Research on the Mechanism and Treatment Technique of Invert Floor Heave After the Penetration of Large Cross-section Tunnel in Slight Inclined Stratum

Abstract

The newly built Chengdu–Guiyang Railway Gaopo Tunnel crosses through an interstratified rock mass of slightly inclined mud-sand. Sometime between the completion of the main structure and the construction of the monolithic ballast-less track, some sections began to bulge or even crack with a maximum deformation of up to 46 mm. At the same time, the vault jet concrete began to peel, causing significant risks in and challenges to the construction of the ballast-less track and the succeeding operations. In this study, the mechanism of and treatment techniques for invert floor heave following penetration of a large-cross-section tunnel into a slightly inclined stratum were investigated. In the study, the in-situ stress values and physical and mechanical parameters of the surrounding rock were obtained through field testing, and the extracted parameters were used in theoretical analysis based on discrete element numerical calculation to determine the applicable mechanism of invert floor heave. Finally, the implementation effects of different treatment techniques were compared and analyzed using the control variable method to identify final treatment measures. The results of this study reveal the following. (1) The heaving section of the invert floor in the tunnel area primarily comprised thin to medium-thick or interbedded mudstone and sandstone with argillaceous cement and was, in general, gently inclined. The joints of the rock mass were relatively well developed and broken. The effects of tectonic stress were not strong, and vertical stress was dominant. The swelling of the surrounding rock was weak. (2) The rock layer cracked as a result of the bending deformation of the layered surrounding rock in the unfavorable force direction. The argillaceous cement was lost or diluted in the presence of water. The weakening of the interlayer connection up to and including failure was an important contributory factor to the invert floor heave. (3) In treating the invert floor heave problems induced by joint surface weakening, the control effect of deepening the inversion was determined to be superior to those obtained by increasing the bolt length or strengthening the support stiffness. Based on these conclusions, a comprehensive management method involving the construction of a circular cavern using double linings and a 12-m long bolt was selected as a replacement treatment to effectively control the deformation. These results provide a reference for the design and construction of similar projects in the future.