Safety and environmental impact control of cross passage construction in soft soil strata using tunnel boring machine method

Abstract The Tianjin area is covered by soft, water-bearing silt and silty sand strata and has a high groundwater table. The excavation of a cross passage in this area entails a high risk of water and sand inflows and a significant impact on the structural integrity of the mainline tunnels—particularly during the excavation of the break-out and break-in openings. However, few cross passages have been excavated in water-bearing soft soil strata using the tunnel boring machine (TBM) method, and the necessary safety control techniques and environmental impact of TBM tunnelling of cross passages in such soil strata remain unclear, which limits the application of this tunnelling method. In this study, a cross passage excavated using the TBM method—the first of its kind in the Tianjin area—was investigated. We identified the key risk control measures for the construction process, particularly the excavation of the break-out and break-in openings, and analysed the TBM operating parameters, monitoring ground and building settlements, and monitored mainline tunnel deformations and mechanical responses at key points of the construction process, revealing the ground and tunnel structure deformation patterns during the cross passage excavation. Our major conclusions are as follows. 1) The ground surrounding the cross passage break-out opening was stabilised by performing two rounds of grouting and small-range freezing, and the break-in opening was excavated using a completely enclosed steel sleeve. These measures effectively prevented the risk of water and sand inflows during the excavation of the break-out and break-in openings in the silt and silty sand strata. 2) The thrust of the TBM was approximately 2000 kN during the break-out phase, and it increased to 3500 kN after the break-out opening was excavated. The torsional moment of the cutter disc was large during the break-out phase. Break-out mainline tunnel displacement monitoring data indicated that the thrust had a significant effect on the mainline tunnel during the break-out phase. 3) The TBM tunnelling of the cross passage caused ground loss, and the ground settlement increased as the excavation proceeded. The ground settlement exhibited a U-shaped distribution along the cross passage axis, with the maximum settlement being 10 mm. During the break-out phase, the deformation of the break-out mainline tunnel exhibited a duck-egg-shaped distribution. The clearance convergence of the break-out mainline tunnel was within ± 4, and the displacement and clearance convergence of the break-in mainline tunnel were controlled within ± 1 mm.