Deformation of Existing Highway Induced by Close Undercrossing of Shield Tunnel with Steep Slope: A Case Study

As Earth Pressure Balance (EPB) shield machine crossed with steep slopes beneath an existing highway in sandstone–mudstone alternating strata, case studies of changes in vertical displacement, settlement trough evolution, and tunnel stress induced by shield tunnel construction were investigated. The quality of synchronous grouting was evaluated using ground penetrating radar (GPR) technology. The results showed that highway settlement could be categorized into four stages: initial settlement, uplift, secondary settlement, and stabilization. The secondary settlement caused by shield tail detachment was significantly greater than the initial settlement induced by distant shield construction. The settlement trough evolved throughout construction; the maximum settlement point shifted from the tunnel centerline but it consistently remained within 3 m. During the early phase of shield tail detachment, the circumferential stress of shield tunnel changed rapidly. The circumferential stress was primarily compressive, tensile stress was observed at some monitoring points. The tensile stress at the monitoring points gradually transitioned to compressive stress. After the tunnel undercrossed, the circumferential stress gradually stabilized. The GPR detection revealed that in groundwater-rich strata, poor grouting quality areas were prone to appear at the tunnel crown, while grouting quality in other areas performed better. This engineering case can serve as a positive reference for similar EPB shield tunnels passing in close proximity beneath existing highways.