# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 7 Challenges and Future Innovation Directions  
A comprehensive review of various technologies for ultra-deep shaft construction highlights significant challenges in their application, particularly in soft UUS. Conventional shaft construction methods, including open caissons, diaphragm walls, sheet pile walls, and secant piles, have been employed in diverse geological conditions. Open caisson shafts are favored for their simplicity and cost-effectiveness but encounter limitations in soft soils due to improper sinking techniques, excessive settlement, and challenges in maintaining verticality. Diaphrag walls, often constructed with reinforced concrete panels, provide excellent stability in deep excavations; however, the labor-intensive construction process, which relies on consistent circulation of bentonite slurry for trench stability and guide wall templates for verticality control, results in prolonged construction times, rendering it unsuitable for soft UUS. Similarly, secant pile walls, which consist of overlapping concrete piles, offer robust lateral support but are less effective in managing groundwater seepage in soft soils. Sheet pile walls, typically utilized for temporary or shallow shaft construction, are quick to install yet present several disadvantages, including low strength and durability, high noise and vibration, considerable ground movement and settlement, low water tightness, and limited excavation depth and width. These conventional methods may be effective in specific scenarios but often struggle to meet the demands of soft UUS characterized by confined areas, advanced depths, and complex subsurface conditions. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Location** & **Shaft Type** & **Internal Diameter (m)** & **Depth (m)** & **Geological Conditions** \\ \hline Girona, Spain (2010–2011) & Ventilation & 5.25 & 20 & Soft ground; loamy coarse boulders \\ \hline Barcelona, Spain (2009–2012) & - & 9.2 & 47 & Heterogeneous ground: sand, gravel, quartz, slate \\ \hline Naples, Italy & - & 4.5–5.5 & 45 & Silt, clay, sand, gravel \\ \hline Grand Paris Express, Paris, France (2018) & - & 11.9 & 53 & Soft and heterogeneous ground \\ \hline Honolulu, HI, USA (2013) & Microtunneling shafts & 10 & 36 & Hard basalt as well as coral \\ \hline Seattle, WA, USA (2012) & Launch shaft & 9.8 & 45 & Soft ground; clay, sand, gravel, silt \\ \hline St. Petersburg, Russia (2010–2012) & Sewage collector shafts & 7.7 & 85 & Heterogeneous ground; clay, sand, boulders \\ \hline Dortmund, Germany (2017) & Launch shaft & 9.0 & 23 & Soft ground; sand, silt, marl \\ \hline Singapore (2018) & - & 12 & 60 & Heterogeneous ground \\ \hline North Yorkshire, England (2018–2019) & Pilot shaft & 9.0 & 115 & Soft to medium-hard rock \\ \hline Jeddah, KSA (2006) & Sewage & 11 & 45 & Soft ground; loam, sand, gravel \\ \hline Nanjing, China (2020–2021) & Parking garage shafts & 12.8 & 68 & Sand, gravel, limestone \\ \hline Pudong, Shanghai (2022) & Sewage Connecting Pipe Project & 6 & 43.22 & Soft ground; sandy silt, silty clay, clay \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of application of VSM technology. A summary of these conventional shaft construction methods is provided in Table 6. Due to the inherent limitations of these methods in addressing soft ground conditions, significant progress has been made in mechanized construction technologies. However, regardless of the level of automation, their performance under soft soil conditions remains significantly below that in hard soil contexts. The primary challenges include the complex requirements of shaft lining construction, the inherent instability of soft soils, and the constraints posed by confined urban spaces. These challenges highlight the urgent need to develop mechanized shaft sinking systems capable of meeting customer expectations while effectively addressing the demands of soft ground excavation.