# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 2 Early Development and Evolution of Shaft Construction Technology  
Shaft technology has developed since the early 1960s and is widely used in underground mining shafts and roadways [12]. Shafts may be sunk to depths exceeding 3000 m in a single lift for mineral extraction [13,14], whereas depths are typically tens or hundreds of meters for other engineering applications, such as access tunnels for roads and railway systems. Japan pioneered the use of inclined auxiliary shaft tunnels in the construction of long tunnels, thereby accelerating the construction progress of main tunnels and enabling the possibility of constructing extra-long mountain tunnels [15]. In 1972, China achieved the highest global ranking in inclined shaft construction, with construction progress reaching 364.5 m/month in Hunan Province [16]. The limitations of traditional self-sinking shafts have become increasingly evident, leading to a significant decline in their use since the 1990s. They have been replaced by more efficient methods such as the space system (SS) caisson method [17], the press-in open caisson method, and the super open caisson system (SOCS) method [18]. In the SS method, the outer contour of the cutting edge extends 15-20 cm beyond the shaft wall, creating a gap that is filled with gravel to reduce peripheral friction without compromising ground stability. In contrast, the press-in open caisson method employs ground anchor reaction devices and through-hole jacks to apply downward pressure on the shaft, enhancing sinking speed and allowing for adjustments in the shaft's attitude, making it ideal for projects with strict environmental controls [19]. Open caisson technology is commonly used due to its simple caisson structure and small construction site requirements, relying on the shaft's gravity and additional loads to overcome wall friction or using thixotropic mud jackets, air curtains, and other methods to achieve the design depth. However, as a dynamic construction foundation project, issues such as non-verticality, abrupt settlement, immobility, and over-settlement frequently arise during the construction process [7,20]. To address the limitations of conventional caissons, including high wall friction, uncontrolled positioning, and significant environmental disturbance, an automated sinking well construction method (SOCS method) has been developed in Japan using underwater automatic bucket excavators. This method reduces wall friction by filling the gap between the outer wall of the well and the section with gravel, achieving high-accuracy posture control of open caissons [21]. SOCS is suitable for rapid, environmentally sensitive urban projects, with recent improvements enabling its use in non-standard shafts [22]. With the construction of transportation infrastructure for long and large railways or highway trunk lines, shafts are extensively used in elongated mountain tunnel engineering projects to solve tunnel construction challenges, operational ventilation, and reduce construction periods [11,12]. Several deep shafts exceeding 2 km in depth have been constructed and are operational in South Africa, Canada, the United States, and Russia. In China, few engineering examples of shafts deeper than 1500 m exist. Notable deep shafts include the main and auxiliary shafts of the Jianlong Group of Sishanling Iron Mine, with depths of 1505 and 1503.9 m, respectively; in Yunnan Province, the no. 3 shaft of the Huize Lead-Zinc Mine at a depth of 1526 m; and the new main shaft in the Shandong Gold Group, Xincheng Gold Mine, at a depth of 1527 m [13]. The deepest auxiliary shaft in China, located at the Sanshandao Gold Mine, is also the fourth deepest in Asia, with a depth of 1915 m [13,14]. The largest vertical shaft tunneling machine in China has been developed [23], utilizing support devices for positioning and driving a conical cutterhead for rock breaking, achieving comprehensive mechanized drilling.