Reinforcement of Interlocked Steel Pipe Pile Cofferdam for Chongqing Guanzhanhe Bridge Main Pier Foundation

Literature Overview

The paper by Qian Ji and colleagues from Chongqing Jiaotong University, published in Bridge Construction in 2024, documents a critical and highly practical engineering case involving the emergency reinforcement of an interlocked steel pipe pile cofferdam during the construction of the Guanzhanhe Bridge. The main bridge is a three-span continuous rigid frame bridge with span configuration of 62+110+62 meters, and the main pier foundation employs interlocked steel pipe pile cofferdams. The project was supported by the China Postdoctoral Science Foundation (2021M702782) and the Chongqing Natural Science Foundation (CSTB2023NSCQ-MSX0633). The case is particularly instructive because it addresses a real-world emergency scenario where extreme weather events threatened the structural integrity of the cofferdam during active construction.

Engineering Background and Problem Statement

The cofferdam was designed with a normal construction water level of +325.300 m and a construction control water level of +330.500 m. After the four-side interlocked steel pipe pile installation and the first four internal struts were completed, an extreme weather event caused the Changshou Lake water level to rise to +332.200 m, exceeding the design water level by 1.7 meters and posing an imminent threat to cofferdam safety. This scenario is not uncommon in mountainous bridge construction projects in southwestern China where rainfall patterns can change rapidly and dramatically, making it essential to have contingency reinforcement plans in place.

The reinforcement solution adopted involved underwater construction of internal struts after equalizing the internal and external water levels. The interlocked steel pipe piles were extended to elevation +334.000 m, the existing four internal struts were removed, and six new internal struts were installed. This approach effectively transformed the cofferdam from a four-strut configuration to a six-strut configuration while simultaneously increasing the pile embedment height.

Finite Element Analysis Results

The structural analysis was performed using MIDAS Civil software with finite element models developed for both the pre-reinforcement and post-reinforcement conditions. The results demonstrate a substantial improvement in structural performance:

The 49.3% reduction in maximum positive stress is remarkable and demonstrates the effectiveness of the reinforcement approach. The increased bearing capacity to +333.500 m provides a safety margin of 1.3 meters above the actual peak water level of +332.200 m that occurred during the emergency. Importantly, both strength and stiffness reserves were maintained after reinforcement, ensuring that the cofferdam could safely withstand subsequent construction loads including the pouring of the raft foundation.

Technical Analysis of the Reinforcement Methodology

The decision to perform underwater strut installation represents a pragmatic engineering solution that balances safety, schedule, and cost considerations. The alternative of dewatering the cofferdam before reinforcement would have required significant time and resources and might not have been feasible given the urgency of the situation. By equalizing internal and external water pressures first, the reinforcement operations could proceed without the risk of cofferdam failure during the critical transition period.

The extension of the steel pipe piles to +334.000 m serves a dual purpose: it increases the overall structural height to provide additional freeboard above the maximum anticipated water level, and it extends the embedment depth to improve the passive resistance against overturning and sliding. The increase from four to six internal struts reduces the unsupported span length of the steel pipe piles, which directly reduces bending moments and lateral deflections in accordance with beam-on-elastic-foundation theory.

Engineering Practice Recommendations

This case study offers several valuable lessons for engineers involved in cofferdam design and construction in regions susceptible to extreme weather events. First, the design water level should incorporate a sufficient safety margin against forecasted extreme events, ideally based on probabilistic hydrological analysis rather than historical maximums alone. Second, pre-planned reinforcement schemes should be developed during the design stage rather than improvised during emergencies, with detailed calculations and material procurement plans in place. Third, the finite element modeling approach used here provides a reliable tool for rapid assessment of reinforcement options, and engineers should maintain proficiency in such tools for emergency response scenarios.

The successful completion of the raft foundation construction after reinforcement confirms that the proposed solution was technically sound and practically feasible. The case demonstrates that even when design assumptions are exceeded by unforeseen environmental conditions, appropriate engineering judgment combined with analytical tools can restore structural safety without abandoning the project.

Summary and Concluding Remarks

The Guanzhanhe Bridge cofferdam reinforcement case stands as an excellent example of practical engineering problem-solving under adverse conditions. The combination of increased pile height, additional internal struts, and careful finite element verification produced a solution that reduced peak stresses by nearly half while providing substantial capacity reserves. For engineers working on bridge foundations in mountainous regions with variable hydrological conditions, this case reinforces the importance of designing cofferdams with adequate margins and maintaining flexibility for adaptive reinforcement during construction. The methodology presented here—particularly the use of underwater strut installation as an emergency measure—should be incorporated into standard practice guidelines for cofferdam construction in regions prone to extreme weather events.