angle is equal to 0, with the maximum relative difference being less than 0.2% in road tunnels. With the dilation angle's increase, the loose plasticity zone area of surrounding rock reduces obviously,and the break angle increases gradually. When the dilation angle is equal to the internal friction angle, the materials follow associated flow rule, and the numerical solution of the break angle is larger than the analytical solution, with the maximum relative difference being greater than 16.7% in road tunnels. Therefore, associated flow rule leads to overestimating break angle, and the dilation angle has an important influence on the failure surface shape for shallow tunnels in the loose surrounding rocks.