# Water Basin Effect of Cofferdam Foundation Pit
## 1 Introduction
The cofferdam foundation pit needs to address not only its own structural and constructional effects [15; 16; 19] (including the stability of the cofferdam foundation pit itself, stress concentration, material fatigue, and additional impacts during construction and demolition), but also hydraulic effects [5; 17; 7] (such as changes in flow direction and velocity, local scouring and siltation, water level rise, and flood risks), geological and foundation effects [25; 26] (including foundation settlement and displacement, seepage, and piping risks), as well as environmental and ecological effects (such as water quality and pollution, ecosystem disturbance, noise, and dust pollution). [PERSON] integrated saturated-unsaturated seepage theory and strength reduction methods within finite element systems to analyze fluid-solid coupling stability and seepage failure during water-level fluctuations in dump-fill cofferdams [27]. [PERSON] & [PERSON] utilized fluid dynamics methods to analyze the influence of external water flow characteristics on cofferdam deformation and stability [5; 7]. [PERSON] evaluated random wave pressure on sea-crossing bridge cofferdams under tropical cyclones using measured and numerical data [13]. [PERSON] simulated flow field variations and scour depth around cofferdams using two-dimensional hydrodynamic analysis and bed-load transport models [28]. [PERSON] & [PERSON] investigated seabed response around dumbbell cofferdams under combined wave-current loading [29], along with deformation characteristics and stability during super-historical floods, proposing emergency reinforcement strategies [17]. Other research numerically explores complex wave hydrodynamics under multidirectional extreme waves, systematically analyzing key factors including significant wave height, peak period, water depth, spreading angle, incident wave angle, and submergence depth [30]. However, these studies frequently overlook the influence of hydro-mechanical coupling characteristics on bottom-seal concrete deformation and rarely employ theoretical approaches to analyze cofferdam support structure system deformation. Bottom-sealing concrete placement is a pivotal construction phase, preventing hydraulic seepage and maintaining cofferdam structural equilibrium. The geometric dimensions and construction quality of this concrete layer profoundly impact its functionality. Determining an optimal thickness for bottom-sealing concrete under substantial hydraulic heads remains challenging, compounded by limited relevant investigations and a lack of consolidated theoretical frameworks [31]. Cofferdam stability is paramount for bridge construction safety, also requiring comprehensive consideration of hydrological parameters, geological conditions, and environmental protection requirements. Under complex hydrodynamic conditions, steel-concrete composite single-wall ciasson cofferdams may experience bottom-seal concrete leakage, internal support instability, or even structural collapse [32]. These issues persist as significant challenges in pier construction within deep sand layers, fundamentally stemming from the water basin effect [33]. However, few people deeply realize that the inherent mechanism and essence of deformation and failure of the cofferdam foundation pit is a problem of water bowl effect, and so, almost no one has particularly proposed and studied the water bowl effect problem unique to the cofferdam foundation pit. This effect significantly impacts the support structure system's mechanical behavior and stability. Although prior research has addressed seepage mechanisms [34; 35], failure modes, and flood-control measures [23] via hydro-mechanical coupling analyses, the specific \"water basin effect\" within the steel pipe pile-bottom-seal concrete system formed after sealing in underwater sand layers remains inadequately explored. Derived from the principle of buoyancy or Archimedes' principle, the water basin effect was initially proposed in the context of building foundation pits [33].