Abstract: This study investigates the mechanical behavior of pile foundation underpinning and shield tunnel construction in water-rich pebbly sand stratum, using the Chengdu Metro Line 13 shield tunnel at the Jiaozi Overpass as a case. First, large-scale direct shear tests are conducted on pebbly sands at three water contents (2%, 12%, 22%) in air-dried, natural, and saturated states. Subsequently, numerical simulations analyze the deformation and force mechanisms during pile foundation underpinning and shield tunnel construction. Finally, deformation control measures for the pile foundations and tunnels are proposed. The simulation results are validated against field monitoring data. The results show that the shear strength of pebbly sands decreases with increasing water contents. The cohesion and internal friction angle of saturated pebbly sands are 62.5% and 82.8% lower than those of air-dried pebbly sands, respectively. The air-dried and natural pebbly sands show dilatancy, while the saturated pebbly sands show contraction. After pile foundation underpinning, the axial force of the pile is significantly reduced. Following shield tunneling, the axial force of the pile near the tunnel increases significantly, while higher water content reduces lateral friction resistance and increases pile settlement. Pile foundation settlement is primarily caused by pile foundation underpinning and shield tunnel construction. The settlement induced by pile foundation underpinning accounts for 73.2%-96.8% of the total settlement, with the calculation results closely matching the field measurements. Tunnel vault settlement is influenced by the water content. When the water content of the pebbly sand layer is controlled around 12%, the proposed control measures can effectively limit the settlement of both the bridge and tunnel.