Abstract: Strata structure of loess area mostly features a dualistic structure of loess and mudstone. When tunnels traverse mudstone and loess-mudstone composite strata, groundwater tends to redistribute and accumulate at the base of the inverted arch. The interaction between mudstone and water causes tunnel heave, and the lithological differences between the strata significantly affect the deformation and stress characteristics of the tunnel lining. Based on a tunnel project in Tianshui City, Gansu Province, model tests were conducted on tunnel in pure mudstone and loess-mudstone composite strata with the interface at the arch waist. Displacement was applied beneath the inverted arch to simulate the swelling effect of mudstone-water interaction. Differences in internal force, deformation, and failure characteristics of the lining under two loading conditions were investigated, and numerical simulations were conducted for comparison and verification. The differences in internal force, the deformation and failure characteristics of lining under two loading conditions were investigated, and the corresponding conditions numerical simulation was conducted for comparison and verification. The results show that: (1) Expansion deformation due to forced displacement at the model’s base can be divided into two stages: the base soil compaction stage and the equivalent transfer stage of base displacement. (2) With the elastic modulus ratio of mudstone to loess approximately 2:1, the loess stratum exhibits smaller overall stiffness, resulting in larger lining deformation and even larger heave displacement in inverted arch. The arch waist at the interface is more prone to outward expansion, with an expansion deformation of 9.2 mm, about three times that of the pure mudstone stratum. Stress concentration tends to occur at the lining section near the interface. (3) In both strata, the inverted arch and arch foot positions are prone to cracking and damage. Additionally, in the composite strata, the lining arch shoulder shows a high risk of cracking, with cracks at the arch foot tending to expand towards the arch waist. (4) Compared to pure mudstone stratum, the loess near the interface in loess-mudstone strata exerts a load release effect on the heave of inverted arch. The loess at the interface compresses and absorbs some of the heave force, thereby delaying the critical tensile failure time of the filling layer. The results can provide valuable references for theoretical analysis, design and construction of tunnels in similar composite strata.

Key words: inverted arch floor heave, interface strata, model experiment, numerical simulation