A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space

Abstract

With the ongoing urbanization and densification of cities worldwide, the planning and utilization of urban underground space (UUS) have become crucial for developing urban underground infrastructure. Given the limited construction space within dense urban areas and the influence of declining groundwater levels, technologies such as open caissons and various vertical shaft methods have been introduced for UUS development. However, the dissemination of these technologies remains fragmented across different domains, lacking systematic summarization. A comprehensive, up-to-date overview of open caisson and vertical shaft technologies is essential for their effective application. In the manuscript, a systematic analysis of vertical shaft technologies, specifically focusing on their use in soft ground conditions, is conducted. The analysis is based on an extensive literature review and case study evaluation. It addresses the unique challenges posed by high compressibility, low bearing capacity, and groundwater sensitivity. Conventional shaft technologies and mechanized systems, including open caissons, drilled shafts, and the novel pressed-in ultra-deep assembled shafts (PIAUS), are evaluated systematically. Key aspects such as design principles, construction techniques, and stability in soft soils are discussed. The limitations of conventional methods in soft UUS are highlighted, while the advantages of advanced mechanized systems—such as rapid construction, reduced environmental impact, and improved safety—are emphasized. A detailed comparison of case studies demonstrates that PIAUS construction technology is particularly efficient in urban areas with confined spaces, dense building conditions, and ground conditions up to 200 MPa, with shaft diameters up to 12.8 m and depths of 115.2 m. Additionally, its suitability for rapid construction in soft and medium ground conditions is supported by undrained excavation with parallel excavation and liner sinking techniques. The PIAUS technology shows considerable potential for future projects, including shield construction shafts, ventilation shafts for tunnels, underground parking garages, and stormwater storage wells. This manuscript also highlights emerging mechanized methods in underground space development, their advantages, limitations, and areas for future research and improvement.

1. Introduction

Over the first three decades of this century, global urban land cover is projected to expand at a rate exceeding all prior urban growth in human history. Cities with populations ranging from 500,000 to 10 million residents will continue to evolve, intensifying land demand in high-density areas [1]. In response, underground construction emerges as a viable solution, offering new opportunities for sustainable urban development. The economic, environmental, and social effects of underground infrastructure are substantial, with well-planned and strategically developed urban underground space (UUS) potentially improving quality of life by fostering compact, efficient, and sustainable urban environments. In civil, architectural, and hydraulic engineering, the development and utilization of UUS rank among the top 10 engineering development fronts [2].

This trend is particularly evident in the last two decades, where rapid economic growth and urbanization have driven the need for increased infrastructure, including underground space excavation for metro, road, and railway tunnels, as well as a diverse array of utility lines. This has created a mounting demand for the construction of shafts. The global shaft sinking equipment market size was USD 213.4 million in 2023 and is expected to reach USD 324.1 million by 2030, exhibiting a compound annual growth rate (CAGR) of 6.5% during the forecast period [3], as illustrated in Figure 1. This growth reflects the increasing demand for efficient underground construction technologies, driven by the need for optimized urban development. As cities expand, achieving effective urban development becomes essential for promoting new infrastructure and digital and intelligent construction technologies, which play a vital role in the development and utilization of underground space.

Where there is initially no access to the bottom, a shaft is defined as a vertical or near-vertical tunnel extending from the top downward [4]. The application of vertical shaft technology in soft soil conditions presents significant engineering challenges due to the complex geotechnical characteristics of these soils, including low shear strength, high compressibility, and susceptibility to instability during construction and long-term use. Factors such as restricted space and groundwater make shaft excavation or sinking one of the most challenging development methods, with various measures complicating the process [5].

1.1. Conventional Shaft Construction Structures

Shaft excavation methods vary according to diameter, depth, and lithologic conditions. In soft grounds, such as underground diaphragm walls and pile foundations, conventional open-cut shaft construction techniques are commonly employed [6], as illustrated in Figure 2.

1.1.1. Open Caissons

Open caissons are utilized for constructing shafts in soft or loose soils. To stabilize the ground around the shaft periphery, sheet piles are first driven to reduce the risk of basal heave in soft clays by mitigating pressure differentials and to prevent hydraulic uplift in loose sands by increasing the groundwater flow path. Cast-in situ concrete caisson rings, typically 1.3 m high and 300 mm thick, are progressively installed from top to bottom, providing structural support to the surrounding soil, allowing excavation inside the shaft to proceed.

Friction between the caisson wall and the soil helps support the weight of the concrete rings during excavation. The primary advantage of this construction method is that the completed shaft remains strut-free, facilitating construction activities within the shaft, such as removing excavated material. However, limitations of open caissons include high friction resistance in dense sands, improper sinking techniques, challenges with excavation bed cleaning, underwater concrete placement, and the risk of tilting due to unsymmetrical work, which can result in construction delays and increased costs [7].

1.1.2. Diaphragm Walls

The traditional open-cut method, using long and deep diaphragm walls as the retaining structure, occupies a large amount of UUS. In shaft construction, particularly when the ground consists of very dense sand with a high-water table, conditions often make other shaft types impractical for achieving the required safety factors. However, due to costs, panel geometry, and time constraints, shafts constructed with cast-in situ diaphragm wall panels have become limited. Moreover, the labor-intensive construction process, which requires maintaining trench stability through consistent circulation of bentonite slurry, verticality control via guide wall templates, specialized machinery, highly skilled operators, and heavy reinforcement for crack control [8], makes this method less feasible in UUS.

1.1.3. Sheet Piling

Sheet piling consists of driving interlocking steel sheets into the ground to form a wall. This fast and straightforward technique is suitable for soft and sandy soils. However, sheet piling has several disadvantages, including low strength and durability, high noise and vibration, significant ground movement and settlement, low water tightness, and limited excavation depth and width. Open caisson shaft construction and retaining structures are depicted in Figure 3.

1.1.4. Secant Piling

Secant piling is a method that entails drilling and concreting overlapping piles into the ground to form a wall. This technique is more robust and watertight than sheet piling and can be applied in hard and rocky soils. However, secant piling also presents several drawbacks, such as high costs and time requirements, difficult site access and logistics, and quality control and inspection issues.

As depth increases and groundwater levels fluctuate, these methods encounter numerous limitations. Conventional shafts are typically restricted to smaller depths and often face challenges in soft or unstable soils, particularly with high groundwater levels. Their use can also be time-consuming due to the need for careful excavation and dewatering, as these processes may induce instability. To address UUS development, novel and effective construction methods are urgently needed [11]. In many Asian countries, the geology of megacities primarily consists of soft grounds, such as clay, silt, sand, and gravel layers with high groundwater tables. Therefore, new technologies related to design, monitoring for safety management, environmental preservation, groundwater strategy, and soil improvement are essential for UUS.

Under limited ground construction space within intensive buildings and the influence of lowering confined groundwater, open caisson and various vertical shaft technologies were introduced in UUS development. However, publication and information regarding these technologies are distributed across fragmented domains in different fields, lacking systematic summarization. An advanced and up-to-date systematic summarization of open caisson and vertical shaft technology is necessary and urgent for quick understanding and application. The primary aim of this review is to present a comprehensive overview of the early development in shaft technology and advancements in mechanized methods, design, and construction technology of UUS in the perspective of soft ground conditions. To develop underground constructions economically and efficiently with minimal environmental impact in megacities, reviewing these new technologies is beneficial not only for developed countries but also for cities in developing nations. The PIAUS construction technology, including the vertical sinking shaft machine (VSM) and ACPP, will be discussed with their design and construction methods, compared to conventional shaft sinking methods used in deep excavations and complex settings, particularly in soft ground conditions.

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. An assembly-type circular shaft support structure [24] directly utilizing tunneling segment pieces as the shaft wall structure offers advantages such as good stress resistance and convenient construction.

