# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 5 Technological Advancements in Design Theory
### Prefabrication Lining Structures  
The construction process of ultra-deep assembled shafts includes the off-site fabrication of individual components within controlled environments. This process typically includes casting concrete segments, installing reinforcement, and incorporating specialized features such as conduits or access points. However, in the development of underground urban spaces, the conventional cast-in-place concrete construction method results in significant material loss and unstable construction quality [79]. Prior to using VSM, ground freezing techniques and retaining structures such as secant pile walls or diaphragm walls are often necessary to support weak soils or rock [28]. Field tests validate the integrity, convenience, recyclability, and effectiveness of this novel supporting structure. This technology allows for simultaneous excavation and support, making it suitable for urban construction due to its low noise levels. A model of four ring shell joints for sinkhole-type prefabricated assembly structures, designed for precast open caissons and segment joint tests, was used for model verification [80]. A spiral supporting structure, which was prefabricated in a factory and assembled on site, was designed by [81; 82]. In conjunction with the characteristics of the VSM method during the sinking process, [PERSON] et al. [83] examined the load distribution design of the assembled sinking well and derived the variation law of VSM suspension force and sidewall frictional resistance with the sinking of the well. The equipment utilized for ultra-deep vertical well excavation comprises exceptionally integrated mechanical systems that operate automatically based on preset values. The vertical well, which includes the blade foot and well wall structure, employs prefabricated assembly that significantly enhances efficiency while ensuring accuracy, marking a prospective trend in the design and construction of ultra-deep vertical wells [29]. Variations in competence, stiffness, and joint properties of the rock during shaft submersion can lead to deviations in the liner. Upon dewatering, new loading conditions and joint properties of the disturbed rock result in increased external pressure on the lining segments and trigger internal hydraulic gradients in the rock mass. Therefore, consolidation settlement occurs, which affects the structural integrity of the liner [28]. The use of prefabricated pipe segments for assembly markedly enhances efficiency. Generally, assembling four rings, i.e., 6 m in height of pipe segments, takes about 8 h, whereas constructing the same height of cast-in-place rings requires several times more time (including steel reinforcement binding, steel formwork installation, concrete pouring and curing, and formwork removal), with significantly less labor needed. Furthermore, compared to cast-in-place structures, the roundness of prefabricated assembled vertical wells is superior, which is vital for ensuring a uniform over-excavation process during successive well sinking. Comparatively, the structural strength and overall integrity of cast-in-place concrete structures exceed those of assembled structures. Therefore, it is advisable to select flexibly between the two forms based on different application plans. The prefabricated shaft support structure offers several advantages over conventional designs. Its circular structure optimizes load distribution, leveraging the high compressive strength of concrete to enhance structural safety. The use of C50 high-strength concrete for shield segments, compared to the C25 strength grade of shotcrete, significantly improves the capacity to withstand greater earth pressures, making it suitable for deeper shafts. Additionally, the absence of internal supports within the shaft increases the available working surface, facilitating more efficient construction operations. The prefabricated design also ensures faster construction with superior quality, reducing project timelines and improving overall efficiency.