# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 5 Technological Advancements in Design Theory
### Shaft Support Structures  
Shafts under construction require timely wall support to reinforce surrounding rocks and maintain shaft wall stability. During support type selection, support measures are typically unnecessary for stable surrounding rocks in the excavation area. However, if the stability of surrounding rocks is poor, support measures must be implemented to ensure shaft wall stability. The selection of shaft support methods depends on the characteristics of the geological formations. Currently, anchor bolt shotcrete technology is predominantly used in rock formations, while soil formations typically adopt internal support and enclosure structure methods [69]. Anchor bolt shotcrete technology is commonly utilized in the support design of ultra-deep shafts due to its advanced technology, reliable quality, and economic viability [70]. Compared to conventional shaft support methods, shotcrete support offers significant advantages in minimizing construction costs, expediting construction speed, reducing labor demands, and creating suitable conditions for subsequent mechanized support construction [71]. The load-bearing capacity of shaft walls consistently increases with excavation depth. However, using plain concrete for shotcrete support is no longer sufficient for construction safety. It is necessary to increase the strength of shaft wall support to ensure structural stability. By adding 25 to 35% fly ash or reinforcing steel, high-strength concrete linings can be developed, increasing the uniaxial compressive strength (UCS) of concrete up to 100 MPa [72; 73]. Despite the development of high-strength concrete linings, more than 200 shafts in China have deformed and been impaired since the 1980s [74; 75]. Reinforced concrete support methods enhance stability, but the additional reinforcement binding process prolongs well construction speed. Therefore, recent years have seen novel research on new types of steel fiber-reinforced concrete support to address these issues [76]. Incorporating steel fibers can improve the crack resistance and toughness of concrete [77; 78], thereby securing the structural integrity of support under load. The support method for shafts relies on ground characteristics. In hard grounds, bolt shotcrete technology is generally used, while piles and internal support are adopted in soil layers. The conventional Shenyang Metro shaft, primarily constructed using the upside-down wall method with dimensions of 4.6 m \(\times\) 6 m, was selected based on structural demands and operational requirements. This design incorporates a 350 mm initial support layer (grid steel frame and shotcrete) with piles along the side walls. Steel supports were employed to reduce bending moments at the shaft corners due to significant lateral pressure from local geology of medium-coarse and gravel sands. However, this design reduces the actual construction area to approximately 23.1 m\({}^{2}\), leading to material inadequacies at the corners (Figure 9) [68]. To address these limitations, a circular shaft structure is proposed. The circular design generates an arching effect, transforming external earth pressure into axial force, thus utilizing concrete's high compressive strength. Even with uneven earth pressure, bending moments can be managed by increasing initial reinforcement rather than adding internal supports, effectively expanding the usable shaft area and improving construction efficiency. Vertical shaft support is typically designed with uniform thickness from top to bottom for computational simplicity, which can lead to excessive safety margins for upper shaft wall support. To address this concern, research on varying-thickness support lining based on theoretical calculations of shaft wall depth can minimize construction costs. Moreover,  
Figure 9: Rectangular shaft excavation support structure in soft grounds, modified after [68]. the advent of mechanized drilling construction presents advanced requirements for vertical shaft support technology.