On July 19, the 9.73-meter-diameter shield machine "Shuxin" developed by the company, after 843 days and nights of arduous struggle, successfully bored through the tunnel on the Dujiangyan-Yongfeng Station section of the Sishan Mountain Rail Transit Project. This marks that the technical challenge of tunneling in the water-rich, large-diameter boulder "extreme stratum" has been fully overcome by the two shield machines developed by Tianhe. The underground shield tunnel section in Dujiangyan urban area has a total length of 6.67 kilometers and was bored jointly by "Shuxin" and its sister ship. Three months earlier, "Shuguang" had successfully broken through.

Breakthrough site

The Dusi Mountain Rail Transit Project is the country's first high-altitude mountain rail project using rack rails as its running track, and is also a key rail transit tourism pilot poverty-alleviation project in Sichuan Province. It starts at Dujiangyan Station on the Chengdu-Guanxian high-speed railway and ends at Siguniangshan Town in Xiaojin County, Aba Prefecture, with a total line length of 123.18 kilometers and a designed operating speed of 120 kilometers per hour. Once completed, the project will open a new chapter in China's mountain rail transit, connect the rich tourism resources of western Sichuan with the Chengdu Plain, create a new benchmark for China's integrated transport-tourism poverty-alleviation model, inject lasting momentum into the efforts of people along the line to shake off poverty and build a moderately prosperous life, and have important strategic significance and practical value for extending and supplementing China's comprehensive multi-dimensional transport network.

The project geology consists mainly of highly water-rich sandy gravel and boulder strata, with boulders 200 to 1,500 millimeters in diameter accounting for more than 80% of the gravel, equivalent to drilling holes in a pile of gravel. The gravel strength is as high as 150 MPa, making it the most difficult shield tunneling project in gravel strata under construction in China, hailed by the industry as the "extreme stratum" and "shield tunneling forbidden zone" of sandy gravel strata. The stratum characteristics of high proportions of gravel and boulders, large particle sizes and high strength may bring challenges such as frequent cutterhead jamming, chipping and detachment of cutters, and screw blockage and broken shafts.

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The company undertakes the mission of pioneering and demonstrative exploration. Based on the geological and stratum conditions of the project, and following the principle of "discharge as the main method, crushing as the auxiliary, and combined discharge and crushing," it carried out targeted and adaptive design of the supplied shield machine. The cutterhead adopts a 6-spoke + 6-panel layout, with an opening rate of 40%, equipped with 352 cutters of various types, a muck conditioning system, a belt-type (shaftless) screw and stable conveying system, and overall wear-resistant treatment of the surfaces where all parts of the shield machine contact gravel, so as to cope with tunneling in complex geological strata. During tunneling, "Shuxin" achieved an average daily advance of 9 rings (14.4 meters) and a monthly advance of 321 rings (513.6 meters), setting a new record for tunneling in the extreme stratum of water-rich large-diameter gravel and boulders.

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The project is constructed by China Railway Tunnel Group, and the company provides the shield machines required for project construction. During tunnel tunneling, the company's technical team closely focused on key and difficult issues, emphasized technological innovation in the construction field of water-rich large-diameter boulder and isolated rock extreme strata, and achieved 8 QC results, 5 utility model patents, 10 invention patent applications and a number of other honorary certificates.