On July 19, good news came from the Dujiangyan to Siguniang Mountain Mountain Rail Transit Poverty Alleviation Project, a key project in Sichuan Province and China's first mountain rail transit project. After 843 days and nights of arduous struggle, the "Shuxin" shield machine of the Dusi Project—invested by Shudao New System Rail Group Dujin Company and constructed by China Railway Tunnel Group—reached an exciting moment after tunneling 3,412 meters underground. The rumbling sound of breaking through the wall marked the successful breakthrough of the tunnel on the Dujiangyan Station–Yongfeng Station section, marking the full breakthrough of the 6.67-kilometer underground shield tunnel section in Dujiangyan urban area of the Dusi Project and achieving the ** full-section breakthrough of the entire Dusi Project. This indicates that the technical challenge of tunneling in the "extreme stratum" of water-rich large-diameter boulders has been fully overcome, bringing the coordinated development of the Chengdu Plain Economic Zone and the Western Sichuan Ecological Demonstration Zone one step further forward.

The Dujiangyan Station–Yongfeng Station section that broke through this time has a total length of 3,412 m. An earth pressure balance shield machine with an excavation diameter of 9.73 meters was launched from Yongfeng Station. The route runs along Yong'an Avenue from northeast to southwest and finally reaches Dujiangyan Station.

Key and difficult points of project construction

The shield tunnel section needs to pass through "extreme strata" such as water-rich sandy gravel, boulders, and isolated rocks. The boulders range from 200 to 1,500 mm in diameter, with strength reaching up to 150 MPa and content as high as 80%, which means the shield machine is almost entirely tunneling through piles of gravel and boulders, making it extremely difficult. The surrounding environment of the tunnel is extremely complex. The section crosses hundreds of municipal pipelines, including urban main roads and main sewage pipes. During the process, major difficulties were encountered, such as abnormal ground settlement, screw conveyor blockage, muck gushing, tool wear, cutterhead wear, and even cutterhead blockage.

Multiple measures taken to overcome difficulties

First, under the condition of a very complex surrounding environment for the shield machine, the project team overcame difficulties and organized scientifically, inviting well-known domestic experts to hold 6 consultation meetings and 4 demonstration meetings, and organizing project technical personnel to hold more than 10 technical seminars and technical summary meetings. Second, relying on the "Mu Yongqi Shield Extreme Stratum Research Base Craftsman Studio," and closely focusing on key and difficult issues in the construction process, project construction has continued to develop positively. Technological innovation was carried out around construction in the extreme stratum of water-rich large-diameter boulders and isolated rocks, special scientific research topics were established, processes and methods were explored, and the results were transformed into technical strength to guide shield tunneling. Third, creatively, in this "extreme stratum," the method of "pressurized opening" for tool replacement in an earth pressure balance shield was adopted for the first time. Compared with the previous method of opening under atmospheric pressure for tool replacement, this greatly improved both economy and time limits, ultimately achieving the safe and smooth crossing of structures and pipelines, setting a tunneling record of 513.6 meters per month in the extreme sandy gravel stratum, and achieving a comprehensive victory in shield tunneling in the "extreme stratum."

The full breakthrough of the underground shield tunnel section in Dujiangyan urban area of the Dusi Project greatly encourages the accelerated construction of the entire project. Subsequently, it will enter the track laying and electromechanical installation stages, laying a solid foundation for the project to open to traffic on schedule. After completion, the Dusi Project will have important strategic significance and practical value for extending and supplementing China's comprehensive multidimensional transportation network. It will open a new chapter in mountain rail transit, create a new benchmark for the integrated development model of "transportation + tourism," connect the rich tourism resources of western Sichuan with the Chengdu Plain, and greatly drive economic and social development along the line.