# A Review of Vertical Shaft Technology and Application in Soft Soil for Urban Underground Space
## 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. [PERSON] proposed the original idea and contributed to the revision of the whole manuscript; [PERSON] wrote and prepared the original manuscript draft; S.N.A., W.P., H.L., B.A. and A.A. reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.