During the sinking process, various assembly forms derived from the prefabricated concept have been introduced to achieve speedy construction of the structure. However, the current prefabricated segment design includes numerous special functional blocks, complicating connection operations. Additionally, conventional methods for aiding caisson sinking conflict with the assembly of prefabricated segments [25]. The VSM, a new fully automatic mechanical shaft excavation technology developed by Herrenknecht, addresses these challenges [26]. VSM technology is primarily employed to excavate overburden soil or weak- to strong-strength rocks with compressive strength less than 140 MPa and has proven capable of cutting through boulders with compressive strength up to 200 MPa in the St. Petersburg Sewage Project. It operates predominantly in submerged conditions, utilizing high specific gravity bentonite slurry for temporary excavation support and prefabricated concrete segments for permanent lining [27].

VSM employs undrained excavation and complete suspension sinking of caissons. The mechanical arm, combined with the concept of a milling barrel and wellbore prefabricated assembly, demonstrates robust compliance in unstable operational settings prone to disturbances. It has proven efficient and beneficial in terms of economy, construction time, and work safety. Since its first operation in 2006, 80 VSM projects with a total depth of approximately 4630 m have successfully completed shaft sinking [28].

A shaft project [29] in Shanghai, where soft clay is widely distributed, serves as an engineering background. Due to the undrained operation of VSM technology, minimal disturbance to surrounding soil and negligible creep was observed. Furthermore, measured results and numerical analysis revealed that ground settlement and in-depth deformation were significantly smaller compared to other construction techniques. The design concept and benefits of VSM shaft construction, as compared to caisson shaft construction, present numerous advantages, including enhanced construction safety, productivity, cost efficiency, effects on surrounding structures, and efficient utilization of site space [30]. The development of shaft construction technology has broadened the application scope of vertical shaft excavation in various ground conditions for multiple purposes. To the best of the authors’ knowledge, the distribution percentage of these technological systems and their geographical applications are shown in

Table 1. Percentage distribution of mechanized systems and their geographical applications.

Technology	Percentage Distribution	Geographical Application
Shaft boring system (SBS)	~15% of mechanized excavation methods	Commonly used in Europe, North America, and Australia for mining and infrastructure projects
Shaft boring machine (SBM)	~15% of mechanized excavation methods	Primarily used in Europe, North America, and mining regions like Chile, South Africa, and China
Blind shaft boring	~10% of mechanized excavation methods	Used in urban areas in Europe, Asia, and North America for metro and ventilation shafts
Raise boring machine (RBM)	~23% of mechanized excavation methods	Widely used in mining regions (e.g., South Africa, Australia, Europe) and hydropower projects in Asia (e.g., China)
Shaft boring road header (SBR)	~10% of mechanized excavation methods	Commonly used in Europe for mining projects
Shaft boring cutterhead (SBC)	~5% of mechanized excavation methods	Primarily used in mining regions (e.g., South Africa, Australia) and deep infrastructure projects
Vertical shaft construction (VSM)	~20% of mechanized excavation methods	Widely used in Asia (e.g., China, Japan, Singapore), Russia, the Middle East, and Europe in urban infrastructure projects for ventilation, parking garages, and launch shafts
Active controlled prefabricated caisson (ACPP)	~2% of mechanized excavation methods	Primarily used in China for urban infrastructure projects

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3. Methodology

This research study employs a comprehensive and systematic methodology to evaluate the design, construction, and performance of ultra-deep assembled shafts. This section consists of extensive literature research and case study analysis, allowing for detailed examination of mechanized systems, shaft design, and construction techniques. The methodology framework for reviewing vertical shaft technologies is illustrated in Figure 4.

3.1. Literature Review

An extensive review of existing literature was conducted in the initial phase to collect data on conventional and mechanized systems, press-in methods, and the construction of ultra-deep assembled shafts. Key sources included peer-reviewed research articles, industry reports, conference proceedings, and technical notes. A systematic search was performed across key databases to categorize related studies, including Google Scholar, Scopus, CNKI, and Web of Science. The literature review focused on studies discussing the following:

• Conventional and press-in techniques in soft soil conditions;
• Development of mechanized systems;
• Technological advancement in design principles for ultra-deep shafts;
• Case studies of shaft applications in UUS;
• Advancements and challenges in PIAUS construction technology.

Through this literature review, methodologies related to mechanized systems, as well as the design and construction of ultra-deep assembled shafts, are explained, and limitations in the research are identified.

3.2. Case Studies Analysis

Case studies were analyzed to evaluate the practical application of PIAUS construction technology in constrained urban environments. The selection of case studies was based on the following:

• Accessibility of technical data on shaft construction projects;
• Ground condition variations, ranging from soft soils to high-strength formations (up to 200 MPa);
• Diverse project dimensions, with shaft diameters up to 12.8 m and depths extending to 115.2 m;
• Documented construction challenges and solutions implemented.

Through comprehensive analysis of case studies, the effectiveness of PIAUS construction technology in addressing space limitations, soil stability issues, and construction safety was evaluated. This assessment enabled a comparative estimation of technological effectiveness across various UUSs.

4. Technological Advancements in Mechanized Systems

Developments in mechanized techniques have shifted vertical shaft excavation, improving productivity, safety, and feasibility. Mechanized excavation methods enhance overall shaft construction performance by allowing concurrent processes, such as excavation, muck removal, and rock support installations. The mechanical excavation tools used are of two main types: cutter bits and disc cutters. The advantages of cutter bits include high-impact loads on the rock surface, lower cutting forces, and kinematics that make them suitable for more flexible excavation geometries. However, when the unconfined compressive strength of rock exceeds the range of 100–120 MPa, rock excavation with cutter bits becomes progressively challenging due to low penetration rates and high bit consumption. Typically, disc cutters penetrate hard ground perpendicularly, operate by rotating over the surface, and can excavate materials with a compressive strength exceeding 300 MPa [31]. For shaft construction, especially in soft UUS, additional challenges arise primarily due to soil instability and groundwater presence. In urban settings, shaft construction must account for confined spaces, existing infrastructure, and potential settlement effects. The use of mechanized systems, including technologies like assembled shaft technology in soft UUS, allows for mechanized excavation, reducing the risk of ground settlement while achieving a balance between excavation and structural safety.

The shaft serves as ventilation and is a crucial supporting project of the tunnel. To date, shaft engineering has accumulated substantial engineering experience in design methods and construction technologies for diameters less than 8.0 m and depths less than 300 m, employing methods such as drilling and blasting, freezing in soft ground, and reverse shaft techniques. Researchers increasingly recognize the importance of comprehensive matching technologies for mechanized drilling of ultra-large-diameter deep shafts, given the rising number of shaft projects with diameters around 8.0 m and depths exceeding 500 m. A brief review of mechanized methods for ultra-deep shafts is presented, followed by a discussion of assembled shaft technology.

