## 1 Introduction  
Bag grouting pile reinforcement technology is an advanced foundation reinforcement technique derived from the original grouting reinforcement technology. It innovatively restrains cement mortar inside a geotextile bag, thus achieving the same reinforcement effects as the original grouting technology and preventing the cement mortar from polluting the surrounding soil. Moreover, the column formed by the solidified cement mortar integrates with the surrounding soil, leading to the formation of a composite foundation. This innovative technology significantly improves the foundation bearing capacity and reduces foundation settlement [1; 2]. It has great potential to reduce disturbances and pollution on an existing line during pile foundation construction. In recent years, most studies on bag grouting piles have focused on field tests, theoretical analysis, numerical simulation, and indoor tests. In terms of field tests, [PERSON] [3] conducted a study on the reinforcement of a saturated soft clay foundation project using bag groutingpiles, analyzed the soil displacement around the pile and super static pore water pressure during the pile forming process through a field test, and studied the soil-crowding effect of bag grouting piles under the condition of a saturated soft clay foundation. [PERSON] [4] pointed out that the use of bag grouting piles can effectively control foundation settlement by observing the internal settlement of the roadbed and the horizontal displacement of the foundation soil. [PERSON] [5] investigated the working characteristics of a composite foundation of bag grouting piles by combining the traditional cement mixing pile and bag grouting pile reinforcement technologies and studied the changes in lateral displacement, settlement, and pile-soil stress of a composite foundation by testing the Ningbo-Taizhou-Wenzhou ballast rail passenger railroad. In terms of theoretical analysis, [PERSON] [6] discussed the foundation settlement calculation method, carried out research on the calculation method for a bag grouting pile foundation, proposed a set of calculation methods for bag grouting pile settlement for the special case of a sandwich composite foundation, and verified the calculation results with each other and the measured data; [PERSON] [7] discussed the settlement calculation method for a bag grouting pile composite foundation, supplemented with calculation examples, and then conducted an analysis of the bag grouting pile composite. The traits of the composite foundation of bag grouting piles were analyzed and some conclusions were drawn. In terms of numerical simulation, [PERSON] [8] used Plaxis finite element software to build two numerical simulation models: with and without bag grouting pile treatment. Through consolidation analysis, settlement, superhydrostatic pressure, and stress were compared and analyzed to qualitatively derive the partial action mechanism of bag grouting piles; [PERSON] [9] used Flac3D finite difference software to build a numerical membrane bag grouting pile action model. Through software simulation, the expansion process of the membrane bag grouting pile was studied and compared with a theoretical solution to verify the applicability of the theoretical model. As for model tests, [PERSON] [10] investigated the construction process, pile formation law, and reinforcement effect of polymer bag grouting piles through model tests of pile formation by grouting in soils with different densities and different layering conditions; [PERSON] [11] used model tests as a research tool to investigate the pile formation mechanism on the basis of experimental research on the feasibility of pile formation by bag grouting piles. In conclusion, most previous studies have used a single method to study the bearing characteristics, without considering a comprehensive use of multiple methods. This paper takes Ningbo Hub's Zhuangqiao-Ningbo section, consisting of additional three- and four-line projects as an example.