## A Case Study of the Application of Proposed Support System
### Structural Elements  
#### 3.2.1 Construction Process of the Support Structure  
Several steps are involved in the construction of the proposed supporting structure based on combination of STS and PBA methods. These steps are demonstrated in details in Figure 6.  
(1) The steel tubes are pushed into the soil by the jacking machine, and then the soil inside the steel tubes are extracted using horizontal directional drilling (Figure 6a).  
(2) Lateral connecting bolts are installed between adjacent steel tubes by artificial operations. Blocking plate at both ends of the steel tubes are then installed, and concrete is poured through grouting holes on the blocking plates (Figure 6b).  
(3) The top pilot tunnels (Z1) and bottom pilot tunnel (Z2) are then excavated, respectively. Then the cylindrical steel column (C1) is constructed. Then do the right parts (Z3, Z4 and C2) (Figure 6c).  
(4) The inter grid of the top pilot tunnels is removed, and the waterproof layer at the top and the middle floor slab are constructed. When the strength of the floor slab reaches to the 80% of the design strength, the soil above the middle floor slab is excavated for the whole cross section of the station, followed by the construction of the middle longitudinal beams and side walls (Figure 6d).  
(5) When the strength of the middle floor slabs and side walls of the station hall reach up to 80% of the design values, the soil from the middle span to the bottom slab is excavated. Followed by the construction of the waterproof layer and the bottom slab (Figure 6e).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Index Soil** & **Medium Coarse Sand** & **Gravel Sand** & **Round Gravel** & **Boulder Clay** \\ \hline Count & 6 & 49 & 125 & 97 \\ maximum value & 14.1 & 19 & 17.4 & 17 \\ minimum value & 10.6 & 7.3 & 7.0 & 7.2 \\ average value & 12.9 & 12.3 & 10.5 & 10.4 \\ standard deviation & 1.265 & 3.293 & 2.267 & 3.324 \\ variation coefficient & 0.098 & 0.268 & 0.216 & 0.262 \\ correction factors & 0.919 & 0.934 & 0.967 & 0.955 \\ standard value & 11.9 & 11.5 & 10.2 & 12.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The results of super heavy dynamic cone penetration test.  
Figure 5: Geological profile of the soil.  
(6) The rest of side walls and waterproof layers of top slabs are constructed to complete the construction of the subway station (Figure 6).  
Figure 6: Construction stages in the new support system. Note: Z1, Z2, Z3 and Z4 represent pilot tunnel; C1 and C2 represent cylindrical steel column.  
The characteristics of this construction method combining STS and PBA methods are as follows:  
(1) The support structure can be designed to have adequate bearing capacity to provide support for the overburden pressure when soil below the structure is excavated. So other temporary supports may not be required resulting in a flexibility of the method and potential for wide applications.  
(2) Not only can the proposed supporting system effectively control the soil surface deformation, but also the intensive temporary supports during excavation stage can be avoided owing to support provided by the cylindrical columns involved in the proposed technique.  
(3) The proposed method results in a rectangular cross section for the excavated area, which increases the utilization of the underground space compared to the traditional arch formation.