4.1. Shaft Boring System

The shaft boring system, a development for the mechanized excavation of deep vertical blind shafts in hard rock conditions, represents a large, complex shaft excavation equipment. Its main functions include rock cutting, muck removal, and shaft support. Due to its high excavation efficiency, geological adaptability, and quality of completion [32], it can be widely applied in the construction of shafts in UUS and mineral resources development, such as underground parking lots, water storage tanks, and mine shafts. With the increasing utilization of underground space, mechanized excavation has become a prevailing trend to replace traditional methods [33]. This system employs a rotating cutting wheel to excavate the full shaft diameter in a two-stage semi-full-face sequential excavation process. The two-step excavation process includes trench excavation, where the cutting wheel rotates around its horizontal axis to a depth of one stroke, followed by downward pushing in the shaft direction. In the second step, the entire bench area is excavated by slewing the rotating cutting wheel 180° around the shaft’s vertical axis. The SBS machinery consists of three key operations and components: the shaft boring machine (SBM), which manages excavation, muck transportation, and gripping systems; equipment for primary rock support and probe drilling; a primary platform deck for SBS infrastructure and power packs; and a secondary platform deck for final lining installation, muck handling, and service extension. Power packs supply energy to the systems used in shaft boring, enabling cutting tools and other operational systems to function efficiently. The machinery also includes hydraulic systems, control systems, and other equipment vital for the progression of shaft boring. The infrastructure on the primary platform deck is designed to facilitate the efficient and safe operation of the boring equipment. After excavation is complete, the secondary platform of the SBS aids in the installation of the final shaft lining, essential for maintaining structural integrity and preventing failures, as well as protecting the excavation from groundwater and geological pressures. In addition to lining installation, it supports efficient muck handling, ensuring the timely removal of excavated material to maintain a clear working area and smooth operations. The platform also allows for the extension of essential utilities required for the safe and effective management of the shaft environment, such as water supply, ventilation, and drainage.

The application of the SBS in soft UUS faces numerous limitations, primarily due to unstable soils and high groundwater tables. In such circumstances, soft ground necessitates additional support measures, such as soil stabilization or grouting. The presence of a high groundwater table in soft grounds complicates operations further, necessitating dewatering systems and increasing the risk of land subsidence. Additionally, the SBS typically employs rotating cutters or drills for shaft advancement, which are not effective in soft soil, making cutting and aligning the shaft face more challenging, reducing productivity and potentially increasing costs and delays. Furthermore, urban environments complicate SBS implementation due to confined spaces, existing infrastructure and utilities, vibrations, and the potential for differential settlement in densely populated areas. Alternative shaft construction technologies are necessary, as SBSs often require more complex and expensive solutions, increasing safety and environmental risks in soft UUS.

Shaft Boring Machine (SBM)

Herrenknecht, in partnership with Rio Tinto in the “Mine of the Future” program, developed the SBM, designed for shaft excavations with diameters up to 12 m and depths up to 2000 m in compact rock formations. The SBM is engineered for the excavation of large–deep shaft structures characterized by mechanization, high efficiency, automation, and modularity [34]. The machine utilizes a rotating cutter wheel with disc cutters for shaft excavation via static pressure [35]. Hydraulic cylinders allow the wheel to thrust against the rock and rotate around the shaft bottom. A clam-shell device, similar to traditional shaft mucking, removes cuttings, which are lifted by buckets. The machine moves down and up the shaft against the shaft wall using a system of thrusting grippers. The grippers and their connected cylinders also help maintain the verticality and stability of the machine. The machine operates on the same principles as the TBM but in a vertical mode [36]. Although the working principle and structural composition of the SBM and TBM are similar, significant differences exist in the cutterhead structure. The SBM cutterhead features a conical structure, while the TBM cutterhead has a planar structure, primarily determined by their respective working characteristics, as illustrated in Figure 5.

Additional shaft construction operations, such as utility installation, rock bolting, and shaft concrete lining, can occur simultaneously with shaft boring. This method, to a depth of approximately 460 m, is comparable in cost to traditional sinking, after which the SBM provides a distinct cost advantage. The SBM demonstrates considerable productivity advantages, as it excavates much faster than drill-and-blast methods [31,38]. The significant benefit of the SBM lies in its capability to perform drilling and support operations concurrently, making it adaptable to strata with adverse geological conditions. Currently, it achieves drilling depths exceeding one thousand meters [39].

The SBM facilitates high-speed excavation of deep vertical shafts with diameters up to 8.5 m, making it suitable for various boring operations. It is considered very safe, as it requires fewer personnel in the process. However, the SBM has some limitations; its straight-line drilling approach enhances precision but renders the method relatively inflexible. The process is restricted to specific sizes and lengths, which may become expensive on a cost-per-meter basis. Fast drilling necessitates high-capacity chip removal and is best suited for reasonably stable ground conditions.

4.2. Blind Shaft Boring

Blind shaft drilling refers to the mechanical excavation of large-diameter vertical shafts [40] into the earth using a full-face drill head, with the excavated material transported to the surface through the shaft. In urban infrastructure, it serves as access and ventilation shafts for metro tunnels and parking facilities. This method facilitates the transportation of excavated material through the shaft to the surface. A blind SBM is employed when a pilot hole is not feasible. To date, the South Deep Shaft in South Africa is the deepest lift blind vertical shaft, reaching a depth of approximately 2991 m below collar [41].

Blind shaft drilling presents several advantages over conventional shaft sinking techniques. The process is highly automated, requiring only a crew of three to four individuals. During development and operation, personnel do not enter the shaft; all work is conducted on the surface. However, some drawbacks of blind shaft boring include limited access for ground support installation, constrained remedial support access, and the necessity for precise descriptions of rock mass conditions. Over the past decade, significant advancements in blind shaft boring technologies have emerged (Figure 6).

4.2.1. Raise Boring Machine (RBM)

The raise boring machine (RBM) enhances conventional shaft construction, providing an efficient, mechanized method. This mechanized approach, compared to traditional drill-and-blast methods, significantly reduces the number of operating workers, enhances safety during shaft construction, minimizes worker injuries, and improves overall productivity. Therefore, the advancement and operation of raise boring technology hold promise [42]. Compared to traditional methods, RBMs exhibit improved efficiency, reducing construction time by approximately 20% [43].

In the 1950s and 1960s, the United States and Germany initially developed raise drilling rigs with drill pipes. Since 1968, raise boring has been routinely utilized in the mining industry for mechanically cutting excavations to various diameters and depths, ranging from 0.7 m to 7.1 m in diameter and depths up to 1260 m. In this method, a pilot hole is created by a drilling rig, followed by the use of a specialized cutting head to create a large-diameter hole in an upward direction. In 1971, the first SBM was designed and manufactured in the Federal Republic of Germany, successfully boring a 4.88 m-diameter, 231 m-deep shaft [44]. Since the prototype’s initial use in 2010 at the Vianden hydropower plant in Luxembourg, Herrenknecht RBMs have proven their reliability in various projects worldwide [45].

This method significantly enhances drilling speed and addresses the limitations of low productivity and continuous slag removal associated with traditional drilling methods. However, it also imposes constraints on the maximum diameter and length of the hole. In this context, the shaft diameter is limited to 6.0 m, while its depth can reach up to 2 km [46]. Raise drilling rigs are primarily suited for stable rock environments; inadequate self-supporting capabilities may lead to failures during excavation, necessitating careful application or section modification treatment [47].

China began advancing raise drilling rigs and technology in the 1980s, progressing through three stages [48]. From 1980 to 1989, the equipment reproduction stage focused on underground coal mine construction, characterized by small size, explosion-resistant capabilities, and lightweight design suitable for underground transport. The strata typically consisted of soft rock, with diameters ranging from 1.0 to 5 m and drilling depths generally not exceeding 100 m, requiring compressive strength of the broken rock to be less than 60–100 MPa. From 1990 to 2005, the scope of drilling applications expanded from coal engineering to hydropower, mining, and other sectors, leading to increased drilling depth and diameter, with compressive strength requirements rising to approximately 250–300 MPa. The period from 2006 to the present represents the maturation phase for processes and equipment.

Figure 6. Overview of blind boring machines; (a) SBR and (b) SBC modified after [26], (c) blind shaft boring rig after [41], (d) RBM modified after [45].

Figure 6. Overview of blind boring machines; (a) SBR and (b) SBC modified after [26], (c) blind shaft boring rig after [41], (d) RBM modified after [45].

