# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 4 Technological Advancements in Mechanized Systems
### Advancement in (PIAUS) Construction Technologies  
#### 4.3.1 Vertical Shaft Construction Technology (VSM)  
Currently, conventional caisson construction for deep shafts in urban areas is rare due to its significant environmental impact. As an alternative, deep foundation pits employing underground diaphragm wall construction are typically used. However, issues related to leakage pose significant challenges to the quality and depth of diaphragm wall construction. Moreover, dewatering in soft soil areas is often not environmentally friendly. Therefore, a new in situ assembling caisson technology should be employed to overcome these  
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issues. The deformation mechanism and control of this technology need specific analysis, particularly where thick, soft mucky clays are prevalent.  
The new in situ PIAUS technology (VSM) consists of excavating the stratum underwater using a mechanical arm while simultaneously assembling prefabricated caisson segments. It is designed for the mechanized excavation of shallow shafts in water-bearing soils and applies to soft ground, heterogeneous grounds, and rocks [30]. This technology relies on a road header boom with a cutter drum equipped with cutter bits or soft ground chisels and can drill up to diameters of 4.5-18 m. A typical setup and details of component functionality are shown in Figure 7 and Table 3, respectively.  
A mainframe road header boom is fixed to which gripper pads can be attached for stabilization and support. The entire machine is remotely controlled from the surface and is designed to operate in submerged conditions. For rock support, two options are available: segmental lining or precast concrete segments, as well as wire mesh, rock bolts, and sprayed concrete. The segmental lining, a typical option for soft ground conditions, is assembled on the surface and then lowered as the shaft is sunk.  
During VSM construction, waterproofing layers are used for highly jointed or water-bearing rocks because access to the shaft for remedial work is unavailable until it is dewatered. In hard to weak soft rock conditions, support elements such as rock bolting,  
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**VSM Components** & **Functionality** \\ \hline Cutting edge & Used for cutting the soft soils when lowering the shaft structure into ground, and eliminates the need for an overcut \\ \hline Energy cable towers & Used to guide the cables and conduits of the energy chain into the shaft integrated with lowering of the shaft structure \\ \hline Telescope boom & It can swivel up and down and rotate with telescopic adjustment, and the whole cross-section of the shaft can be excavated gradually \\ \hline Energy chain & Comprises the all-supply lines from ground surface to the boring machine \\ \hline Machine arms & For securing of the shaft boring machine to the bottom segments of the shaft structure \\ \hline Machine frame & Supports all the supply units for the machine and rotary drive of the telescopic boom \\ \hline Shaft & Entire shaft liner is composed of reinforced concrete segments or cast-in-place concrete \\ \hline Lowering units & Lower the whole shaft structure on steel cables connected to the shaft cutting edge \\ \hline Recovery winches & If necessary, machine is retrieved using steel cables and recovery winches \\ \hline \hline \end{tabular}
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Table 3: Description of VSM components and their functionality.  
Figure 7: Components of VSM after [27].  
installation of wire mesh, and shotcreting can be executed from specially designed working operations with permanently established rock drills and shotcreting equipment.  
VSM technology has been applied in numerous projects in soil and soft-rock conditions with compressive strengths up to 120 MPa. For conveying purposes, a slurry circuit is employed in soft soil below the groundwater level, while a pilot hole is used for dry conditions [38].