## 1 Introduction  
In metropolitan areas such as Beijing and Shanghai, booming populations and a scarcity of urban land lead to excessive congestion. To help alleviate such congestion, cities are tending to build more high-rise buildings and develop the space available underground. This, in turn, is leading to an increase in the number of deep excavation projects being undertaken (for building basements and other underground engineering structures). However, excavation induces soil movement and, sometimes, collapse of the land. This can result in adjacent buildings and utilities becoming damaged and even produce casualties [1, 2]. Hence, suitable retaining structures need to be employed in these deep excavations and so engineers are facing severe challenges with respect to the design and construction of excavation support measures. Three of the excavation methods most commonly used for deep excavation are the cantilever full open cut method, braced excavation method, and anchored excavation method [3]. Each of these methods has its own characteristics and application conditions. Cantilever retaining walls keep the excavation stable and can be embedded in the soil without the temporary installation of struts. They thus provide a large amount of room for excavation and are cheap. However, the cantilever full open cut method is not able to sufficiently limit the deformation that occurs when the excavation is deep. The braced excavation method is the one most commonly used because it can limit the movement induced by the excavation process and be used in excavations of any width and at any depth [3]. However, the installation of struts may result in excavation activity being obstructed. In addition, the cost of constructing and demolishing struts is pretty high [4]. Moreover, accidents may occur, such as a strike by dropping strut and the collapse of the structure, due to strut failure [2; 5; 6]. For example, in 2010, some steel struts dropped during the excavation of Shunyi Station (part of Beijing's metro system, see Figure 1) which resulted in 2 deaths. The failure of struts also resulted in the collapse of Nicoll Highway in Singapore (see Figure 2) and 21 people died during the excavation of the Xianghu subway station (part of Hangzhou's metro system, see Figure 3) because of a collapse that occurred due to the poor state of the connection between struts and wales. Anchors can be substituted for struts to free up excavation space and improve the efficiency of the excavation and structure construction processes. However, anchors cannot work well if the soil layers are weak or lie below the groundwater table in granular soils. (An excavation collapsed in Foshan in 2015, for example, because the bearing capacity of an anchor-supported wall structure became degraded, see Figure 4. Another collapse happened in Nanning in 2019 for the same reason, see Figure 5) The use of anchors is also restricted in private areas. The retaining walls used in the excavation methods mentioned above can be divided into four types according to their construction characteristics, including soldier piles, steel sheet piles, column piles and diaphragm walls. Soldier piles and steel sheet piles are employed in some excavations for their fast construction with lower cost. Furthermore, the piles can be pulled out after the completion of the basement and be reusable in the following excavations. However, the two methods have their own drawbacks, such as the considerable noise caused during striking piles, low stiffness with large deformation during excavation which make them inappropriate for deep excavations [3; 7]. The column pile method is to construct rows of concrete piles as retaining walls by either the cast-in situ pile method or the precast pile method. They can be designed as different patterns, such as an independent pattern, the S pattern, the line pattern, the overlapping pattern and the mixed pattern to satisfy different requirements, and the stiffness can be great with appropriate design.