In the last decade, significant progress has been made in raise boring technology and equipment research [49,50]. Following the widespread adoption of small raise drilling rigs, China has initiated the research and design of large-scale raise drilling rigs suitable for diverse geological and engineering conditions. These large-diameter rigs are equipped with substantial push-and-pull propulsion systems, multi-cylinder lifting mechanisms, high torque rotation capabilities, and multi-motor driving equipment. The integration of large-diameter reaming drill bits meets the requirements for underground transportation and assembly in confined spaces. Furthermore, advancements in large-diameter raise drilling technology, including measurement while drilling and deviation correction control technology, enhance deflection control precision. This technological progress has led to the successful development of the ZFY5.0/600 RBM, which facilitates large-diameter shaft construction, contrasting with prior RBMs that were limited to small-diameter raises. The RBM ZFY5.0/600 constructed a 5 m diameter ventilation shaft in a coal mine and drilled numerous 3.5 m diameter deep shafts in hard basalt at the Baihetan hydropower station [42].

In shaft construction, raise boring offers several advantages, minimizing rock mass damage, preserving inherent strength, and providing smooth-finished shafts without the need for lining, which is particularly significant for ventilation shafts.

However, raise boring also presents several drawbacks. Its remote excavation face restricts immediate ground support in unfavorable conditions, unlike conventionally sunk shafts, where support is integrated early in the cycle. Additionally, access for remedial support installation is limited due to the vertical nature of raise-bored shafts, in contrast to tunnel boring, which allows for access behind the advancing face for support purposes. Lastly, raise boring operations lack hands-on access to the working face, necessitating precise rock mass conditions to ensure compatibility with specific diameters and accurate evaluation of rock conditions.

4.2.2. Shaft Boring Road Header (SBR)

The shaft boring road header (SBR) is developed for the mechanized sinking of blind shafts in soft to medium-hard rocks with a compressive strength of up to 120 MPa. The SBR is equipped with a road header boom and a rotating cutting drum for efficient excavation of soft to medium-hard consolidated rock. The excavated material from the bottom of the deep shaft is drawn and pneumatically transported to intermediate containers, from which it is cyclically hoisted to the surface using typical tubes. The SBR machine enables sinking of shafts with diameters of 7 to 12 m to a depth of approximately 1000 m, without the need for a pilot hole.

In 2018, for the first time in Europe, the Nezhinsky Mine in Belarus utilized this mechanized technology to sink two large-diameter service and production shafts, each with a diameter of 8 m, to depths of 697 and 750 m, respectively [51].

Due to the limitations of the cutting devices employed, neither the Herrenknecht SBR excavator nor the Russian or Polish shaft excavators are suited for excavation in hard rock. The European shaft borers and SBR machines utilize pencil point tools for rock excavation, constrained by rock strength. To address this limitation, Herrenknecht, in collaboration with Rio Tinto under the “Mine of the Future” program, developed an alternative shaft excavation machine known as the SBM. Subsequently, the shaft boring cutterhead (SBC), another shaft excavator, was developed. Both the SBM and SBC are equipped with disc cutters, contrasting with the pencil bits used in the SBR [52]. In comparison to conventional shaft excavation, the SBR is safely operated from elevated working decks, ensuring no personnel are present at the bottom of the shaft during mechanized excavation. Overall, the SBR design establishes new safety standards for personnel.

4.2.3. Shaft Boring Cutterhead (SBC)

The SBC is the third machine in Herrenknecht’s range of blind boring equipment. This machine, measuring 40 m in length and weighing 350 tons, is ideal for excavating deep blind shafts ranging from 7 to 12 m in diameter in hard rock conditions up to 2000 m deep, achieving a progress rate of 6 m per day. The SBC employs a conical full-faced cutterhead assembled with disc cutters, featuring a highly automated cutting sequence. Shaft ropes are utilized for the suspension and movement of this mechanical shaft sinking unit [53]. A summary of the shafts drilled using raise boring technology and blind boring shafts is provided in

Table 2. Summary of shafts drilled through raise boring and blind shaft system.

Location	Shaft Technology	Diameter (m)	Depth (m)	Advantages	Shortcomings	Market
Shanghai Jingan Smart Garage Project	SBM (Dream)	12–23	50.95	High prefabrication ratio, fast construction speed, extensive excavation depth, and less disturbance to the surrounding environment	Structural anti-floating, sensitive environmental impact, small spacing mutual influence, and underwater bottom sealing in soft soil layers	Civil
Bosjespruit Mine, South Africa	RBM	7.1	178	Efficient excavation with minimal disruption	Limited to moderate depths	Mining
Prismulde Project, Germany	-	1.83	1260	Narrow shaft reduces excavation volume, making it cost-effective for small operations	Challenging to work in limited space, risks increase with depth due to small diameter	Civil
Oryx Gold Mine, South Africa	-	6.5	972	Large diameter supports structure stability	Ventilation, cooling challenges	Mining
Sedrun, Switzerland	-	7.1	785	Capable of handling significant overburden	Advanced equipment needed for granitic rock excavation	Civil
Roxby Downs, Australia	-	4.5	730	Suitable for ore-rich operations, balanced size for stability and functionality	Depth and ore composition increase cost and time of maintenance	Mining
Madisonville, KY, USA	-	6.7	205	Easily accessible and manageable depth for coal extraction	Limited scalability for deeper coal seams	Mining
Vianden, Luxembourg	-	5.5	282	Integral for hydropower, ensuring efficient water flow for energy generation	Depth may limit future expansion of hydropower capacity	Hydropower
Baoxin coal mine, China	ZFY5.0/600 (RBM)	4.7	482.2	New technology ensures precision and safety in coal seam conditions	Moderate diameter limits equipment sizes for certain operations	Mining
Jansen Mine	SBR	11	1005	Large diameter supports high-capacity potash mining, SBR allows efficient boring	Expensive technology, high initial capital investment required	Mining
Nezhinsky Mine, Belarus	-	8	750	Well suited for potash extraction with stable shaft design	Requires specialized ventilation for potash mining	Mining
North Yorkshire, UK	-	12	1600	Largest diameter supports high-capacity operations for polyhalite mining	Extreme depth increases safety risks and ventilation challenges	Mining

.

From the revised case histories, there is significant advancement in mechanized technologies utilized in hard grounds, particularly in mining. However, well-documented case histories for their application in soft UUS are lacking. Despite advancements in mechanized shaft sinking technologies, which exhibit varying degrees of automation, challenges persist when excavating in soft UUS. The progress achieved remains unsatisfactory compared to performance in hard grounds. The primary obstacle to achieving proper progress stems from various activities related to shaft lining construction technology, soft grounds, and confined urban spaces. Therefore, there is a pressing need to explore mechanized shaft sinking systems that meet customer expectations and to identify solutions for shaft excavation in soft UUS. To address this challenge, the following paragraphs present the PIAUS construction technologies, a novel approach designed to optimize shaft sinking in soft UUS environments.

4.3. Advancement in (PIAUS) Construction Technologies

4.3.1. Vertical Shaft Construction Technology (VSM)

Currently, conventional caisson construction for deep shafts in urban areas is rare due to its significant environmental impact. As an alternative, deep foundation pits employing underground diaphragm wall construction are typically used. However, issues related to leakage pose significant challenges to the quality and depth of diaphragm wall construction. Moreover, dewatering in soft soil areas is often not environmentally friendly. Therefore, a new in situ assembling caisson technology should be employed to overcome these issues. The deformation mechanism and control of this technology need specific analysis, particularly where thick, soft mucky clays are prevalent.

The new in situ PIAUS technology (VSM) consists of excavating the stratum underwater using a mechanical arm while simultaneously assembling prefabricated caisson segments. It is designed for the mechanized excavation of shallow shafts in water-bearing soils and applies to soft ground, heterogeneous grounds, and rocks [30]. This technology relies on a road header boom with a cutter drum equipped with cutter bits or soft ground chisels and can drill up to diameters of 4.5–18 m. A typical setup and details of component functionality are shown in Figure 7 and

Table 3. Description of VSM components and their functionality.

