# Ground Deformation Associated with Deep Excavations in Beijing, China
## 2 Typical Details concerning Deep Excavation
### Support System  
Generally, support systems for excavations can be divided into three categories, as follows: unrestrained systems, back-restrained systems, and front-restrained systems [4]. Contiguous pile walls (CPWs), deep soil mixing (DSM) columns, diaphragm walls (DWs), and compound soil nail (CSN) walls, among others, are commonly used in Beijing. These systems are composed of both unrestrained and restrained support systems (see Figure 2).  
In Beijing, low-cost and quickly constructed CPWs are commonly utilized as temporary walls. The DSM columns, which are consist of in situ soil mixed with cement, serve as  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Line No.** & **Service Time** & \begin{tabular}{c} **Mileage** \\ **/km** \\ \end{tabular} & **Station No.** & \
\begin{tabular}{c} **Open Cut** \\ **Method** \\ \end{tabular} &
\begin{tabular}{c} **Proportion** \\ **/\%** \\ \end{tabular} \\ \hline
5 & 2007 & 27.6 & 23 & 17 & 73.9 \\
10 & 2008 & 24.7 & 22 & 9 & 40.9 \\
4 & 2009 & 28.2 & 24 & 12 & 50.0 \\
9 & 2011 & 16.5 & 14 & 7 & 50 \\
7 & 2014 & 23.7 & 20 & 15 & 75 \\
14 & 2015 & 47.3 & 37 & 21 & 57 \\
6 & 2021 & 52.9 & 35 & 23 & 66 \\
8 & 2021 & 51.6 & 35 & 22 & 63 \\ Total & 272.5 & 210 & 126 & 60 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Information about Beijing subway lines.  
Figure 1: Diagram of the layered and segmental construction method in Beijing.  
Figure 2: Typical support system in Beijing. (**a**) Bored pile wall with steel struts in Beijing Subway Line 6; (**b**) Bored pile wall with an anchor in Beijing Subway Line 8.  
waterproof curtains, and they are typically built behind the CPWs. Since the 1990s, DSM columns have gained popularity for their effectiveness as retaining structures. Due to their low tensile strength, the DSM walls are usually very thick and they lack struts. The practical maximum depth for the cantilever pile and deep soil mixing (DSM) columns are generally less than 6-7 m, although depths of up to 10 m have been achieved in Beijing. Compared with CPWs and DSM, DWs can offer effective water tightness and relatively high stiffness, which are often reserved for use as both a retaining wall and a basement wall due to their high cost and complicated construction technology. The diaphragm wall is widely used in soft clay areas. Few cases with abundant groundwater in Beijing also adopt this support system.  
CSN walls combine single or double rows of DSM columns with soil nails. By reinforcing and fixing the soil, it forms a co-acting system with the soil. It has good soil quality and it has developed quickly in Beijing in recent years. The depth of excavation supported by soil nail or composite soil nail can reach more than 7 m in clay soil or soft clay soil, and the excavation depth in plastic or hard plastic clay soil can reach 14 m. In Beijing, depths of up to 8-15 m have been achieved.  
Bored pile walls are applications of a sequence of concrete piles that are cast in drilled holes as CPWs. Bored pile walls with internal support are the most commonly used bracing system for deep excavations of subway stations in Beijing (Figure 2). In situations where environmental regulations are particularly stringent, or when there are limitations upon time and workspaces, a bottom-up approach is usually used with CPWs, which are propped up by temporary concrete or steel struts.