Investigation on the evolution characteristics and transfer mechanism of surrounding rock pressure for a hard-rock tunnel under high geo-stress: case study on the Erlang Mountain Tunnel, China

Similar to the squeezing soft-rock tunnel, the surrounding rock pressure for a deep-buried hard-rock tunnel under high geo-stress also possesses certain long-term evolution characteristics, which have a significant impact on the safety performance of the supporting structures. This paper investigated the evolution characteristics and transfer mechanism of the surrounding rock pressure for a deep-buried hard-rock tunnel through the combinations of the field measurements and numerical simulation. Firstly, the triaxial compression and uniaxial creep tests for granite samples indicated that the hard-brittle rock could exhibit the rheological properties under high geo-stress. Secondly, the field measurements showed that the surrounding rock pressure continued to increase throughout a period of 1200 days. Thirdly, a new composite viscoelastic-plastic creep constitutive model was used to calculate the creep damage degree and scope of the surrounding rock. The results showed that the evolution characteristics of the surrounding rock pressure can be divided into three typical stages, i.e., phase I: rapid growth stage; phase I : decelerating growth stage; and phase III: basically stable stage. From the beginning of phase II to the end of phase III (about 1100 days), the pressure on the supporting structures increased by nearly 40% compared to phase I (about 50 ~ 150 days), and the average pressure-sharing proportions of the primary support and the secondary lining finally were stabilized at 0.6 and 0.4, respectively. The damage of surrounding rock was distributed mainly from the haunch to the vault and the bottom, with the degree and scope of the bottom being the most serious.