VSM Components	Functionality
Cutting edge	Used for cutting the soft soils when lowering the shaft structure into ground, and eliminates the need for an overcut
Energy cable towers	Used to guide the cables and conduits of the energy chain into the shaft integrated with lowering of the shaft structure
Telescopic boom	It can swivel up and down and rotate with telescopic adjustment, and the whole cross-section of the shaft can be excavated gradually
Energy chain	Comprises the all-supply lines from ground surface to the boring machine
Machine arms	For securing of the shaft boring machine to the bottom segments of the shaft structure
Machine frame	Supports all the supply units for the machine and rotary drive of the telescopic boom
Shaft	Entire shaft liner is composed of reinforced concrete segments or cast-in-place concrete
Lowering units	Lower the whole shaft structure on steel cables connected to the shaft cutting edge
Recovery winches	If necessary, machine is retrieved using steel cables and recovery winches

, respectively.

A mainframe road header boom is fixed to which gripper pads can be attached for stabilization and support. The entire machine is remotely controlled from the surface and is designed to operate in submerged conditions. For rock support, two options are available: segmental lining or precast concrete segments, as well as wire mesh, rock bolts, and sprayed concrete. The segmental lining, a typical option for soft ground conditions, is assembled on the surface and then lowered as the shaft is sunk.

During VSM construction, waterproofing layers are used for highly jointed or water-bearing rocks because access to the shaft for remedial work is unavailable until it is dewatered. In hard to weak soft rock conditions, support elements such as rock bolting, installation of wire mesh, and shotcreting can be executed from specially designed working operations with permanently established rock drills and shotcreting equipment.

VSM technology has been applied in numerous projects in soil and soft-rock conditions with compressive strengths up to 120 MPa. For conveying purposes, a slurry circuit is employed in soft soil below the groundwater level, while a pilot hole is used for dry conditions [38].

4.3.2. Actively Controlled Prefabricated Caisson Construction Technology (ACPP)

A new in situ assembled caisson technology, actively controlled prefabricated caisson (ACPP), has been developed by Shanghai Tunnel Engineering Co., Ltd. in Shanghai, China, to enhance construction efficiency. This construction method is specifically tailored for soft soil layers, utilizing non-draining sinking throughout the entire process. Similar to traditional sinking wells, the blade foot remains embedded in the soil layer to a certain height, and the water level in the well is automatically controlled to maintain a balance of internal and external water pressure.

With the introduction of an automatically injected new form of mud sleeve, the technology can pass through shallow soft and silty soil layers with minimal settlement, eliminating the need for further foundation reinforcement. The construction team can employ the latest technology by deploying underwater robots and an intelligent propulsion system to efficiently complete the construction of ultra-deep vertical shafts without foundation pit excavation, dewatering, and support structures. The propulsion system serves two functions: lifting and sinking, and lowering and sinking. The sinking coefficient is generally large in the initial sinking stage, and to ensure complete vertical sinking, a suspension sinking mode may be utilized. The normal sinking stage transitions to an excavation pressure sinking cycle, and under normal operating conditions, equal stroke control and adaptive thrust modes may assure exceptional verticality. The schematic of ACPP, featuring an automatic track recording system and excavation interface, is depicted in Figure 8.

The ACPP technology system presents substantial potential for various construction applications, including escape shafts, railway ventilation shafts, underground parking facilities, pumping stations, and deep underground pipeline systems. This innovative technology offers a solution for constructing super-deep vertical shafts in urban environments, facilitating enhanced efficiency, structural integrity, and safety. Implementing ACPP technology can address the challenges associated with traditional sinking methods, thus optimizing resource utilization and minimizing environmental impact. Key advantages include the following:

• Underwater mechanized soil extraction enables visual unmanned construction;
• A counterforce device is positioned at the top to facilitate active control of sinking, and a cross-beam structure is retained in the wellbore, aiding in achieving soil pressure and breaking;
• The well structure is prefabricated and assembled to enhance construction efficiency;
• Tracking pressure injection of anti-friction mud outside the shaft wall reduces drag, stabilizes the wall surface, and minimizes environmental impact.

To date, ACPP technology has been utilized in the west extension of Shanghai Metro Line 13 for the escape shaft project at Yunle Road Station, demonstrating the strength of this advanced technology. Measurements taken during the project revealed notable results. The verticality of the sinking well achieved a precision of 0.65%, with maximum ground settlement of 4.95 mm and a corner height difference of only 2 mm. Furthermore, the segment lining exhibited no leakage or misalignment, representing exceptional overall performance [54]. Continued research and extensive case studies are necessary to fully elucidate the potential of ACPP technology and to explore its broader applicability in various civil engineering projects, particularly in ultra-deep shaft construction. A description of mechanized shaft technologies, including their advantages and limitations, is provided in

Table 4. Description of mechanized shaft technologies including their advantages and limitations.

Technology	Description	Advantages	Limitations	Applications
Shaft boring system (SBS)	Mechanized excavation system for deep vertical shafts in hard rocks; includes rock cutting, muck removal, and shaft support	High excavation efficiency; widespread geological adaptability	Limited efficiency in soft grounds; requires dewatering in high groundwater conditions; high cost and complexity in urban settings	Underground parking Water storage tanks; mine shafts
Shaft boring machine (SBM)	Established by Herrenknecht for large–deep shaft excavation (up to 12 m diameter, 2000 m depth); uses rotating cutter wheel with disc cutters	High efficiency, automation, and modularity; simultaneous drilling and support operations	Inflexible for non-linear drilling; high cost-per-meter for shorter depths; requires stable ground conditions	Deep shaft construction; mining; infrastructure projects
Blind shaft boring	Mechanized excavation of large-diameter vertical shafts without a pilot hole; excavated material is transported to the surface	Highly automated; no personnel in the shaft during operation	Limited access for ground support installation; requires precise rock mass conditions	Metro access tunnels; ventilation shafts
Raise boring machine (RBM)	Mechanized construction method for shafts using a pilot hole and reaming; suitable for stable rock conditions.	High efficiency; reduced construction time; minimal rock mass damage	Limited to stable rock conditions; restricted diameter (up to 6 m); limited access for remedial support	Ventilation shafts; mining; hydropower projects
Shaft boring road header (SBR)	Mechanized sinking of shafts in soft to medium-hard rock (up to 120 MPa compressive strength); uses a road header boom and rotating cutting drum	Safe operation with no personnel at the shaft bottom; efficient excavation in soft to medium-hard rock	Limited to soft to medium-hard rock; not suitable for hard rock conditions	Service and production shafts; urban infrastructure
Shaft boring cutterhead (SBC)	Mechanized excavation of deep blind shafts in hard rock (up to 2000 m depth, 7–12 m diameter); uses a conical full-faced cutterhead with disc cutters	High progress rate (6 m/day); highly automated; suitable for hard rock conditions	Limited case histories in soft ground; high cost and complexity	Deep shaft construction; mining
Vertical shaft construction (VSM)	In situ assembling caisson technology for shallow shafts in water-bearing soils; uses a road header boom with cutter bits or soft ground chisels	Suitable for soft and heterogeneous conditions, operates in submerged conditions; remote-controlled operation; waterproofing layers for jointed or water-bearing rocks	Limited to shallow shafts; prior to remedial work requires dewatering for stability and access	Shallow shafts in urban areas; soft ground conditions
Active controlled prefabricated caisson (ACPP)	In situ assembled caisson technology for soft grounds; deploys underwater robots and an intelligent propulsion system to complete excavation; uses non-draining sinking and automated mud sleeve injection	Active sinking control and unmanned construction; prefabricated wall linings; reduced environmental impact	Applicable to soft soil conditions	Escape shafts and ventilation shafts; underground parking; pumping stations

.

