## 1 Introduction
The fluid-structure interaction model was employed to quantitatively investigate non-Darcy effects on natural convective flow and heat transmission in a square enclosure filled with porous media [44]. Figure 1: The schematic diagram of the deep excavation pit with an unclosed waterproof. The study considered different flow models for porous media, such as [PERSON]'s law model and the Darcy-Forchheimer model. The importance of soil characterization and the need to perform a water tightness assessment test were emphasized before the excavation stage, as applied to the excavation of a deep shaft of a high-speed train tunnel in Barcelona. However, their study could not accurately predict pumping settlements. To model the soil behavior during dewatering, a 3D hydromechanical numerical model and prior knowledge of the initial stress distribution would have been necessary [19]. Furthermore, the blocking effect on groundwater seepage under varying insertion depths of retaining walls in the aquifer was investigated through laboratory tests and numerical simulation. The results demonstrated that the drawdown of the groundwater level decreases with an increase in the insertion depth ratio of retaining structures in an aquifer. The optimal value of the insertion depth ratio (approximately 70%) was obtained for the retaining wall [45]. In the context of a water-rich gravel layer within the aquifer, the potential for safety accidents is increased due to several factors. These include the limited compressibility, absence of cohesive forces, and high permeability of the gravel layer. Groundwater seepage in the pebble stratum of water-rich sand is likely to carry away fine sand particles, leading to substantial water inrush and consequent settlement of the surrounding strata. This settlement poses a direct threat to the safety of nearby structures. In the case of deep foundation pits situated in ancient riverbed areas with gravel-confined aquifers, the prominent hydraulic conductivity, distinct anisotropy, and the presence of 3D flow patterns require the implementation of strategies to regulate groundwater levels inside and outside the foundation pit. By using such measures, groundwater levels can be managed, thereby reducing the potential risks associated with the unstable characteristics of the gravel-confined aquifer. However, most research conducted thus far has utilized numerical modeling techniques to study the 3D seepage flow of a deep excavation pit with an unclosed waterproof curtain. Few field test studies or physical experiments have been reported, implying that the seepage mechanism of a deep excavation pit with an unclosed waterproof curtain remains unclear and lacks concrete verification and evidence. Especially for the foundation pits constructed in gravel-confined aquifers, lowering groundwater levels in the aquifers with high permeability near a river is quite difficult. Pumping tests have been performed to verify whether the groundwater level can be lowered effectively in gravel-confined aquifer in a deep foundation pit near Giantang jiang River in Hangzhou. Super large pumping rates and limited drawdowns were observed. The results indicated that the pumping well group with a long filter tube could not lower the groundwater level to the designed drawdown where groundwater flowed horizontally. The expensive and dangerous underwater-excavation method had to be used later. Whether the groundwater level in gravel-confined aquifer can be lowered to designed drawdown becomes a vital question for the underground engineering construction in Hangzhou. In this paper, a three-floor underground land platform of Qianjiang Century City Station in Hangzhou Metro Line 6 was selected as the study site. Field tests were conducted to evaluate the 3D flow pattern of the dewatering of a deep foundation pit with an unclosed waterproof curtain. To understand the drawdown control mechanism of an unclosed waterproof curtain, two pumping and dewatering tests were carried out in an underground land platform pit.