# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 6 Application Cases Overview  
A comprehensive overview of various technologies for ultra-deep shaft construction has been carried out to highlight the significance and challenges in their application, particularly in soft UUS. Researchers have extensively investigated the sinking process of open caisson technology through various methods, including theoretical analysis, laboratory tests, numerical analysis, and field monitoring data. Their aim was to understand the mechanical responses and deformation characteristics of the surrounding soil induced by the installation of these caissons [84; 85; 86; 87; 88; 89]. Many case studies of caissons were presented by [90; 91], used for bridge piers in Bangladesh, most of which exceed 100 m in depth. During construction, significant ground subsidence became evident due to upward heave of soil inside the caisson, leading to potential damage to nearby infrastructure. Through four construction case histories, [PERSON] et al. [92] documented the process of sinking both dry and wet open caissons. They highlighted the critical role of control measures in ensuring caisson verticality and structural integrity during the construction process. [PERSON] et al. [93] conducted a field study on the deformation and stress characteristics of large open caissons during excavation in deep marine soft clay. During caisson excavation, they analyzed the deviations of caisson geometric posture, cone tip resistance, and lateral soil pressure. The findings provide valuable technical guidance for accurate modeling to predict ground movement and stress redistribution. [PERSON] et al. [7] documented a case history of a caisson with a 20 m diameter that became wedged due to the development of excessive soil-structure frictional stresses during sinking. From the extensive review of case studies, it is evident that installation problems, including slow sinking, sudden or super sinking, tilting, and freezing, are often encountered in the caisson installation process. To improve installation controllability, some new caisson installation technologies were also reported [94; 95; 96], but they remain inadequate to meet the criteria for ultra-deep shaft construction in complex geological conditions, especially in soft UUS. PIAUS technology has proven to be an efficient and effective solution for soft UUS, presenting a range of benefits, including reduced construction time, cost savings, enhanced safety, and improved quality control. Moreover, off-site construction reduces noise, vibration, and other disruptions while also lowering worker exposure to hazardous conditions, thereby decreasing the risk of on-site accidents common with traditional excavation methods. For UUS projects, these advantages make PIAUS technology an ideal solution where minimizing ground disruption, maintaining structural stability, and ensuring long-term performance are critical considerations. Several successful projects worldwide demonstrate the efficiency and benefits of ultra-deep preassembled shafts in various applications. These projects show innovative design solutions and efficient construction methodologies. A significant project is the Grand Paris Express (2018), where VSM was employed to construct four ventilation and rescue shafts for approximately 200 km of new metro tunnels. In this project, the use of VSM in densely populated areas with restricted site access is especially noteworthy [26]. For the excavation of the launch shaft in the Ballard Siphon Project in Seattle (2012), the contractor [PERSON] chose VSM technology due to its various advantages regarding noise emission safety and cost efficiency. The shaft, with a 9 m inner diameter and a depth of 125 ft, was completed in approximately twenty-eight days of excavation time, with an average excavated depth of 6 to 7 ft per shift [97]. To date, the deepest underwater VSM shaft was one of the sewage collector shafts completed in St. Petersburg, Russia, in 2012.