5. Technological Advancements in Design Theory

5.1. Shaft Section

In designing vertical shafts, the cross-sectional shape significantly influences structural stability, load distribution, and construction feasibility, particularly in soft UUS. The shaft shape directly affects factors such as lining mobilization and soil–structure interaction. Compared to rectangular or polygonal shafts, circular shafts offer several advantages, including efficient load distribution, reduced lining mobilization, minimized deformation, and decreased shaft reinforcement. In soft UUS, where soil compressibility and settlement are critical concerns, circular shafts present an economical and sustainable solution for shaft construction.

The shaft structure section form is the primary consideration in designing the shaft to determine its dimensions and support structure [55]. When designing the shaft section, the shape is determined based on factors such as geological and hydro-geological conditions, shaft depth, and construction costs [56,57].

Theoretically, shafts can adopt various geometric shapes; however, the most common shapes are circular, square, rectangular, and elliptical. The shaft shape is influenced by its intended purpose and ground conditions [58]. Currently, shaft sections predominantly feature rectangular, circular, oval, and elliptical shapes [59].

In analyzing the prevalence of vertical shafts compared to inclined ones, Holl and Fairon [60] reviewed the shapes of vertical shafts sunk over the past sixty years and concluded that the rectangular shape may be phased out, with circular and elliptical shafts becoming the future trend. To develop design guidelines for rectangular- and circular-shaped deep shafts and assess nearby mining effects on shaft stability, Li et al. [56] investigated an approach to field testing and design for deep mine shafts in the Western U.S.A. High stress concentration occurs at the corners of rectangular vertical shaft cross-sections, leading to the failure of surrounding rock. Therefore, the design of rectangular cross-sections is gradually becoming obsolete and is being replaced by circular vertical shaft cross-section structures [61].

Through numerical calculations, Han et al. [55] considering factors such as layout, load, function, depth, and construction difficulty of the vertical shaft, provided guidance for engineering scenarios suited for various cross-sectional forms by comparing the cross-sections of vertical shafts. Parametric numerical analysis of shafts with different shapes, construction sequences, stiffness, and embedded lengths concluded that circular shafts resulted in lower lining mobilization than oval and polygonal shafts [62].

According to Muramatsu and Abe [63], circular diaphragm walls represent self-stable structures due to their geometry, which forms a stable ring under compression. Case histories demonstrate that this design usually results in smaller settlements compared to rectangular excavations. Moreover, a circular diaphragm wall can present preferable structural stability compared to a rectangular diaphragm wall due to its inherent rigidity and reduced radial deflection or deformation [64,65]. Recent project reports of a shaft for an underground car park in Nantes, France, with a diameter of 46 m [66], and a shaft for TBM extraction in Rome, Italy, with a diameter of 15.7 m [67], indicate that such structures remain operational with substantial diameters. According to Zhao [61], analysis of various vertical shaft cross-sectional forms concluded that as the depth of vertical shaft excavation increases, stresses such as self-weight stress, maximum horizontal stress, and additional stress from surrounding rocks also increase.

In ultra-deep vertical shafts with depths of hundreds of meters, the uneven distribution of horizontal structural stress gradually increases with depth. A circular vertical shaft cross-section may not meet requirements, so to ensure the stability of the wellbore surrounding rock under high-stress conditions and to optimize cross-section use, an elliptical cross-section structure can be adopted based on the direction of the principal horizontal stress [68].

The design of ultra-deep shaft cross-sections must consider several factors, including geological conditions, construction difficulties, and the specific purpose of the shaft. The demand for efficient and robust shaft designs becomes critical as urban development expands into soft underground urban spaces in dense settings. Due to their capability for uniform stress distribution, which enhances structural stability and reduces failure risk in high-stress environments, circular and elliptical geometries are increasingly preferred. For underground urban spaces, circular cross-sections aligned with the principal direction of horizontal stress offer significant advantages over rectangular shapes. Future advancements in shaft design using prefabricated integrated advanced underground support construction technology will likely focus on further optimizing these shaft sections to meet engineering specifications and constraints imposed by complex subsurface conditions, ensuring the stability, safety, and long-term sustainability of UUS.

5.2. Shaft Support Structures

Shafts under construction require timely wall support to reinforce surrounding rocks and maintain shaft wall stability. During support type selection, support measures are typically unnecessary for stable surrounding rocks in the excavation area. However, if the stability of surrounding rocks is poor, support measures must be implemented to ensure shaft wall stability. The selection of shaft support methods depends on the characteristics of the geological formations. Currently, anchor bolt shotcrete technology is predominantly used in rock formations, while soil formations typically adopt internal support and enclosure structure methods [69]. Anchor bolt shotcrete technology is commonly utilized in the support design of ultra-deep shafts due to its advanced technology, reliable quality, and economic viability [70]. Compared to conventional shaft support methods, shotcrete support offers significant advantages in minimizing construction costs, expediting construction speed, reducing labor demands, and creating suitable conditions for subsequent mechanized support construction [71].

The load-bearing capacity of shaft walls consistently increases with excavation depth. However, using plain concrete for shotcrete support is no longer sufficient for construction safety. It is necessary to increase the strength of shaft wall support to ensure structural stability. By adding 25 to 35% fly ash or reinforcing steel, high-strength concrete linings can be developed, increasing the uniaxial compressive strength (UCS) of concrete up to 100 MPa [72,73]. Despite the development of high-strength concrete linings, more than 200 shafts in China have deformed and been impaired since the 1980s [74,75]. Reinforced concrete support methods enhance stability, but the additional reinforcement binding process prolongs well construction speed. Therefore, recent years have seen novel research on new types of steel fiber-reinforced concrete support to address these issues [76]. Incorporating steel fibers can improve the crack resistance and toughness of concrete [77,78], thereby securing the structural integrity of support under load.

The support method for shafts relies on ground characteristics. In hard grounds, bolt shotcrete technology is generally used, while piles and internal support are adopted in soil layers. The conventional Shenyang Metro shaft, primarily constructed using the upside-down wall method with dimensions of 4.6 m × 6 m, was selected based on structural demands and operational requirements. This design incorporates a 350 mm initial support layer (grid steel frame and shotcrete) with piles along the side walls. Steel supports were employed to reduce bending moments at the shaft corners due to significant lateral pressure from local geology of medium-coarse and gravel sands. However, this design reduces the actual construction area to approximately 23.1 m², leading to material inadequacies at the corners (Figure 9) [68].

To address these limitations, a circular shaft structure is proposed. The circular design generates an arching effect, transforming external earth pressure into axial force, thus utilizing concrete’s high compressive strength. Even with uneven earth pressure, bending moments can be managed by increasing initial reinforcement rather than adding internal supports, effectively expanding the usable shaft area and improving construction efficiency.

Vertical shaft support is typically designed with uniform thickness from top to bottom for computational simplicity, which can lead to excessive safety margins for upper shaft wall support. To address this concern, research on varying-thickness support lining based on theoretical calculations of shaft wall depth can minimize construction costs. Moreover, the advent of mechanized drilling construction presents advanced requirements for vertical shaft support technology.

Currently, concerns persist regarding unclear safety factor reserves for shaft wall support structures in the construction of ultra-deep vertical shafts. Inadequate support strength can lead to structural instability, while excessive material strength may result in resource wastage. Current design approaches for shaft wall support structures, relying on theoretical or empirical methods, are relevant to shallow vertical shaft excavations, yielding conservative design outcomes. To overcome these challenges, it is necessary to establish a scientific calculation theory suitable for ultra-deep shaft design methods in soft underground urban spaces, ensuring optimized performance and cost efficiency. Moreover, the focus of current research is on the application of prefabricated steel support structures and lining designs for unconventional shafts. These prefabricated solutions enhance construction efficiency and advance standardization, requiring precise customization to meet site-specific demands and emphasizing the need for further innovation and refinement in design methodologies.

