Study on Mechanical Properties of Surrounding Rock and Combined Stress of Composite Lining in the Diversion Tunnel

To determine the mechanical properties of the rock mass in the Nansha Branch, three in-situ tests (on-site pressure plate, borehole TV and acoustic wave, and borehole deformation methods) were carried out. Based on the deformation characteristics of the rock mass, the finite element numerical method is used to study the combined stress of the composite lining in the diversion tunnel. The results demonstrate that the deformation modulus values are similar by comparing them to tests of the pressure plate and borehole deformation method. It shows the accuracy and reliability of the deformation modulus obtained by the borehole deformation method. In addition, the deformation modulus of the borehole deformation method and wave velocity were fitted to obtain a power function relationship. The deformation modulus of the pelitic siltstones was estimated. After the Nansha Branch tunnel is filled with water, the maximum principal stress of the pipe piece is 1.953 Mpa in tensile stress, located at the outside of the waist on both sides of the pipe piece. The maximum principal stress of the reinforced concrete lining is 5.407 Mpa, located at the top and bottom inside. The maximum principal stresses have exceeded the standard tensile strength of both the pipe piece and reinforced concrete and are vulnerable to cracking. Special attention should be paid to the deformation of the above parts with regard to long-term operation.