# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 7 Challenges and Future Innovation Directions
Additionally, its operational depth requires optimization, considering the outlined risks and limitations for an efficient and safe operation process. VSM operations conducted in submerged conditions necessitate a thorough understanding of fractured or weak rocks subjected to high water pressure and the joint characteristics of the rock. Transitioning from hard layers to weak layers can result in tilting or deviation of linings due to differences in rock stiffness and competence. Upon dewatering, the joint properties of the disturbed rock under new loading conditions become evident. This condition increases pressure on the lining segments and triggers internal hydraulic gradients within the rock mass, resulting in consolidation settlement that affects the structural integrity of the lining. Despite these limitations, the assembled VSM technology has gained prevalence in soft UUS due to its advantages over traditional manual excavation and liner installation methods. The new ACPP is specifically designed for soft soil layers, employing non-draining sinking throughout the entire process. This innovative technology provides a solution for constructing super-deep vertical shafts in urban environments, enhancing efficiency, structural integrity, and safety. The application of ACPP technology addresses challenges inherent in traditional sinking methods. By utilizing underwater mechanized soil extraction, ACPP facilitates visual, unmanned construction. The integration of a counterforce device and a cross-beam structure allows for active sinking control and soil \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Parameters** & **PIAUS Construction** & **Caisson Shaft** \\ \hline Ground conditions & Applicable to any type of soft or heterogeneous ground conditions with UCS up to 120 MPa. & Appropriate for soft grounds having adequate self-supporting capability during excavation process and hard grounds of any strength; however, soft and weak rocks/soils like very weak clay and loose sand need additional measures \\ \hline Groundwater & Undrained excavation; excavation is carried out in submerged condition so no pumping test or recharge wells are required & Prior to excavation work, pumping test and recharge wells are required especially where critical structures are located near the shaft, as a contingency measure \\ \hline Excavation time & Fast excavation speed due to parallel excavation and installation of segment lining & In situ casting works require installation of formwork, rebar, and concrete casting; after required strength is achieved for last cast ring, excavation to next step can only started \\ \hline Ground modifications & Required only if bearing capacity is not sufficient for a few meters beneath the ring beam & If highly permeable nature of residual soil and rocks are present it will be required around the entire shaft perimeter as well as shaft base \\ \hline Stability & Stability of shaft is increased as at all times it is balanced by hydrostatic pressure & Depends only on soil arching capacity for stability unless additional measures are implemented \\ \hline Excavation damage zone or disturbance & During excavation minimizes ground displacement and eliminates heaving resulting from ground modification works & Due to ground improvement and during excavation, ground movement increases and excessive heaving is encountered \\ \hline Section design & Only excavate circular shaft & Any type of shaft including circular, rectangular, elliptical, etc. \\ \hline Safety & Operational process is safe as excavation is monitored and controlled from surface, eliminated need for personnel inside shaft & Personnel needed inside the shaft to perform various tasks related to construction and excavation, which increases safety concerns \\ \hline \end{tabular}
\end{table} 
Table 7: Comparison of VSM shaft and caisson shaft. pressure management.