5.3. Prefabrication Lining Structures

The construction process of ultra-deep assembled shafts includes the off-site fabrication of individual components within controlled environments. This process typically includes casting concrete segments, installing reinforcement, and incorporating specialized features such as conduits or access points. However, in the development of underground urban spaces, the conventional cast-in-place concrete construction method results in significant material loss and unstable construction quality [79]. Prior to using VSM, ground freezing techniques and retaining structures such as secant pile walls or diaphragm walls are often necessary to support weak soils or rock [28].

Field tests validate the integrity, convenience, recyclability, and effectiveness of this novel supporting structure. This technology allows for simultaneous excavation and support, making it suitable for urban construction due to its low noise levels. A model of four ring shell joints for sinkhole-type prefabricated assembly structures, designed for precast open caissons and segment joint tests, was used for model verification [80]. A spiral supporting structure, which was prefabricated in a factory and assembled on site, was designed by [81,82].

In conjunction with the characteristics of the VSM method during the sinking process, Bian et al. [83] examined the load distribution design of the assembled sinking well and derived the variation law of VSM suspension force and sidewall frictional resistance with the sinking of the well. The equipment utilized for ultra-deep vertical well excavation comprises exceptionally integrated mechanical systems that operate automatically based on preset values. The vertical well, which includes the blade foot and well wall structure, employs prefabricated assembly that significantly enhances efficiency while ensuring accuracy, marking a prospective trend in the design and construction of ultra-deep vertical wells [29].

Variations in competence, stiffness, and joint properties of the rock during shaft submersion can lead to deviations in the liner. Upon dewatering, new loading conditions and joint properties of the disturbed rock result in increased external pressure on the lining segments and trigger internal hydraulic gradients in the rock mass. Therefore, consolidation settlement occurs, which affects the structural integrity of the liner [28].

The use of prefabricated pipe segments for assembly markedly enhances efficiency. Generally, assembling four rings, i.e., 6 m in height of pipe segments, takes about 8 h, whereas constructing the same height of cast-in-place rings requires several times more time (including steel reinforcement binding, steel formwork installation, concrete pouring and curing, and formwork removal), with significantly less labor needed. Furthermore, compared to cast-in-place structures, the roundness of prefabricated assembled vertical wells is superior, which is vital for ensuring a uniform over-excavation process during successive well sinking. Comparatively, the structural strength and overall integrity of cast-in-place concrete structures exceed those of assembled structures. Therefore, it is advisable to select flexibly between the two forms based on different application plans.

The prefabricated shaft support structure offers several advantages over conventional designs. Its circular structure optimizes load distribution, leveraging the high compressive strength of concrete to enhance structural safety. The use of C50 high-strength concrete for shield segments, compared to the C25 strength grade of shotcrete, significantly improves the capacity to withstand greater earth pressures, making it suitable for deeper shafts. Additionally, the absence of internal supports within the shaft increases the available working surface, facilitating more efficient construction operations. The prefabricated design also ensures faster construction with superior quality, reducing project timelines and improving overall efficiency.

Moreover, the prefabricated approach directly utilizes standard shield segments, eliminating the need for specialized design or manufacturing, thus lowering costs. Recycled segments from shield tunneling projects can further reduce expenses. The elimination of shotcrete operations minimizes environmental pollution, enhances the construction environment, and protects worker health. Dismantling is safer and more convenient, especially for vertical structures within 3 m below ground. With minimal application restrictions, this method is highly adaptable and can be implemented across various regions nationwide.

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, Allenby 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.

Yan 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.

Abdrabbo 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 James W. Fowler 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. A total of four shafts were constructed, with depths ranging from 65 to 83 m. In St. Petersburg, VSM technology demonstrated exceptional efficiency, particularly in meeting time schedules. The first shaft, with a depth of 85 m and a 7.7 m inner diameter, was successfully accomplished in 50 days. The lithology of the underground area consists of soft soils like sand and loam. The upper layer, up to a depth of about 60 m, is groundwater-bearing, while the lower layer consists of hard but dry loam. In the transition zone between these two formations, boulders of up to 2.5 m in length are present, as shown in Figure 10.

Shaft lining is conducted on the shaft surface simultaneously with the excavation. Four segments, each 1 m in height and designed to withstand pressures of 8.5 bar, are installed as a ring segment. These segments are held by a crane attached to the shaft sinking unit and placed onto the previously installed ring segment [98].

In the Middle East, various microtunneling projects include the construction of deep target or launch shafts with depths ranging from 25 to 45 m. Initially, in 2006, a shaft with an inside diameter of 6.5 m and an outside diameter of 7.3 m was successfully sunk. The construction area consists of heterogeneous ground, including sand, sandy clay, hard coral, and limestone. The high porosity of corals and limestone results in significant groundwater intrusions, posing challenges to shaft construction efforts. Due to the considerable volume of water, drainage is often difficult or impossible, necessitating the deployment of VSM equipment. Through the groundwater-flooded shaft, the excavated material is mucked out, supplemented by a slurry system for soil transport. Due to local regulations, the use of prefabricated segments is prohibited during this project. Shafts are constructed using in situ casting. Every two days, 2 to 4 m are cast using a quick handling formwork and reinforcement system, achieving an average production rate of 1 to 2 m per day.

Another sewage shaft was constructed in Jeddah, Saudi Arabia, using VSM technology. The vertical shaft, with an inner diameter of up to 10 m and an outer diameter of 11 m, and a depth of 45 m, was successfully sunk in soft grounds, including loam, sand, and gravel.

By utilizing VSM technology, 13 emergency and ventilation shafts with inner diameters ranging from 4.5 to 5.5 m and depths of up to 45 m were constructed for the subway line in Naples, Italy. The project site is located in a densely populated urban area with high traffic levels. The shafts were built on soft ground consisting of silt, clay, and sand gravels. Rapid completion was achieved, with sinking rates reaching up to 5 m per day due to the parallel excavation and lining process using precast concrete segments.

In Spain, from 2010 to 2011, four shafts were constructed in Girona using a Herrenknecht VSM as ground stabilization shafts prior to tunneling works, featuring an internal diameter of 5.25 m and a depth of 20 m for the high-speed rail link from Barcelona to the French border. Additionally, in Barcelona, one shaft with an internal diameter of 9.2 m and a depth of 47 m was built for ventilation and as an emergency exit. Extremely restricted working conditions posed significant challenges for construction contractors. For instance, one of the shafts in Girona was situated between rows of houses with only 12 m of spacing. Typical shafts from Naples, Italy and Girona, Spain are illustrated in Figure 11. This project demonstrates the major advantage of VSM’s capability to operate under confined space conditions in urban environments [27].

In Singapore, under the Deep Tunnel Sewerage System (DTSS) Phase 2 Contract T11, twenty-one deep circular shafts connecting the tunnels were constructed. Five shafts—K1, K2, K2-DS1, Ki, and L-LS4—were constructed using VSM technology, each with an internal diameter of 10.4 m and depths of 42.2 m, 57.6 m, 55.9 m, 38.6 m, and 41.8 m, respectively. The construction of shafts in highly permeable sand layers underlain by fractured rock presented considerable challenges. In Asia, VSM was first employed for this project to address these challenges and demonstrates several advantages over caisson shaft construction [30].

