## 4 Conclusions  
After conducting two multi-depth pumping tests with different filter lengths to study the 3D flow mechanism during deep excavation within the gravel layer of Qianjiang Century City Station of Hangzhou Metro Line 6, the following conclusions can be drawn:  
1. During the pumping tests, a robust 3D flow phenomenon was generated around the filter, forming a drawdown cone with a significant hydraulic gradient. The maximum drawdown and hydraulic gradient inside the pit were located 3 m below the bottom of the filter. The observed 3D flow confirmed the existence of 3D flow deduced in theoretical analysis. The 3D flow combined with anisotropic gravel-confined aquifers was significant in controlling drawdown during foundation pit dewatering.
2. The dewatering scheme using longer filters, larger pump rates, and filters situated closer to the aquifer roof could enhance the dewatering effectiveness while minimizing the impact on the surrounding geological environment. This conclusion is supported by the study conducted by [PERSON] et al. [29], which found that the pumping volume inside a pit increased with increasing filter length.
3. The diaphragm wall-pumping well effect efficiently controlled the water head outside the pit and the subsidence, which aligns with the findings of [PERSON] et al. [35; 36; 45; 46; 47]. This effect can regulate the drawdown of the deep foundation pit of a gravel-confined aquifer in an ancient river distribution area with an unclosed waterproof curtain.
4. The efficiency of the diaphragm wall-pumping well effect depended on the anisotropy of gravel-confined aquifers. For ellipsoid gravels, their long axis generally followed the horizontal direction to obtain the minimum potential energy during its sedimentation process, and the vertical hydraulic conductivity was at least one order of magnitude smaller than the horizontal direction, with significant anisotropy. The effect was obvious and can be utilized to control the drawdown inside and outside a foundation pit. For spherical gravels with less anisotropic, the diaphragm wall-pumping well effect was relatively weak. The penetrating depth of the diaphragm wall had to be improved to increase the length of the permeability path to reach the same effect.
5. When the penetrating depth of the diaphragm wall into gravel-confined aquifer was limited, a short filter with high strength and large porosity was necessary to reach the designed effect.  
**Author Contributions:** Conceptualization, [PERSON], Y.W. and Z.Y.; methodology, [PERSON] and Y.W.; software, Y.W.; validation, J.W., Y.W. and N. X.; formal analysis, [PERSON], Y.W. and N. X.; investigation, J.L., Y.S., X.H, Z.Y. and R.Z.; resources, J.W. and J.L.; data curation, [PERSON], Y.S., X.H. and Z.Y.; writing--original draft preparation, [PERSON], [PERSON] and [PERSON]; writing--review and editing, [PERSON] and N.X.; visualization, [PERSON]; supervision, [PERSON]; project administration, [PERSON]; funding acquisition, [PERSON],Y.S., X.H. and J.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Shanghai Municipal Science and Technology Project (18 DZ1201301; 19 DZ1200900); Shanghai Institute of Geological Survey (2023(D)-003(F)-02); Key Laboratory of Land Subsidence Monitoring and Prevention, Ministry of Natural Resources of the People's Republic of China (No. KLLSMP202101; KLLSMP202201); Shanghai Municipal Science and Technology Major Project (2021 SHZDZX0100) and the Fundamental Research Funds for the Central Universities.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.