Discussion on Reasonable Clear Spacing of Twin-Tunnels in Weak Surrounding Rock: Analytical Solution and Numerical Analysis

Due to the limitations of the terrain and other reasons, twin-tunnels with small clear spacing are becoming more and more common. The reasonable clear spacing of twin-tunnels in weak surrounding rock is an urgent problem. In this paper, based on pressure-arch theory (PAT), the method used to calculate the loosening pressure on the surrounding rock of twin-tunnels under three cases was derived by considering the additional disturbance of the excavation on the middle rock pillar through the amplification factors of the sliding angles, k(1) and k(2). Taking the weak surrounding rock as an example, we discussed the effect of the clear spacing variation on the loosening pressure of the surrounding rock, and obtained the dangerous clear spacing of twin-tunnels. The accuracy and applicability of this approach were verified by the in-site data that were measured. Considering the limitation of the pressure-arch theory on a deep-buried tunnel under high geo-stress, based on the Jiuzhai valley-Mianyang highway in China, the reasonable clear spacing of twin-tunnels in weak surrounding rock at different buried depths, i.e., 400 m-1,600 m, is discussed using the numerical simulation method. The analytical solution indicated that the loosening pressure of the inner side of the first tunnel was greater than the one of the second due to the amplification factors of the sliding angles, k(1) and k(2). The in-site data showed that in the weak surrounding rock mass, when k(1) and k(2) are taken as 1.15 and 1.3, respectively, the loosening pressure distribution law is closest to the actual situation.