Investigation of the settlement mechanism and control measures of a super-large section tunnel in a giant karst cave using ultra-thick backfill method

A giant cave was encountered during the construction of the Yujingshan Tunnel. There were deep deposits at the bottom of the giant cave, including soft soil, crushed stone soil, and fine sand. A comprehensive construction scheme, comprising an ultra-thick backfill and a continuous rigid frame bridge, was adopted when constructing the large-section tunnel. However, there were still problems regarding the post-construction and differential settlements of the tunnel (the differential settlement is defined here as the difference between the left corner settlement and the right corner settlement of the tunnel bottom). These problems led to significant challenges for the long-term safety of the tunnel. Control measures were proposed to address these settlement problems. Centrifugal model tests were mainly conducted to analyze the leading causes of the post-construction and differential settlements. And numerical simulation was mainly used to investigate the differences of various grouting depths in the base. Finally, regression analysis and trend simulation were performed based on in-situ monitoring data. The results indicate that the ultra-thick backfill and soft clay are both the causes of the base settlement, and the soft clay accounts for a larger proportion. The differential settlement is mainly due to the uneven soft clay at the bottom. After research, the final scheme is grouting to a depth of 30 m, the postconstruction and differential settlements can be controlled at 35.3 mm and 9.6 mm respectively, and the tunnel structure will have higher safety. On this basis, it is proposed that the maximum allowable settlement is 40 mm, and the maximum allowable differential settlement is 10 mm in long-term operation. The monitoring results and regression analysis show that the construction of the Yujingshan Tunnel is significantly improved, and the post-construction and differential settlements can be controlled at approximately 12.2 mm and 6.6 mm, respectively, which are both less than the maximum allowable values proposed above.