INTERACTION MECHANISM BETWEEN PRIMARY SUPPORT AND RHEOLOGICAL SURROUNDING ROCK CONSIDERING HARDENING PERFORMANCE OF SHOTCRETE

Shotcrete is widely used in tunnel engineering as an essential part of initial support. When tunnels are excavated in deep-buried soft rock, the deformation of rock mass and the stress on the structure are complicated due to the hardening characteristics of shotcrete and the rheological effect of rock mass. For this purpose, a coupled analytical model considering the hardening characteristics of shotcrete and the rheological effect of surrounding rock is established in this study. The dynamic evolution mechanism between shotcrete support and surrounding rock is investigated. Furthermore, the impacts of shotcrete parameters, tunnel engineering parameters and rheological properties of surrounding rock on the mechanical performance of tunnel structures are analyzed. The time-dependent support characteristics of different support types are discussed for specific problems. The results show that the deformation rate of surrounding rock is large under the rheological characteristics of the surrounding rock and the spacing effect of excavation at the initial stage due to the lower stiffness of shotcrete. However, the stress in shotcrete is small compared with the case negeccting the hardening characteristics. Therefore, early strength concrete should be used to limit the early deformation of surrounding rock for rheological rock tunnels. The rheological load of surrounding rock and the stiffness of shotcrete increase gradually with time. If the rheological effect of surrounding rock is not considered, the control effect of shotcrete on surrounding rock deformation and the stress level could be underestimated. The support system composed of a section steel frame and shotcrete has large stiffness that restricts the surrounding rock's deformation. However, the support system composed of a grid steel frame and shotcrete has less rigidity, which is more suitable for the stress release of the surrounding rock. The influence of surrounding rock rheological effect, support type, shotcrete hardening characteristics and tunnel excavation rate should be comprehensively considered to select a suitable support time. Furthermore, the limit relation curves between support time and excavation speed for different support types are proposed. The research results can provide a theoretical basis for support design and construction of similar tunnel projects.