## 1 Introduction  
In the past, bored cast-in-place piles, precast-driven piles, and steel-reinforced composite piles were widely used in pile foundation projects both domestically and internationally. However, bored cast-in-place piles often caused mud pollution, and problems such as mud skin on the pile side, soft soil collapse, and pile bottom sediment were also common. Precast-driven piles faced difficulties in driving into hard layers, significant soil displacement effects, high noise levels during pile driving, and insufficiently secure pile joint connections. Steel-reinforced composite piles required high construction precision, and the post-inserted piles had low shear strength and were prone to tilting, overstepping, and poor concentricity. Their construction time was limited, insertion was difficult, the pile driver was too tall, and the cost was relatively high. In addition, most existing piles were of equal diameter, [1, 2, 3, 4, 5, 6, 7] resulting in waste of pile body strength in the deep part and poor economy. [PERSON] [8] systematically analyzed the vertical bearing characteristics of large-diameter steel tube concrete composite piles in the pile foundation of a highway bridge over a river. The results showed that due to the superior engineering properties of steel casing materials compared to reinforced concrete materials, the mechanical properties of steel tube concrete composite piles were superior to those of reinforced concrete piles. Under the same working conditions, the safety reserve of the bearing capacity of steel tube concrete composite piles was much higher than that of reinforced concrete piles, which was obviously not conducive to economic and social benefits. At that time, steel tube concrete piles [9; 10; 11; 12; 13] generally adopted the mode of driving in the steel tube first and then pouring the pile core concrete. It was difficult to ensure the verticality of the steel tube. The steel tube was usually an open pile, and after driving, there was a lot of soil in the pile. It was difficult to remove the soil in the steel tube. After cleaning, there was still a lot of residual soil and groundwater in the hole, and the quality of concrete pouring in the steel tube was not easy to ensure. The construction was difficult, the process of removing soil in the steel tube was complex, and the construction cost was high. Aiming at the problem of soft soil subgrade reinforcement treatment near operating lines and the defects of traditional composite piles and bored cast-in-place piles, Hohai University independently developed a new type of steel tube concrete mixing composite pile (Patent No. : ZL.2017 1 1294958.6). This new pile type could be completed with a single-machine tool. The conventional mixing pile machine was improved in terms of power to achieve the synchronous sinking of steel pipes and rotary jet grouting mixing operations to form piles. The steel tubes were connected by a threaded joint rotation, which was fast and had high connection strength. After pouring concrete into the steel tube, the shear strength at the joint was consistent with that of the pile body, and there was no weak connection point between sections. The steel tube was connected by a rod method for rotary spraying, mixing, and pouring in one operation, avoiding the problems of high working space requirements, easy eccentricity, and weak shear capacity in conventional core-inserting pile operations. The construction machine could be adjusted in height, ranging from 4 m to 40 m, and the length of each section of the steel tube could be adjusted according to the site environment. The machine could also use new mixing blades to crush and cut underground geotechnical materials such as drainage boards, adapting to various site environments. Before pouring concrete, the center of the composite pile was an empty steel tube, and a measuring rope could be placed in the empty steel tube to measure the pile length one by one. The detection method was simple and reliable.