On August 21, as the east-line shield machine broke through precisely, the east line of the shield tunnel of the Beijing East Sixth Ring Road Reconstruction Project—a national-level innovation project designed by China Railway Siyuan Survey and Design Group and constructed with the participation of China Railway 14th Bureau Group—was completed, marking the full completion of China's longest shield-driven expressway tunnel.

The Beijing East Sixth Ring Road Reconstruction Project is a key project for implementing the Beijing Master Plan and the Regulatory Detailed Plan for the Beijing Municipal Administrative Center. It bears the important mission of "creating an innovative development axis that stitches together urban functions, effectively repairing urban space, and solving the problem of expressways dividing the city."

The reconstruction project has a total length of 16.3 km, including 7.4 km of shield tunnel for the underground reconstruction. The shield tunnel has a diameter of 15.4 m and a maximum burial depth of 59 m. Each tunnel bore accommodates three lanes and is divided into three levels: the upper level is a smoke exhaust passage, the middle level is the traffic passage, and the lower level is an evacuation and rescue passage. It is the largest-diameter shield tunnel under construction in northern China.

Plan and profile schematic of the East Sixth Ring Road Reconstruction Project

In May 2021, during the first section of excavation of the west-line tunnel of the Beijing East Sixth Ring Road Reconstruction Project, the "Jinghua"—the largest domestically developed shield machine, jointly developed by China Railway 14th Bureau Group and China Railway Construction Heavy Industry—successfully overcame 11 technical challenges, including shield launching under shallow cover, high-risk lifting of the shield machine, and long-distance crossing of high-density sand layers, and successfully passed through more than 30 risk sources such as the Beijing-Harbin Railway and the Beijing Municipal Administrative Center Comprehensive Transportation Hub.

During construction, the project builders set an excavation record of 542 meters in a single month, applied for 23 research and development items including construction methods and patents, and pioneered the super-large-diameter synchronous dual-liquid grouting technology, which filled a domestic gap and accumulated valuable experience for similar shield construction.

On June 28, 2023, the "Jinghua" shield machine broke through precisely, marking the successful completion of the west-line tunnel of the Beijing East Sixth Ring Road Reconstruction Project undertaken by China Railway 14th Bureau Group and of China's longest shield-driven expressway tunnel under construction (7.4 km total length).

The Beijing East Sixth Ring Road has four characteristics: "long, large, deep, and numerous."

1. Longest

The longest single continuous shield excavation distance reaches 4,770 meters, making it one of the tunnels with the longest single excavation length for a super-large-diameter shield in China.

2. Largest

The designed cross-sectional diameter of the tunnel reaches 15.4 meters, making it the largest-diameter shield tunnel under construction in northern China.

3. Deepest

The maximum burial depth of the tunnel is about 75 meters, and the maximum water head reaches 65 meters.

4. Numerous risk points

It crosses as many as 115 risk sources, including the Beijing-Harbin Railway, the Municipal Administrative Center Hub Station, Metro Line 6, and the Tongzhou-Sanhe Railway.

Schematic of risk sources

Facing the above difficulties, the construction team of China Railway 14th Bureau Group applied a series of technological innovations that are novel in concept, safe and reliable, and convenient to construct.

01

Innovation in grouting materials

In view of the project characteristics of long-distance excavation in high-pressure water-rich sand layers and crossing dense risk sources, the design team and all participating parties held multiple discussions and proposed the method of synchronous grouting with cement-water glass dual-liquid grout.

Principle of dual-liquid grout

Builder demonstration

Compared with traditional single-liquid grout, this process shortens the solidification time of the grout outside the tunnel lining from 8 hours to 1 hour, promptly and effectively filling the gaps behind the segment rings, avoiding segment uplift, misalignment, and the resulting segment damage and water leakage, achieving millimeter-level control of ground settlement and "zero leakage" in the completed tunnel.

02

Innovation in structure and waterproofing

The shield tunnel is assembled from multiple segments, and each segment serves both structural and waterproofing functions. Water leakage is prone to occur and has become the greatest hidden danger to operational safety.

Schematic of segment concave-convex tenon

This project adopts a new connection method of "distributed concave-convex tenons + bolts," and uses "outer primary and inner auxiliary" double sealing gaskets at the joints, effectively controlling joint misalignment, greatly improving the reliability of joint waterproofing, and significantly enhancing the overall bearing capacity.

03

Innovation in the ultra-deep foundation pit of the intermediate shaft

The intermediate shield working shaft is used during construction for shield machine passage and maintenance, and during operation as an equipment room area for the tunnel. The foundation pit excavation depth reaches 43.4 meters, and the retaining underground diaphragm wall is 73.4 meters deep, mostly in complex and sensitive silty clay layers and fine silty sand layers.

Structure of the intermediate shaft foundation pit

In view of this, various innovative design schemes such as composite walls, curved base slab structures, and top-down construction were adopted, effectively preventing risks such as water and sand ingress into the foundation pit.

04

Innovation in reception under ultra-shallow cover

The cover soil at the side reception working shaft is only 7.44 meters, less than 0.5 tunnel diameters. The stress, waterproofing, and anti-flotation performance of the tunnel lining are key concerns of the project. The construction team adopted a series of measures, including dewatering during construction, enhanced synchronous grouting outside the segments, and steel plate tying of segments within the end reinforcement zone, effectively ensuring reception under ultra-shallow cover at the south working shaft.

As a controlling node of the East Sixth Ring Road Reconstruction Project, the completion of both lines of the shield tunnel has laid a solid foundation for the project's opening to traffic next year. After the project opens to traffic, it will promote the integration of the Tongzhou urban area with the administrative office area of the Municipal Administrative Center, and is of great significance for improving Beijing's traffic service level, promoting regional transportation integration in the Beijing-Tianjin-Hebei region, and building a comprehensive transportation system.