Editor's Note:Recently, the Beiqing Highway escape shaft of the western extension of Shanghai Metro Line 13 is undergoing ring beam construction. This shaft is another engineering application of the Active Control Precast Press-in Shaft (ACPP) following the Xinle Road escape shaft on Line 13. Supported by Shanghai Shentong Metro, this technology was independently developed by Shanghai Tunnel Engineering Co., Ltd. and Shanghai Tunnel Urban Construction Equipment Co., Ltd., and designed by Shanghai Tunnel Engineering Rail Transit Design and Research Institute. Today, let's follow the editor to learn about this "ancient yet modern" technique!

Digital-intelligent renewal of an "ancient technique"

The open caisson method was already used in the 19th century for deep bridge foundations and harbor structures. Owing to its high stiffness, large cross-section, high bearing capacity, strong impermeability, and wide applicability to various strata, it is an excellent construction method for building small and medium-sized shaft structures. However, traditional open caissons sink passively entirely under their own weight, resulting in low construction efficiency, difficult attitude control of the shaft body, and generally large settlement of surrounding strata. Therefore, with the improvement of socioeconomic conditions, the open caisson technique gradually faded away, with fewer and fewer applications in urban environments.

But this ancient method is now gradually gaining new life. In recent years, a series of new shaft construction methods combining mechanized and intelligent approaches with traditional techniques have flourished, including Europe's Segment Shaft and VSM, Japan's SOCS (automated open caisson), Urban Ring, and PC-Well for small-diameter shafts. In China, enterprises such as CRCHI, China Railway Engineering Equipment Group, CITIC Heavy Industries, and Shanghai Construction Group Foundation Engineering Group have respectively developed modern shaft construction equipment with distinctive features, which can be broadly divided into two categories: the urbanized mining drilling method and the mechanized open caisson method. The former is represented by VSM, and most products launched by domestic equipment companies are of this type; the typical representative of the latter is ACPP.

The main technical features of the ACPP method include full-process non-drainage sinking, precast segmental shaft walls, a dual-action lifting and pressing propulsion system, robotic underwater automatic soil excavation, and intelligent friction-reducing slurry injection. It is essentially a prefabricated, mechanized, and intelligent transformation of the traditional non-drainage open caisson, propelling the ancient non-drainage caisson technique toward mechanization and intelligence.

ACPP's "unique skills"

The ACPP method is tailor-made for soft soil strata. It adopts full-process non-drainage sinking; as with traditional open caissons, the cutting edge is always embedded at a certain height in the soil, and the water level inside the shaft is automatically controlled to ensure internal and external water pressure balance. Combined with the automatically injected new-type slurry sleeve, it can cross shallow soft silt and muddy soil strata with almost no settlement and without auxiliary ground improvement. The propulsion system has two functions: lifting sinking and pressing sinking. In the initial sinking stage, the sinking coefficient is generally large, so the suspended sinking mode can be adopted to ensure completely vertical sinking; in the normal sinking stage, it switches to an excavation-pressing cycle, and under normal conditions the adaptive thrust and equal-stroke control mode ensures excellent verticality.

The excavation system uses flexible underwater robots that can excavate in a preset block-scanning manner and can also operate in multi-machine coordination for cluster collaborative operation, solving the problem that conventional shaft boring machines become "harder the larger they are." Simply modifying the underwater robot trajectory database allows application to rectangular shafts, "peanut shafts," and even "caterpillar shafts"; the cutter-suction head module of the excavation system can also be replaced with a milling head module for soft rock strata.

From a structural design perspective, ACPP technology retains the bottom cross beam that is crucial to the stiffness of medium and large open caissons. During construction, it can effectively increase the contact area between the structure and the soil during sinking, preventing sudden sinking in soft strata. Compared with mining-method shafts that cannot be provided with a bottom beam, the thickness of the plain concrete base of ACPP can be greatly reduced, further enhancing safety.

In addition, a large number of sensors are installed in the structure and equipment system, covering everything from excavation and pressing sinking to sidewall friction reduction and beam-bottom reaction force measurement and control. The entire system is highly intelligent, and excavation-sinking construction requires only a single operator, fundamentally curing the chronic problems of traditional open caissons such as sudden sinking and tilting, enabling this ancient traditional technique to leap across the mechanization and informatization eras and radiate new vitality.

Exploring the infinite possibilities of underground space

On May 20, as the water level inside the caisson of the Xinle Road escape shaft of the western extension of Shanghai Metro Line 13 slowly dropped, the 31 m deep shaft wall composed of 13 rings of segments gradually emerged—solid and complete in structure, straight in alignment, and leak-free. Actual measurements showed excellent data for the shaft: verticality of 0.65‰, height difference at the four corners of 2 mm, maximum ground settlement of 4.95 mm, and no segment misalignment or leakage. This marks the first successful engineering application of the ACPP equipment and method technology system!

Since ACPP shafts require no retaining structures and no dewatering, the design goals of small space occupation, high construction efficiency, and environmental friendliness are truly achieved; the prefabricated assembled shaft body with fine prestressed compression joints also ensures structural quality.

In the future, ACPP has broad application prospects in rail transit ventilation shafts, escape shafts, shield and pipe-jacking working shafts for large-depth pipe network systems, pump stations, gate shafts, underground parking garages, and more. Combined with the small-diameter shaft BPSS (Bored Precast Sinking Shaft) method, which was experimentally completed by Shanghai Tunnel Engineering during the same period, it will become a brand-new solution for ultra-deep shaft construction in cities.