# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 7 Challenges and Future Innovation Directions
The introduction of innovative technologies, such as PIAUS construction methods, offers a promising solution by optimizing shaft sinking processes in soft UUS environments. Moreover, the widespread adoption of full-section mechanized construction methods in UUS necessitates novel requirements for shaft support structures, emphasizing the need for prefabricated shaft structure designs utilizing prefabricated segments. Unlike conventional shield tunnel segments, the assembled linings for shafts possess distinct characteristics, necessitating detailed research into the design parameters of prefabricated shaft lining structures. The advancements in PIAUS, VSM, and ACPP construction technologies for shafts in softer UUS provide numerous benefits, including cost and time efficiency, minimal environmental risks, and unmanned safe operations, beginning to exceed traditional shaft construction technologies by optimizing shaft sinking processes and integrating advanced prefabricated structures. Studies and reviews have been conducted on the application of VSM in shaft excavation across various projects, comparing it to traditional caisson shaft excavation methods, as summarized in Table 7.  
However, for deeper shafts and excavations in hard rock, this technology presents several limitations [30]: (1) the shaft section design is restricted to circular shafts; (2) it currently accommodates a maximum shaft diameter of 18 m; (3) submerged excavation is performed, and access inside the shaft is unavailable until dewatering occurs; (4) for deeper excavation operations exceeding 200 m, the risks associated with VSM technology and methodology require reassessment, summarized as follows [28]: (a) potential for liner jamming, (b) tilting or verticality issues of the liner, (c) gaskets attached to liners are limited to supporting pressures of 18 bar, (d) increased load-bearing capacity is necessary to support larger linings, and (e) required ground reinforcement in weak rock formations. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**Method** & **Advantages** & **Limitations** & **Ground Conditions** \\ \hline \multirow{3}{*}{Open caissions} & Cost-effective, simple installation process, suitable for large load applications & Prone to excess settlement, difficulties in sinking in soft soils, challenges to maintain verticality & Dense, stable soils (cohesive or granular), low water tables \\ \hline \multirow{3}{*}{Diaphragm walls} & High stability in deep excavations, effective lateral load resistance and waterproofing potential & Laborious construction process, time-consuming, requires bentonite slurry for stability & Cohesive soils, high groundwater levels \\ \hline \multirow{3}{*}{Secant piles} & Strong lateral support, can adopt to complex excavations, effective in groundwater control & Costly installation, difficult to achieve total waterproofing, limited verticality tolerances & Variegated soil types, cobbles and boulders, high water table \\ \hline \multirow{3}{*}{Sheet pile walls} & Quick installation, lightweight and reusable, effective in providing lateral earth support & Limited structural integrity for ultra-deep applications, difficult to install in rocky soils, installation cause noise and vibrations in nearby areas & Loose or unstable soils, temporary or shallow excavations, urban areas with confined space \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of conventional shaft construction structures. Additional limitations of the VSM method's reliance on blind excavation in submerged settings heighten the risk of encountering unforeseen obstacles, such as boulders or hard rock, which can damage equipment and cause construction delays, thus limiting the method's effectiveness [101]. Exploring the potential to expand the application of VSM for larger shaft diameters and non-circular shaft configurations could enhance the use of VSM technology in shaft construction.