Ground subsidence characteristics caused by construction of shallow-buried tunnel in a sandy soil composite formation

Abstract

In dry fine sand and medium-coarse sand strata, large surface subsidence and even collapse are common issues related to tunnel excavation. In the Liu-Huo Section of Line 1 of Shijiazhuang Subway, transverse and vertical surface subsidence were examined by analyzing the measured subsidence data. The width coefficient of the settling tank and the loss rate of the sand mixed stratum were determined using the Peck formula. The settlement of the vertical and transverse surface and vault was further analyzed by establishing a three-dimensional numerical model of the tunnel. Results show that the deformation and surface subsidence vary rapidly for the dry sand mixed stratum, and the subsidence trough is “narrow and steep,” which is different from the general clay stratum. In the Peck formula, the ground settlement tank width coefficient is i = 6.55, and the formation loss rate is Vi = 4.81%. Ground settlement increased nonlinearly with the excavation of the pilot tunnels using the double-sided drift method. The settlement caused by the left No. 1 pilot tunnel was the largest, next was the right No. 3 pilot tunnel, the left No. 2 pilot tunnel, and the right No. 4 pilot tunnel. The settlement caused by the middle No. 5 and No. 6 pilot tunnels was smaller. The vertical settlement is divided into three stages: the small settlement stage, rapid stage, and stable stage. The settlement caused by the excavation of the No. 1, No. 2, and No. 3 pilot tunnels was the largest. After excavation of the No. 3 pilot tunnel, the settlement occupied about 78.3% of the final settlement. Monitoring of the No. 1, No. 2, and No. 3 pilot tunnels should be strengthened during the excavation, and the settlement should be strictly controlled.