Experimental and Numerical Study on the Dynamic Response of a Superthick Backfill Subgrade under High-Speed Railway Loading: A Case Study of Qianjiang–Zhangjiajie–Changde Railway

In this study, a mountain tunnel of the Qianjiang–Zhangjiajie–Changde Railway, which passes through a giant karst cave at a high position, was investigated as a case study. The cave cavity height below the tunnel track is approximately 30–55 m, and the collapse accumulation thickness at the cave bottom is approximately 37–66 m. After solution comparison analysis, a superthick backfill subgrade solution was selected by filling the karst cave with a cave ballast backfill + upper grouting. In connection with railway operation, large vibrations and dynamic settlements may occur under train loads, and the safety of train operations can be compromised. To investigate the vibration response of a superthick backfill subgrade in a giant karst cave, 12 dynamically scaled model tests with a scale ratio of $1:5$ were performed with a high-speed railway subgrade dynamic test platform. The subgrade slab thickness and running speed of the train were investigated as the variable parameters. The working state of each part of the subgrade was studied while the train passed, and the time-history variation and long-term development trend of the dynamic properties were analyzed. The attenuation behaviors of vibration displacement, dynamic stress, and acceleration in the superthick backfill subgrade were analyzed using the three-dimensional finite element method. The results showed that as the thickness of the subgrade slab increased, the dynamic response of the train decreased rapidly. At a running speed of $200\mathrm{km}/\mathrm{h}$, the average acceleration decreased by 75.2% at the bottom of the subgrade slab when the thickness of the slab decreased from 60 to 20 cm at the initial passage. Under a long-term load, acceleration at the same depth below the subgrade slab of thickness 60 cm decreased by 69.5% compared to that of thickness 20 cm. The superthick backfill subgrade with a 3-m-thick RC subgrade slab avoided evident dynamic settlement induced by the running load. When the tunnel was opened to traffic, tunnel structure settlement was monitored. Furthermore, the settlement of the tunnel invert was $<1\mathrm{mm}$ within 1 year, which was consistent with the test results. The results proved that the design parameters and solution selection were reasonable and reliable and could serve as a reference for the development of similar projects and researches. The key contribution of this research is to propose an effective and low-budget treatment for the construction of railways that pass through giant caves. In addition, the findings from the study on superthick backfill subgrades provide industry practitioners with a comprehensive guide regarding the specific applications and mechanical performance of superthick backfill subgrades, which can serve as a stepping stone to facilitate the development of construction technology in the transportation industry.

The fast-growing transportation industry faces various challenges to railway construction, including the surrounding environment, geological condition, and construction technology, for example. Railway tunnels constructed in karst areas often pass through karst caves with different dimensions. The ground under tunnels needs to be treated if the tunnels pass through the caves. Backfill subgrade is a fast and low-budget construction technology. Because the mechanical properties of the backfill subgrade are different from those of the in situ ground, the working performance of the backfill subgrade, particularly the superthick backfill subgrade, must meet strict requirements. Therefore, a new and reliable construction method for superthick backfill subgrade, a solution consisting of cave ballast backfill + upper grouting, is proposed and is used for filling karst caves. To investigate the vibration response of the superthick backfill subgrade in a giant karst cave, scaled model tests, numerical investigation, and in situ observations were conducted. The study determined that superthick backfill subgrade with 3-m-thick RC subgrade slab will avoid apparent dynamic settlement induced by a running load. In addition, the settlement of the tunnel invert was $<1\mathrm{mm}$ within 1 year, reflecting the good performance of the superthick backfill subgrade constructed in the case history. The findings obtained from the study provide guidance on construction technology in the transportation industry.