The world’s largest-diameter vertical shaft was successfully excavated using a “Dream” boring machine with an excavation diameter of 23.02 m and a depth of 53 m in the underground smart parking garage in the Jing’an District of Shanghai [99]. VSM was utilized for the first time in the construction project of a parking garage in the Jianye District of Nanjing City, successfully completed in heterogeneous ground conditions of sand, gravel, and limestone with an outer diameter of 12.8 m and a depth of 68 m. The results indicate that this method achieves minimal construction footprint and negligible effects on the surrounding environment, with construction efficiency exceeding that of alternative methods [100].

In the Pudong New Area of Shanghai, 17 shafts of the Zhuyuan Bailonggang Sewage Connecting Pipe Project were constructed using VSM technology. The internal diameter and excavation depth of the shafts were 6 m and 43.22 m, respectively, with an average excavation rate of 1.9 m per day. The soft ground conditions, including silty clay, clayey silt, and clay, resulted in maximum settlement of approximately 15.2 mm and maximum horizontal displacement of about 3.74 mm at a depth of 40 m. Compared to other construction methods, the disturbance to the surrounding soil was minimal [29]. Successful shaft excavation projects worldwide utilizing VSM technology are summarized in

Table 5. Summary of application of VSM technology.

Location	Shaft Type	Internal Diameter (m)	Depth (m)	Geological Conditions
Girona, Spain (2010–2011)	Ventilation	5.25	20	Soft ground; loamy coarse boulders
Barcelona, Spain (2009–2012)	-	9.2	47	Heterogeneous ground: sand, gravel, quartz, slate
Naples, Italy	-	4.5–5.5	45	Silt, clay, sand, gravel
Grand Paris Express, Paris, France (2018)	-	11.9	53	Soft and heterogeneous ground
Honolulu, HI, USA (2013)	Microtunneling shafts	10	36	Hard basalt as well as coral
Seattle, WA, USA (2012)	Launch shaft	9.8	45	Soft ground; clay, sand, gravel, silt
St. Petersburg, Russia (2010–2012)	Sewage collector shafts	7.7	85	Heterogeneous ground; clay, sand, boulders
Dortmund, Germany (2017)	Launch shaft	9.0	23	Soft ground; sand, silt, marl
Singapore (2018)	-	12	60	Heterogeneous ground
North Yorkshire, England (2018–2019)	Pilot shaft	9.0	115	Soft to medium-hard rock
Jeddah, KSA (2006)	Sewage	11	45	Soft ground; loam, sand, gravel
Nanjing, China (2020–2021)	Parking garage shafts	12.8	68	Sand, gravel, limestone
Pudong, Shanghai (2022)	Sewage Connecting Pipe Project	6	43.22	Soft ground; sandy silt, silty clay, clay

.

The newly developed PIAUS construction technology (ACPP) has successfully completed its first project in constructing the escape shaft at the Yunle Road Station, part of the west extension of Shanghai Metro Line 13. The verticality of the sinking well achieved a precision of 0.65%, with a maximum ground settlement of 4.95 mm and a corner height difference of only 2 mm, illustrating the capability of this advanced technology. Furthermore, the segment lining exhibited no leakage or misalignment, indicating exceptional overall performance.

PIAUS construction technology, VSM, and ACPP have emerged as highly efficient solutions for constructing shafts in UUS, offering reduced construction time, cost savings, enhanced safety, and improved quality control. Off-site prefabrication minimizes noise, vibration, and disruptions while reducing on-site hazards and accidents, making it ideal for UUS projects where stability, minimal ground disturbance, and long-term performance are critical. Numerous aforementioned global projects have demonstrated its advantages across various challenging environments.

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. Diaphragm 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.

A summary of these conventional shaft construction methods is provided in

Table 6. Summary of conventional shaft construction structures.

Method	Advantages	Limitations	Ground Conditions
Open caissons	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
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
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
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 can cause noise and vibrations in nearby areas	Loose or unstable soils, temporary or shallow excavations, urban areas with confined space

. 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. 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. Comparison of VSM shaft and caisson shaft.

Parameters	PIAUS Construction	Caisson Shaft
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
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
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
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
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
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
Section design	Only excavate circular shaft	Any type of shaft including circular, rectangular, elliptical, etc.
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

.

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. 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. 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 pressure management. Prefabricated and assembled well structures streamline construction processes, while the precise injection of anti-friction mud minimizes wall drag, stabilizes the shaft, and reduces environmental impact.

Our findings suggest that addressing the technical challenges and enhancing the sustainability and resilience of PIAUS construction technology requires continued research and development initiatives. These technologies have the potential to significantly contribute to future projects, including shield construction shafts, ventilation shafts for tunnels, underground parking garages, and stormwater storage wells.

The execution of complete mechanization in the sinking process, with potential for simultaneous activities such as excavation, loading, and shaft lining installation, necessitates the development of a novel shaft sinking technology. Continued research and extensive case studies are essential to fully elucidate the potential of VSM and ACPP technologies and to explore their broader applicability in various civil engineering projects, particularly in ultra-deep shaft construction.

Additionally, it is crucial to develop a series of vertical shaft support design theories that are adaptable to diverse environmental conditions. These theories should be based on continuous monitoring of parameters such as stress and deformation of the shaft wall rock, observed in ongoing vertical shaft engineering projects. This approach will provide valuable guidance for design and construction.

Future research should emphasize adopting economically feasible design solutions through comprehensive studies on optimal pile structures, the thickness of plain concrete, and construction technologies for the shaft structure. This should take into account factors such as depth, diameter, geological conditions, and stress effects on segment joints.

8. Conclusions

This review presents a comprehensive overview of the current status of vertical shaft construction technology worldwide. It highlights recent technological advancements and opportunities in design and construction technology for ultra-deep assembled shafts, particularly considering PIAUS construction technology, including VSM and ACPP, discussed with their design and construction methods compared to conventional shaft sinking methods in soft UUS. Current challenges, advancements, and prospective issues in vertical shaft technology can be summarized as follows:

Conventional shaft construction methods, although effective in specific geological conditions, face limitations in addressing the complex demands of UUS. These methods, including open caissons, diaphragm walls, secant piles, and sheet pile walls, encounter significant limitations in soft UUS. Challenges include settlement and verticality issues with open caissons, labor-intensive processes for diaphragm walls, poor groundwater management with secant piles, and reduced strength and durability in sheet pile walls. These methods often struggle to meet the demands of UUS, such as confined spaces, greater depths, and complex subsurface conditions, highlighting the need for advanced construction solutions.

The VSM technology consists of highly integrated mechanical machinery constructed from prefabricated pipe segments, which accelerates the construction speed of shafts. To prevent jamming issues and ensure smooth sinking, overcut excavation and bentonite slurry can be utilized. To minimize the risk of sudden sinking and tilting of the shaft, four sets of settlement units are used and steel wire strands are applied. The joints of the prefabricated vertical shaft segments are sealed with EPDM rubber as a waterproof material.

The new ACPP is specifically tailored for soft soil layers, employing non-draining sinking throughout the entire process. By utilizing underwater mechanized soil extraction, ACPP enables visual, unmanned construction. The integration of a counterforce device and cross-beam structure facilitates active sinking control and soil pressure management. Prefabricated and assembled well structures streamline construction processes, while the precise injection of anti-friction mud minimizes wall drag, stabilizes the shaft, and reduces environmental impact.

The popularization and application of the full-section mechanized construction method puts forward new requirements for the shaft support structure, necessitating the design of prefabricated shaft structures using the method of prefabricated segments. However, the assembled lining used in the shaft differs from segments in conventional shield tunnels, warranting further study of the design parameters of the prefabricated shaft lining structure.

The operational capacity of PIAUS construction technology, including VSM and ACPP, in confined spaces ensures safe operation, accuracy, lower environmental risk, lower cost, and time efficiency. This not only enhances efficiency but also indicates a trend in the construction of complex ultra-deep vertical shafts in soft UUS.