## 1 Introduction  
For the past few years, foreshore land reclamation has been a solution to the problem of land scarcity in coastal regions due to the increasing population and economic boost [1]. Many deep excavations are constructed in the area where the stratums often have a high groundwater head [2; 3; 4]. The dewatering of deep excavation is essential for ensuring the safety of the excavation works, especially in well-permeable confined aquifers [5]. Cut-off walls are widely used to block groundwater seepage into the excavation, which can result in quicksand and internal piping erosion [6; 7]. For deep excavation pits that require groundwater pumping in thick confined aquifers, a suspended cut-off wall is usually used to increase the seepage path [8; 9]. As a result, hydraulic connection inside and outside the excavation pit is reduced, and the groundwater level inside the excavation pit can be lowered below the pit excavation bottom [10]. Therefore, it is crucial to predict the distribution of groundwater head induced by the dewatering of excavation before construction [11]. The existing research on dewatering in the excavation pits with cut-off walls are commonly based on the following methods: the analytical method [12; 13; 14; 15], the numerical method [16; 17; 18; 19] and the experimental method [20; 21]. The commonly used numerical methods are the finite difference method [22; 23], finite volume method [24] and finite element method [25], which can adapt to complex boundary conditions to calculate the head variations during dewatering. However, numerical analysis depends on the appropriate selection of parameters and does not directly reveal underlying mechanisms. Additionally, computational modeling requires significant time, which can be challenging for engineers needing on-site solutions. In order to provide the experimental basis for numerical results, [PERSON] et al. conducted a series of laboratory investigations and studied the distribution of groundwater head during dewatering [21]. However, the experimental method has limitations, including issues with its applicability in general engineering scenarios and challenges in simulating complex hydrogeological conditions. While analytical solutions offer convenience, they are often based on several assumptions [26; 27], which makes them unsuitable for the calculation of groundwater head during dewatering in an excavation pit with a cut-off wall, because of the existence of a mixed boundary condition [28; 29]. With the help of a conformal transformation method, [PERSON] and [PERSON] gave an implicit expression of the seepage field around the foundation pit in a homogeneous soil layer [30]. On this basis, [PERSON] assume that one side of a cut-off wall is a free water surface and obtained an implicit expression of a seepage field [31]. When it comes to complex problems such as group well dewatering in an excavation pit, the simplification of the large well method is necessary [32]. With this method, [PERSON] et al. derived an analytical solution based on the confined-unconfined unsteady groundwater pumping flow model to assess groundwater level trends during excavation [33]. However, the anisotropic permeability of soil has not been considered in these analytical solutions. To solve this problem, [PERSON] et al. derived a semi-analytical solution of two-dimensional steady seepage around the excavation pit in a confined aquifer with anisotropic permeability [34]. In summary, the hydraulic property of the aquifer, hydrogeological conditions, and the embedding depth of cut-off walls, among other engineering parameters, all influence the dewatering effectiveness of excavation pits. Considering these factors is crucial for developing groundwater control schemes that not only meet economic considerations but also fulfill environmental impact requirements. In addition, the problem of variation in groundwater level over time during dewatering has not been addressed.