IIn-situ tests on lining system of double-arch tunnel with shallow large section and span

In order to ensure the security of the construction of a double-arch tunnel with shallow large section and span, which has large span, shallow depth, poor rock stability and complicated geological condition, the extensive measurements of the tunnel are essential. Based on a double-arch tunnel project, the contents, techniques and methods of stress monitoring of the support system are described. The stress characteristics of the tunnel during different excavation stages are monitored and analyzed. Some conclusions are drawn as follows: 1) The construction of the upper stage in the left and right holes, which changes the stress distribution in the support system greatly, is the main point for controlling the stability of the support system; 2) The construction of the upper and lower stages in the right hole is the main point for controlling the stability of the middle wall due to great change of the stress distribution in the middle wall; 3) The longitudinal incidence of the excavation of the upper stage of right and left tunnels are about 1/3 and 1/2 times the tunnel span; 4) The secondary support structure and the enclosing of the lining must be established in time to improve the performance of the lining. The experience and conclusions presented can be referred to in the design, construction and in-situ monitoring of similar tunnels.