First High-Speed Rail Tunnel Under Yangtze River Completed

China has achieved a landmark engineering feat with the completion of the Chongqing Caiyuanba Yangtze River Railway Tunnel, the first-ever high-speed railway tunnel to cross beneath the Yangtze River. The 11.9-kilometer tunnel, which broke through on July 24, 2026, represents a critical milestone for the Chongqing-Xiamen High-Speed Railway and showcases China’s rapidly advancing capabilities in ultra-complex underground construction, according to People’s Daily.

A Tunnel Born of Necessity

Situated between Chongqing’s Yuzhong and Nan’an districts, the tunnel was driven by an inescapable geographic reality: Chongqing’s dense urban core leaves no room for surface rail corridors. The city’s historic Yuzhong District, situated on a narrow peninsula between the Yangtze and Jialing rivers, made traditional bridge or surface approaches prohibitively disruptive.

“Initially we considered a bridge option,” Wu Juncai, the shield tunneling section leader, recounted in a first-hand account published by Guancha (Observer Net). “But one end connects to Yuzhong District, Chongqing’s mother city. The buildings are too dense — the demolition costs, investment, and impact on the city were all too great. After comparisons, a shield tunnel beneath the Yangtze riverbed became the only choice.”

Engineering Against the Odds

The project demanded mastery of what engineers call the “four crossings”: through a mountain (Nanshan), beneath a dense city (126 buildings and 6 operating metro lines), under a major river (the Yangtze’s main channel), and past multiple water reservoirs. The shield tunneling section alone spanned 3,845 meters, including 1,282 meters directly under the Yangtze, where water pressure reached 0.93 MPa (9.3 bar).

At the heart of the operation was the “Yangtze River” mud-water balance shield machine — a 12.66-meter-diameter, 142-meter-long, 3,500-ton behemoth featuring a 340-ton cutterhead with 239 custom cutting tools. What made this project unprecedented was where and how the machine was assembled: inside a mountain cave rather than from a surface shaft, a domestic first.

Wu described the razor-thin margins of that cave assembly: “The clearance between the crane arm and the cave ceiling was only 5 centimeters. If we had carved the chamber even slightly smaller, the consequences would have been unthinkable.”

Rock conditions proved far tougher than geological surveys had predicted. Actual rock strength hit 60–70 MPa versus the 30+ MPa forecast, with quartz content reaching 45%. The team responded by modifying cutting tools with hardened alloys and ceramic coatings, tripling tool life and accelerating daily progress from 4 meters to 10 meters.

Zero Pipe Bursts, Millimeter Precision

One of the project’s most remarkable achievements was managing a 140-meter vertical elevation difference for the mud slurry circulation system — from the riverbed to the surface — which created immense water hammer effects. Wu admitted the pressure took a personal toll: “I became allergic to any abnormal sound. Middle-of-the-night phone calls made my heart skip a beat. My wife later told me I was even arranging work in my sleep. I had nightmares of pipe bursts, mud flooding in, the shield machine drowning.”

The team implemented more than ten safety measures, including emergency flood barriers, pneumatic pressure relief valves, and multiple dam walls. The result: zero pipe bursts throughout the entire tunneling operation.

Above ground, continuous real-time monitoring of 126 buildings and 6 metro lines ensured millimeter-level settlement control, with automated parameter adjustments maintaining near-zero ground displacement.

A Milestone for a National Network

The Caiyuanba tunnel is a critical control project of the Chongqing-Xiamen HSR, itself a major component of China’s “Eight Vertical and Eight Horizontal” national high-speed rail network. The route will eventually connect Chongqing in the southwest through Hunan, Jiangxi, and Fujian to Xiamen on the southeast coast — linking inland regions with the economically dynamic coastal belt.

For Chongqing itself, the transformation is profound. Chongqing Station (Caiyuanba), originally built in 1952 as a terminus and closed to passenger services in 2022, will be reborn as a through-station capable of handling high-speed rail traffic. As the Chongqing Municipal Government noted, the tunnel achieves “high-speed rail entering the city without visible tracks, without bridges, without noise, without fragmentation” — using underground mega-projects to free up above-ground urban space.

Broader Implications

The project’s innovations — cave-based shield machine assembly, high-pressure slurry management, customized cutter technology for quartz-rich rock — create a knowledge base with global applicability, as Xinhua reported. The team published more than 10 academic papers, filed 6 national patents, certified 5 provincial-level construction methods, and implemented over 20 micro-innovations.

The tunnel also offers a template for other cities grappling with the “high-speed rail entering the city” challenge — how to bring HSR into dense urban cores without physical or visual disruption. Similar projects in Beijing, Chengdu, and Suzhou have drawn on comparable techniques.

What Comes Next

With tunneling complete, the focus shifts to ballastless track installation, signaling, and electrification. The full Chongqing-Xiamen HSR line is expected to open in 2027, which will dramatically reduce travel times between Chongqing and provinces to the east. When operational, the corridor will allow through-trains from Chengdu through Chongqing to Changsha and Xiamen, seamlessly connecting the Yangtze River Economic Belt with China’s southeast coast.

For those who built it, the achievement carries deep personal significance. Wu recalled the final moments: “I remember saying, ‘Goodbye, old friend.’ A lump of metal — we had been together for nearly three years. We had touched nearly every part of it, repaired every breakdown… Later, people took photos, applauded, embraced. I noticed many who were usually very tough had slightly reddened eyes.”

As China pushes toward completing 95% of its national comprehensive transportation network framework by decade’s end, the Yangtze tunnel stands as both a technical triumph and a symbol of the country’s approach to infrastructure: ambitious in scale, relentless in execution, and increasingly sophisticated in the details that matter most.