Abstract: The mudstone-sandstone interbed is prone to large deformation, so the reasonable support timing is extremely
important. In order to explore the reasonable support timing of the large-section mudstone-sandstone interbed tunnel and
to guide the construction in practical application, relying on the Huipaishan tunnel of China-Laos railway, the support
timing and full displacement inversion of the large-section mudstone-sandstone interbed tunnel are carried out.Firstly,
the engineering characteristics of the rock mass are studied by triaxial compression test, and the mechanical parameters
of the rock mass are modified based on the Hooke Brown strength criterion to provide the basis for the later numerical
calculation.Further, through the derivation of the total displacement, combined with the field monitoring data, the
relationship between the total displacement and the measured displacement is obtained.Finally, the relationship between
the settlement of the vault and the radius of the plastic zone is obtained by numerical simulation, and the characteristic
points are found to determine the timing of the initial support and the strengthening support. According to the
relationship between the measured displacement and the full displacement, the best support timing based on the field
monitoring data is obtained.The results show that :(1) The condition of mudstone-sandstone interbedded surrounding
rock is poor, and the mechanical index parameters are about half lower than that of single 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 vault settlement reaches 170 mm and there is no convergence trend, the support measures should
be strengthened.

Key words: railway tunnel, supporting time, mudstone sandstone interbedded strata, triaxial compression test, numerical
simulation