On August 12, after more than two years of effort by railway builders, the Dazhuang Tunnel on the Mile-Mengzi High-Speed Railway in Yunnan was successfully holed through. With this, all 10 tunnels along the entire line have been completed, laying a solid foundation for the line to open to traffic in 2022.

The Dazhuang Tunnel is located in Kaiyuan City, Honghe Hani and Yi Autonomous Prefecture, Yunnan Province. With a total length of 2.7 kilometers, it is a double-track railway tunnel designed for a speed of 250 kilometers per hour, and it is also the last tunnel on the entire line to be holed through.

Since construction began in July 2019, the tunnel has passed through soft shale and karst development zones, posing high risks such as large deformation of soft rock, mud and water inrush, and deformation-induced collapse. Construction was therefore highly difficult, and a total of seven sliding collapses occurred, with the maximum deformation of the initial support reaching 1.2 meters.

In response to the complex geology and construction difficulties mentioned above, the South Yunnan Railway Construction Command of China Railway Kunming Group organized units including China Railway No. 3 Engineering Group and China Railway Eryuan Engineering Group to optimize the design and construction plans. They strictly followed the tunnel construction principles of "advance pipe forepoling, strict grouting, short excavation, weak blasting, rapid sealing, and frequent measurement," used labor competitions as an entry point to continuously drive construction production, used "Party member red-flag responsibility zones" as a carrier to strengthen construction process control, and used Party member-led technical research as a guide to improve process efficiency indicators.

The Mile-Mengzi High-Speed Railway starts in the north at Mile Station on the Yunnan-Guangxi Railway, runs south through Mile City and Kaiyuan City, and ends at Mengzi City, the seat of the Honghe Prefecture government. It connects with the Kunming-Hekou Railway, part of the China-Vietnam international corridor, and integrates into the railway hub centered on Kunming. The main line of the Mile-Mengzi High-Speed Railway is about 107 kilometers long, with a target passenger train speed of 250 kilometers per hour. Construction began in 2018. The line includes four newly built stations—Zhuyuan, Pengpu, Kaiyuan South, and Honghe—as well as 46 bridges, with a bridge-tunnel ratio of more than 45%.

Peng Faxin, chief engineer of the Mile-Mengzi High-Speed Railway Section Command of China Railway Kunming Group, said that at present, more than 97% of the subgrade, bridges, and tunnels of the Mile-Mengzi High-Speed Railway have been completed, 58 kilometers of track have been laid, and construction of the station buildings has fully started, with foundation work underway. The entire line is expected to meet the conditions for opening to traffic in 2022. After completion and opening, Kunming and Mengzi will be reachable within two hours, and the line will become a convenient passenger transport corridor between the two major urban agglomerations of central Yunnan and southeastern Yunnan.