## 1 Introduction  
In recent years, large-scale damage and even continuous collapse accidents have often occurred in foundation pits; moreover, recovery from such accidents is difficult, and research on the continuous failure mechanism has been limited [1, 2, 3]. Because the scale of foundation pits is increasing and adjacent environmental facilities are becoming increasingly complex, evaluating and controlling the deformation of foundation pits and their impact on the surrounding environment is an important challenge for foundation pit engineering [4, 5, 6]. Researchers have extensively investigated foundation pit excavation and noted that high confined water, poor geology and support structure defects are the main causes of foundation pit damage [7; 8; 9; 10]. For the past few years, researchers have analyzed cases of deep and large foundation pits, providing guidance for the treatment of foundation pit risk [11; 12; 13; 14]. A high water head in a confined aquifer is the main cause of pit surge accidents; [PERSON] et al. [15] used FLAC3D simulation analysis to classify pit bottom uplift deformation caused by confined water into three stages--slow and uniform deformation, accelerated deformation and sharp and uniform deformation--and proposed a numerical method to determine the critical water head (\(H_{\text{wcr}}\)) of a pit surge, providing a reference for foundation pit design. Taking a foundation pit project on the north bank of the Yangtze River as an example, [PERSON] et al. [16] reported that the construction defects of the three-axis mixing pile and the potential weak area around the abandoned pile are the main reasons for a foundation pit surge accident caused by groundwater seepage. [PERSON] et al. [17] further calculated and analyzed the influence of water seepage caused by the low quality of the suspended impervious curtain during the excavation dewatering process and reported that the amount of water seepage, the water level outside the pit, and the surface settlement are closely related to the permeability coefficient at the seepage point. In the event of an accident, effective emergency disposal is particularly important. [PERSON] et al. [18] indicated that improper disposal after a surge accident caused continuous water gushing at the bottom of the pit, intermittent sand transport and severe surface settlement, which effectively promoted emergency disposal. In addition, the excavation earthwork of the foundation pit sometimes fails to leave the site in time, causing great hidden danger to the stability of the foundation pit. To date, research on the loading of foundation pits has focused mainly on the influence of loading outside the pit on the supporting structure and the deformation of the bottom of the pit [19; 20], and research on back pressure soil has focused mainly on analyzing the supporting effect of the foundation pit [21; 22; 23]. Thus, there is a lack of research on the influence of loading in pits on soil deformation and groundwater seepage. Foundation pit disasters continue to occur, and the origins of the accidents are not clear [24; 25]. Most previous studies have focused on the excavation of foundation pits in cohesive soil layers [26; 27; 28], whereas relatively few studies have investigated the excavation of deep foundation pits in coastal water-rich sandy strata [29]. There are many islands and coastal cities in the tropics. With the continuous advancement of urbanization, many deep foundation pit projects need to be constructed [30; 31; 32]. These projects are characterized by high groundwater levels, thick sand layers, large excavation depths and narrow engineering sites [33; 34; 35]. Their unique hydrogeological characteristics restrict the design and construction of such projects. Therefore, this paper takes a foundation pit project in Haikou, China, as an example.