# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 4 Technological Advancements in Mechanized Systems
### Advancement in (PIAUS) Construction Technologies
#### 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:  
1. Underwater mechanized soil extraction enables visual unmanned construction;
2. 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;
3. The well structure is prefabricated and assembled to enhance construction efficiency;
4. 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 tech nology 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.