# Water Basin Effect of Cofferdam Foundation Pit
## 1 Introduction  
The construction of large-span bridge bearing platforms presents significant challenges, including demanding foundation pit excavation, heightened risks of base uplift, critical requirements for secure cofferdam embedment, and susceptibility of mass concrete to cracking [1]. Among these, river-crossing bridge pier cofferdams are particularly crucial for constructing underwater foundations. Cofferdams serve as indispensable temporary structures across diverse domains such as hydrological engineering [2], municipal infrastructure [3], transportation engineering [4], ocean engineering [5, 6], port construction [7, 8],waterway projects [9], and ecological engineering [10]. Their primary function is to isolate construction zones from water ingress, thereby creating dry working environments [11]. Continuous advancements in engineering technology, integrating novel materials and methodologies, progressively enhance cofferdam efficiency and safety. Alongside the sustained development of China's infrastructure capabilities, the proliferation of river- and sea-crossing bridges has stimulated substantial research on pier cofferdams [12; 13; 14]. Key research areas encompass innovative cofferdam configurations [4], structural optimization [15], structural responses during construction phases [16], and stability under extreme loading conditions [5; 17], contributing significantly to advancements in China's water-crossing bridge engineering. Existing studies predominantly employ numerical simulations, model tests, and in situ monitoring to analyze the effects of construction sequences [16], loading scenarios, enclosure structure designs, and stratum properties [18] on cofferdam deformation, stress distribution, and stability. [PERSON] & [PERSON] investigated mechanical properties, deformation characteristics, and design optimization based on actual foundation pit construction procedures, integrating numerical simulation with on-site monitoring [15; 19]. [PERSON] analyzed cofferdam deformation and structural behavior using the principle of equivalent bending stiffness, while detailed numerical simulations examined the influence of parameters such as pile length, dam width, tie rod spacing, and pile spacing [20]. Novel methods simulating penetration processes, accounting for deformation in slender profiles with low bending and torsional stiffness, have been developed and validated via model tests [21]. [PERSON] explored internal deformation mechanisms within double-row pile anti-sliding systems, using numerical simulations and transparent soil model tests to investigate the influence of section shape, row spacing, and pile layout on soil-arching effect formation, development, and failure [22]. [PERSON] developed physical and numerical models to study how short rear piles affect earth pressure evolution in stabilized soil [23], while [PERSON]'s research addressed the bearing capacity of double-row steel sheet piles based on reinforcement depth and inter-pile soil properties [24]. Some studies have formulated multi-objective optimization models to address the lack of systematic, practical evaluation frameworks for selecting steel pipe pile cofferdam construction schemes [4]. [PERSON] investigated failure mechanisms of nonuniform geotextile-reinforced cofferdams under seepage and excavation on soft sediments [25]. [PERSON] demonstrated that spatial heterogeneity and degradation of soil properties also significantly impact cofferdam stability in soft soil foundations [26]. However, there is still a lack of theoretical analysis research on the essential characteristics of the stress and deformation of the cofferdam structure system, especially the evolution theory of the stress and deformation during the cofferdam construction process of drain water, excavation, and support.