Abstract: The mudstonesandstone interbedded tunnel is prone to large deformation, thus the reasonable support timing is extremely important. Study is conducted on the support timing and total displacement inversion of the Huipaishan tunnel of ChinaLaos railway in mudstonesandstone interbedded strata. First, the engineering characteristics of the rock mass are studied using triaxial compression test, and the mechanical parameters of the rock mass are modified based on the HoekBrown strength criterion to provide the basis for the later numerical calculation. Then, through derivation of the total displacement, the relationship between the total displacement and the measured displacement is obtained based on field monitoring data. Finally, the relationship between the crown settlement and the radius of the plastic zone is obtained using numerical simulation, and the characteristic points are found to determine the timing of the primary support and the strengthening support. According to the relationship between the measured displacement and the total displacement, the optimum support timing based on the field monitoring data is obtained. The results show the following: (1) The condition of mudstonesandstone interbedded surrounding rock is poor, and the mechanical index parameters are about half lower than that of singlecomponent surrounding rock. (2) The measured displacement accounts for 60%-70% of the total displacement, and the relationship between the measured displacement and the total displacement is obtained. (3) The tunnel needs to be supported with the excavation, and when the onsite monitoring crown settlement reaches 170 mm and there is no convergence trend, the support measures should be strengthened.

Key words: railway tunnel, support timing, mudstonesandstone interbedded strata, triaxial compression test, numerical simulation, displacement inversion