In a completed shield tunnel with an outer diameter of nearly 9 meters, there is not a single hand hole or bolt on the segment linings. Viewed from a distance, the ring segments are joined neatly and uniformly with no visible misalignment; the tunnel's curved surface is smooth and finely finished, and inspection has found no leakage anywhere...

On June 3, the tunnel of the Nanjing Jiangxinzhou Sewage Collection System Phase II Construction Project (Jiajiang River Crossing Section) — the largest-diameter sewage river-crossing shield tunnel under construction in China (hereinafter referred to as the Nanjing Sewage Phase II Channel) — was completed. The project pioneered a completely new shield construction technology in China: the integrated technology of "new socket-type segments + dual-liquid simultaneous grouting + double water-stop sealing gaskets."

"The Nanjing Sewage Phase II Channel no longer uses traditional bolts to fix segments; instead, segments are assembled directly inside the tunnel, a first in China," said Wu Wenqing, project manager of the China Railway 14th Bureau Group. This not only eliminates a large number of segment joints and saves labor, but also provides technical support for intelligent segment assembly in future shield tunneling.

Removing Bolts to Build an Upgraded Tunnel

To meet the needs of large-volume, long-term operation, standards for the finished quality and later maintenance of future tunnels are continuously rising.

According to Shan Xiaobo, chief engineer of the China Railway 14th Bureau project, the current domestic shield tunnel structure basically consists of reinforced concrete segments secured with bolts, with elastic sealing gaskets installed in the segment joints for waterproofing.

Studies have found that during construction, the extensive use of bolts creates a large number of joints, leading to many problems. "For example, reduced bearing capacity of concrete segments causes local cracking; large misalignment between segments leads to joint leakage and leakage at the bolt hand holes, which affects tunnel operation and maintenance and can even shorten the tunnel's service life," Shan said.

After receiving the Nanjing Sewage Phase II Channel construction task, the China Railway 14th Bureau project team carefully decided to break with traditional construction methods and boldly innovate by adopting the new "socket-type segment" technology, enabling a new technological leap in domestic shield construction and creating an "upgraded" Nanjing Sewage Phase II Channel.

The project technical team, together with all participating units, conducted extensive inspections and research to learn from best practices, overcame multiple key technical challenges, and repeatedly explored ways to successfully apply "new socket-type segments" in a 9-meter-class underwater shield tunnel.

The "new socket-type segments" use a new joint segment structure. "That is, the longitudinal joints of the segments use plug-in connectors, and the circumferential joints use insert-type connectors," Shan said.

Although this only involved a subtraction in segment design — "removing the bolts" — this subtle structural change set off a series of "butterfly effects" across the entire shield tunnel construction.

Grafting New Technologies to Provide Double Leakage Protection

"Using 'new socket-type segments' in underwater shield tunnels has no mature domestic experience to draw on, and places extremely high demands on segment prefabrication and assembly precision, shield machine driving posture adjustment, and the adaptability of shield equipment," Shan said.

Breaking convention is easier said than done. In the field of large-diameter underground shield tunnel construction assembled with bolted segment structures, the China Railway 14th Bureau has mature technical experience. "But this time, we had to 'tear down and start over' with our existing experience, which for us was undoubtedly a 'complete transformation,'" Shan said.

The biggest challenge in building underwater tunnels is preventing leakage.

To explore the "new socket-type segment" technology, the team kept exploring and researching more new technologies to graft onto the "new socket-type segments."

In 2021, after three years of exploration and practice, the China Railway 14th Bureau, together with Southwest Jiaotong University and China Railway Construction Heavy Industry Group, successfully developed the "key technology for simultaneous dual-liquid grouting in super-large-diameter slurry shields," filling a domestic technical gap.

The "simultaneous dual-liquid grouting" technology was successfully grafted onto the construction of the Jiangxinzhou Sewage Collection System Phase II tunnel, providing strong support for the new segment assembly technology. "During construction, the team not only used the new simultaneous dual-liquid grouting process behind the segment walls, but also adopted double waterproof sealing gaskets at the segment lining joints," Shan said.

The two waterproofing systems plus simultaneous dual-liquid grouting provide double insurance against tunnel leakage.

One Ring Assembled in 45 Minutes with Millimeter-Level Precision

The new segments require high assembly precision. To meet the new standards, the design team modified the sealing design of the assembly machine's vacuum suction cups, improving the machine's gripping capacity and segment assembly capability, achieving assembly precision of 1 millimeter.

"If prefabricated segment assembly is like building with Lego bricks, this time the new version of Lego we received came with no assembly instructions at all — we had to figure it out entirely on our own," said Yao Yinbin, the project's shield manager.

Through continuous exploration, the project team finally developed a mature set of assembly techniques. "At first, assembling one ring took more than three days; now, it takes only 45 minutes," Yao said.

The completed shield tunnel is 693 meters long, with 464 rings and 3,712 segments assembled in an orderly manner, solid inside and beautiful outside, successfully achieving a finished tunnel with no seepage or leakage, with segment uplift controlled within 5 millimeters and segment misalignment controlled within 2 millimeters.

"The complete set of socket-type segment construction technology for shield tunnels enriches basic scientific research in the shield field. In the future, we will establish relevant technical systems, control methods, and standards, supplementing new solutions and theoretical support for the construction of large-diameter and underwater shield tunnels in China," Wu Wenqing said.

The Nanjing Sewage Phase II Channel began construction in 2021. Once completed, it can directly transport about 400,000 cubic meters of sewage per day from downtown Nanjing to the Jiangxinzhou Sewage Treatment Plant for treatment, benefiting 1.74 million households and effectively ensuring the safe and economical operation of Nanjing's sewage channels.