Analysis of Steel Pipe Pile Cofferdam Structures Under Unbalanced Loading Conditions

Engineering Background and Challenge Description

This study, published in 2024 in the journal Construction Technology (Vol. 53, No. 19, pp. 8–13), addresses a challenging geotechnical engineering problem encountered during the expansion of the Jing-Tai Expressway main bridge P3 pier at the Qihe-Jinan section. The deep foundation pit is located along the Yellow River bank, presenting unique challenges including significant downstream backwater scouring, proximity to existing bridge foundations, and the action of unbalanced soil and water pressure on the cofferdam structure. The research was supported by the Shandong Provincial Natural Science Foundation (Grant No. ZR2021ME238) and was conducted by researchers from the School of Civil Engineering at Shandong Jianzhu University.

The unbalanced loading condition arises because the cofferdam is subjected to full soil and water pressure on the upstream side while the downstream side experiences reduced loading due to scouring and the presence of the existing bridge foundation. This asymmetric loading creates complex stress states in the steel pipe pile cofferdam and its support structures, posing risks of excessive deformation, structural failure, and settlement of adjacent infrastructure.

Finite Element Modeling and Analytical Framework

A three-dimensional finite element model was developed to simulate the construction sequence and the structural response of the steel pipe pile cofferdam under unbalanced loading conditions. The model incorporates the soil-pile interaction, the support structure behavior, and the construction staging. The construction process was divided into multiple analysis steps, each corresponding to a distinct stage of excavation and support installation. The finite element analysis considered nonlinear material behavior of the soil, geometric nonlinearity due to large deformations, and the progressive loading associated with the construction sequence.

The modeling approach followed a systematic framework that included: site characterization and material property determination, boundary condition definition based on geological and hydrological data, construction staging definition, mesh generation with appropriate element types and densities, and validation against field monitoring data. This methodology ensures that the numerical model accurately represents the physical behavior of the cofferdam system throughout the construction process.

Deformation and Strength Analysis Results

The finite element analysis revealed that the deformation of the steel pipe pile cofferdam increases progressively with each construction stage, with the maximum deformation occurring at the fifth analysis step, which corresponds to the construction of the cap structure. The maximum deformation value was calculated as 30.40 mm, located at the short-side water surface level. The deformation distribution showed a characteristic pattern where the backwater side (upstream) deformation exceeds the downstream side deformation, consistent with the unbalanced loading condition.

The support structure exhibited pronounced asymmetry in the loading distribution, with the long-side left and right sides experiencing significantly different stress levels. The maximum stress in the support structure occurred at the fourth analysis step, during the excavation to pit bottom stage, with a value of 121.47 MPa. This value was compared against the allowable stress of 215 MPa for the structural steel, confirming that the strength requirement is satisfied with a safety factor of approximately 1.77.

Impact on Adjacent Structures and Verification

A critical aspect of this study is the assessment of the impact of cofferdam construction on the adjacent existing bridge. The finite element model was extended to include the existing bridge foundation and the intervening soil mass. The analysis showed that the maximum settlement and maximum inclination of the adjacent bridge were both within the limits specified by relevant design codes. This finding is of significant practical importance, as it demonstrates that the cofferdam design and construction method can be safely implemented in proximity to existing infrastructure without causing unacceptable damage.

The validation of the finite element model against field monitoring data confirmed the accuracy of the numerical predictions. The measured deformation values were in good agreement with the calculated results, with deviations within acceptable engineering tolerances. This validation provides confidence in the model's predictive capability for similar engineering scenarios and supports its use as a design and monitoring tool in future projects.

Summary and Practical Recommendations

This study provides a comprehensive analysis of steel pipe pile cofferdam behavior under unbalanced loading conditions in a complex riverbank environment. The finite element modeling approach, validated against field data, offers a reliable tool for predicting deformation and stress distributions during construction. The key finding that the maximum deformation of 30.40 mm and the maximum support stress of 121.47 MPa are within acceptable limits provides confidence in the structural adequacy of the design. For engineers facing similar challenges with unbalanced loading in deep foundation pit projects near existing infrastructure, this work offers a validated analytical framework and practical guidance for design verification, construction monitoring, and risk management. The emphasis on comparing calculated results with field measurements is a best practice that should be adopted in all such projects to ensure construction safety and structural integrity.