## 1 Introduction  
The east coast of the Zhejiang and Fujian provinces has large deposits of silt and silty soil, as much as 30 to 45 m thick and 70 m high in some areas. The soil is characterized by high water content (40-70%), high compressibility, a porosity ratio (the ratio of the volume of the pores to the total volume) of 1.0 to 2.0, and a compression coefficient that ranges from 0.5 to 2.0 MPa\({}^{-1}\). Some areas have a large amount of gravel or hard plastic clay interlayer and lens distributed in the soil. It is difficult and expensive to break through the composite foundation of a conventional pile to reach the hard layer, especially when using a bored pile or replacing the road with a bridge. Therefore, researchers have developed a special reinforcement technology for deep soft-soil foundations interlayered with hard materials [1]: the bag-grouting-pile method, which was used to reinforce the foundation of the Ningbo-Taizhou-Wenzhou Railway. There are nearly ten reports on its design and construction technology, including design calculation [2], construction technology [3], subsidence characteristics [4], and engineering applications [5, 6]. [PERSON] used the ultrasonic-transmission method and the inclinometer method to detect the pile-diameter change in the bag grouting pile; the small-strain test was used to detect the pile-body integrity of the bag grouting pile [7]. [PERSON] used the inclinometer tube to detect the horizontal displacement of the soil around the pile when the pile was formed and obtained the change trend of the excess pore water pressure during the pile-formation process and the maximum value of the horizontal displacement of the soil around the pile on the upper and lower sides [8]. [PERSON] obtained the settlement deformation and distribution law of bagged piles through layered settlement pipes and inclinometer pipes [9]. [PERSON] obtained the distribution of vertical compressive strength and transverse shear strength of bagged piles through a full-scale loading test [10]. The test showed that bagged piles have strong anisotropic strength characteristics. There are few measured data on site. There is also a lack of research results that systematically analyze the working behavior of bagged-pile-composite foundations, predict post-construction settlement, and discuss the effect of reinforcement.  
The bag grouting pile, which is a new type of composite foundation developed by the author, was applied to the Ningbo-Taizhou-Wenzhou ballast track passenger railway for the first time. However, its reinforcement effect remains to be verified. A situ test was conducted in order to clarify the effect of reinforcement. The working properties of the bagged-pile-composite foundation were obtained by testing parameters such as lateral displacement, settlement, pile-soil stress, etc. Additionally, numerical calculation was used to analyze the influence of different design parameters on the reinforcement effect. The research results of this paper are the experimental data of the first application of bag grouting piles in a high-speed railway. The data can be used to reveal the action mechanism of the bag grouting pile and verify its reinforcement effect; it is helpful for the popularization and application of the foundation-treatment technology of the bag grouting pile, and has important engineering significance for the development of railway construction.