"I have encountered 18 instances of sudden mud and water inrush on this line—water and mud would just burst out all at once. I think we are lucky; facing these dangers, we have averted them time and again. As a member of the people's armed forces, one should have this kind of spirit of dedication." Lei Yu, a technician from the Dali-Ruili Railway No. 2 Branch, said loudly at the noisy drill-and-blast excavation construction site of the Gaoligongshan Tunnel.

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As a key controlling project of the Dali-Ruili Railway, the Gaoligongshan Tunnel has a total length of 34.538 kilometers and has set nine records, including China's longest railway tunnel, longest inclined shaft, and deepest vertical shaft. Since construction began at the end of 2015, after more than nine years of all-out efforts, excavation of the Gaoligongshan Tunnel has passed the halfway point.

The geological conditions of the Gaoligongshan Tunnel are extremely complex, requiring it to pass through 18 different rock strata and 19 active fault zones. It is known as "the world's most difficult railway to build" and a "geological museum." To build the 34.538-kilometer-long, 9.03-meter-wide main tunnel, a parallel pilot tunnel of the same length and 6.36 meters wide must also be built to assist excavation, along with one 3,870-meter-deep inclined shaft and two 640-meter-deep vertical shafts to increase working faces, realizing the "long tunnel, short drive" construction method. Wang Yafeng, project manager of the Dali-Ruili Railway No. 2 Branch, introduced: "During construction, we overcame a series of unfavorable geological zones, including high ground temperature in the long and large inclined shaft, mud and water inrush in the ultra-deep vertical shaft, fully weathered granite, fault fracture zones, densely jointed zones, and lithological contact zones."

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During tunnel excavation, the main tunnel and the parallel pilot tunnel each use an open-type hard rock TBM independently developed by China. Under complex geological conditions such as high geothermal heat, high ground stress, and high seismic intensity, the TBMs advanced in fits and starts, making excavation extremely difficult. "During TBM excavation, we encountered 42 instances of the machine becoming stuck and trapped. We got through and overcame water and mud inrush, densely jointed zones, fault fracture zones, lithological contact zones, and the high-pressure deep-buried alteration zone currently encountered," said Wang Yafeng, project manager of the Dali-Ruili Railway No. 2 Branch.

More than 1,100 meters deep underground in the Gaoligongshan main tunnel, facing harsh working conditions such as darkness, confined space, high temperature and humidity, and harmful gases, batch after batch of engineers have stayed on the job, working tirelessly day and night to advance tunnel construction. Multiple teams, including the excavation team, lining team, drill-and-blast excavation team, and grouting team, work together in coordinated construction, jointly tackling each new geological challenge they face and ensuring the normal progress of every stage of tunnel construction. "Grouting is to reinforce the unfavorable geology ahead that we are about to encounter and ensure the safety of excavation construction. We work continuously 24 hours a day, with each shift lasting 12 hours, in three shifts," said Zhang Fengqi of the grouting team of the Dali-Ruili Railway No. 2 Branch, while monitoring the operation of the grouting machine.

Yu Xingcheng, foreman of the secondary lining team of the Dali-Ruili Railway No. 2 Branch, came down from the arch frame and said: "The secondary lining is the final procedure of the entire tunnel and also the most critical step. Ensuring the entire tunnel does not leak water, that the steel reinforcement is stable, and that arch frame deformation caused by rock stratum changes is handled—all of this must be dealt with in advance by our secondary lining work."

Through the unremitting efforts of more than 1,600 railway builders and over 3,000 consecutive 24-hour battles, excavation of the 34.538-kilometer-long Gaoligongshan Tunnel has passed the halfway point. The overcoming of one tunnel construction challenge after another and the breakthroughs in one railway technology after another have laid a solid foundation for the early completion and opening of the Dali-Ruili Railway, provided valuable experience for the construction of major and difficult tunnels in the future, and are of epoch-making significance in the history of China's railway construction.