The dewatering of foundation pits with a suspended waterproof curtain causes different groundwater drawdowns inside and outside the pit, resulting in the drawdown difference between the inside and outside the pit. Maintaining a groundwater drawdown difference between the inside and outside of a foundation pit can eliminate the adverse effects of dewatering on the surrounding environment. According to previous studies on unsteady flow, an analytical solution of the groundwater drawdown with a suspended waterproof curtain under unsteady flow has been proposed. The analytical solution of unsteady flow and the formula of groundwater drawdown difference with a suspended waterproof curtain were validated by comparing pumping tests and finite-element method (FEM), in which a good agreement was observed. The magnitude of the drawdown difference generally represents the extent of surrounding groundwater affected by groundwater drawdown inside the pit. This paper also investigated the effects of sensitivity parameters on the drawdown difference for minimizing the effect of surrounding environment. During the process of dewatering with a suspended waterproof curtain, the groundwater drawdown (S_h) should not exceed the length of the waterproof curtain (L), and the optimal radius of foundation pit (R_w) and length of waterproof curtain (L) were found, i.e., R_w/H₀ = 0.781 and L/H₀ = 0.813 (H₀ is 32 m). Beyond these values, the drawdown difference tends to be stable. The drawdown difference is also significantly affected by the dewatering time. When t < 48 h, the groundwater drawdown difference decreases rapidly; when t > 48 h, the groundwater drawdown difference stabilizes.

Metro foundation pits are important components of urban infrastructure projects. Dewatering and excavation are essential stages in foundation pit construction; however, this process can significantly induce groundwater drawdown, as well as diaphragm wall deformation and surface settlement. Based on a subway station foundation pit project, in this study, we employ three-dimensional numerical software to simulate the process of dewatering and excavation. A refined model is used to investigate groundwater seepage, the deformation of the retaining structure, and surface settlement under spatial effects. The finite element model accounts for stratified excavation and applied prestress conditions for the support system within the foundation pit. Its accuracy is validated through a comparison and analysis with measured data from the actual foundation pit. The results indicate that foundation pit excavation leads to significant groundwater drawdown around the pit and the formation of a characteristic “funnel-shaped” drawdown curve. Moreover, extending the diaphragm wall length contributes to maintaining a higher external groundwater level surrounding the foundation pit. The horizontal displacement of the diaphragm wall increases progressively during dewatering and excavation, and the bending moment of the diaphragm wall exhibits a trend consistent with its horizontal displacement. Surface settlement decreases as the length of the diaphragm wall increases.

In order to figure out the response of existing tunnel induced by shield tunneling underneath, an enhanced analytical method for estimating the interaction between existing tunnels and surrounding soil is proposed. The Loganathan solution is employed to obtain the free displacement field of the surrounding soil induced by the shield tunneling. Subsequently, the existing tunnel is simplified as a Euler–Bernoulli beam placed on a Vlasov foundation, considering the lateral soil effect on both sides of the existing tunnel. By utilizing the boundary conditions of the existing tunnel and expanding in terms of the Maclaurin’s series, an enhanced analytical solution for the tunnel deformation response can be derived. Comparing the results with measured data from Shanghai engineering, it can be found that the results given by this method closely match the measured data, demonstrating its reliability. When compared to the degenerate analytical solution, our method’s results better align with the measured data. Then a series of parameter analysis is conducted. The parameter study reveals that as the vertical clearance between two tunnels increases, the influence of shield tunnel excavation on the existing tunnel gradually diminishes. Increasing the ground loss ratio leads to a linear increase in the displacement and flexural moment of the existing tunnel. Additionally, enhancing the existing tunnel’s flexural stiffness significantly reduces the impact of shield tunnel excavation on the stress and deformation response of the existing tunnel.