# Water Basin Effect of Cofferdam Foundation Pit
## 6 Conclusions  
This study introduces the water basin effect into the cofferdam foundation pits engineering. Based on the stress and deformation theoretical analysis of the simply supported beam under various loads, the evolution characteristics of the stress and deformation of the steel pipe pile and the bottom-seal are analyzed and derived by using the superposition method and the incremental method for four typical construction stages during the construction of the cofferdam. And the water basin effect mechanism and the evolution of steel pipe pile deformation in cofferdam foundation pits are systematically elucidated. Theoretical derivations are validated and applied through full-process hydrological-mechanical coupling simulation and in situ monitoring of the steel pipe pile cofferdam foundation pit for the 6# pier in Xi'an Metro Line 10's Jingwei River Rail-Shared Bridge. The main conclusions are as follows:  
Water basin effect characterization: Under the combined action of the water basin effect and differential hydro-sedimentary pressures across the cofferdam boundary, the bottom seal exhibits heightened uplift susceptibility and potential anti-uplift failures even at relatively low hydraulic gradients. Dewatering stage classification: The cofferdam dewatering process is systematically divided into four stages: Riverbed excavation for bottom sealing preparation. Dewatering to the elevation of the second internal support. Dewatering to the elevation of the third internal support. Final dewatering to the bottom sealing elevation. Steel pipe pile model: The steel pipe pile structure is simplified as a single-span or multi-span cantilever continuous beam. Using the superposition principle, deformation evolution equations for this statically indeterminate structure are derived for each of the four construction stages, capturing the incremental load effects during dewatering. Bottom sealing model: The bottom sealing concrete is modeled as a doubly fixed-end beam. By applying the principle of superposition, the deflection curve equation is established under combined actions of uniform vertical loads (self-weight, buoyancy) and end-fixed additional loads (differential water and soil pressures), providing a theoretical basis for deformation prediction. Simulation validation: Full-process hydrological-mechanical coupling simulation of the actual cofferdam project confirms the deformation characteristics under the water basin effect. The steel pipe pile exhibits consistent \"convex\" horizontal deformation along the depth direction across all working conditions, with a trend of increasing first and then decreasing as construction progresses. At the main pier completion stage, the maximum horizontal deformation of 20.72 mm occurs at 14 m below the pile top, showing good agreement with both analytical solutions and field measurements. This paper mainly carries out the sublimation of theory and the mining of mechanism on the water basin effect, and the research results of this are more generalizable. The theoretical model of this article is derived based on the homogeneous strata and relatively simple load, and relatively regular shape, the applicability of the relevant analytical results is relatively good and has general applicability but for the occasional load or for large-scale or complex cofferdam projects, it is necessary to conduct specialized research modify or further. The next step can also attempt to break through the theoretical analysis of the water basin effect in terms of layered strata, fully continuous dynamic drain water, and simplification of pile-bottom contact supports. Although simplifications were made in theoretical derivation and numerical modeling, the research results offer valuable insights into the deformation mechanisms of cofferdam structures under dewatering. These findings can serve as a reference for similar engineering projects involving deep foundation pits and water-retaining structures. Conceptualization, Q.Z., X.Z. and N.L. ; methodology, N.L. and G.L. ; software, N.L. and G.L.