# Review of the Double-Row Pile Supporting Structure and Its Force and Deformation Characteristics
## 1 Introduction  
Double-row pile supporting systems are mainly used in temporary engineering as a protection measure to ensure the smooth construction of the underground section of the main part of the structures and the safety of the environment around the foundation pit [1; 2; 3; 4]. During the process of urbanization, the density of high-rise, super-high-rise, and underground buildings in cities has increased, and the utilization rate of underground space has been increasing. Influenced by the foundation of adjacent structures and the complex underground environment, the double-row pile supporting structure has attracted wide attention [5; 6; 7; 8]. Based on the existing single-row pile, and by adding a row of piles behind it, the double-row pile supporting structure evolves from this. The double-row pile structure is mainly composed of the front pile, the back pile, the crown beam, and the connecting beam [9; 10; 11; 12; 13; 14]. As far as the single-row pile-supporting structure is concerned, the new type of pile-row-supporting structure has the good stress and deformation characteristics of the single-row pile-supporting structure and has better stiffness and excellent anti-lateral displacement ability [15; 16; 17; 18; 19; 20; 21]. This makes the horizontal displacement and vertical displacement of the support section better controlled and able to meet the requirements of \"solving the large height difference in a small space\"; that is, it can have more prominent advantages in the case of a narrow construction space, strict deformation, and a deep excavation depth [22; 23; 24; 25]. In recent years, research on double-row pile supporting structures has achieved significant results. For example: (1) The control effect of the pile diameter, the pile length, the crown beam, the connecting beam, and other basic structural dimensions, as well as pile spacing and row spacing, on the horizontal deformation and vertical settlement of the whole double-row pile structure system and the cost control of foundation pit engineering [26; 27; 28; 29; 30]; (2) The influence of the development of the soil arch effect on the deformation of the double-row pile supporting structure during the excavation of the foundation pit [31; 32; 33]; (3) Based on the numerical simulation analysis, the deformation mechanism and deformation characteristics of the overall double-row pile structure, the front pile, the back pile, the crown beam, the connecting beam, etc. [34; 35; 36]; (4) Optimization and innovation of the pile-soil interaction calculation model [37; 38; 39; 40]. Most of the above studies have been based on numerical simulation analysis, experimental research, and other methods. Generally, they researched the dimension parameters of double-row pile supporting structures, pile-soil stress and deformation characteristics, and calculation models. According to the available data, in the same foundation pit wall protection project, the size parameters of the components, such as pile diameter and pile length, and the design parameters, such as pile spacing and row spacing, are often variable. However, there is a node near which the structural parameters can give full play to their supporting role in controlling the deformation of the foundation pit. When it is far away from this node, the supporting effect of the double-row pile supporting structure will not only be greatly reduced but also increase the project cost in terms of cost control [41; 42; 43; 44]. For example, [PERSON] et al. [29] found through numerical modeling that the control effect of a double-row pile supporting structure on the stability of the foundation pit is similar to that of a single-row pile when the spacing of the double-row pile supporting structure is too small. Appropriately increasing the double-row pile gap can have a good control effect on reducing the level shift of the supporting structure. In addition, based on numerical modeling and scale tests, [PERSON] et al.