Mechanical behaviour and damage constitutive model of semi-circular arch tunnels with straight walls under soaking conditions

To study the groundwater influence on the mechanical properties of deep hard rock semi-circular arch tunnels with straight walls, compression experiments were conducted on semi-circular arch tunnel samples treated with soaking. The hydraulic damage and failure behaviour of semi-circular arch tunnels with straight walls under soaking conditions is analysed. The peak stress and stiffness decreased gradually with soaking time. These mechanical properties of samples softened significantly. Compared with those of the natural samples, the pre-peak plastic deformation of the water-soaked samples is larger and the plasticity is stronger. Under high vertical stress, the left and right sidewalls of all samples show the spalling failure character from the shallow section to the deep section. During the accumulation and releasing of elastic strain energy, initial failure typically occurs at arch corners. Moreover, this failure location is not affected by water. The sample failure process under uniaxial loading can be divided into quiescent period, particle ejection period, accelerated crack expansion period and formation period of symmetrical "V"- shaped groove failure zone. The low acoustic emission (AE) energy and high AE energy events occur sequentially with loading time. This phenomenon reflects the gradual increasing in the AE energy concentration within samples. Under uniaxial loading, the influence of water-rock interaction on crack generation and expansion is significant. The longer the soaking time is, the more obvious the influence is. The damage variable increases rapidly firstly. Then, it becomes slow. Finally, it increases rapidly with strain.