Work site of Shanhe TBM File photo: VCG

The world's largest-diameter underwater shield tunnel project, located in Jinan, East China's Shandong Province, successfully completed the Yellow River section excavation on Sunday, a milestone in the project's construction, China Media Group (CMG) reported.

The Jinan Huanggang Road Yellow River Crossing Tunnel spans about 5,755 meters, with a shield tunneling section of about 3,290 meters. It is designed as a single-tunnel, double-deck structure with a speed limit of 60 kilometers per hour, according to the report.

The tunnel is a key part of the "four bridges and four tunnels" project spanning the Yellow River, serving as a major infrastructure undertaking in support of China's national strategy for ecological protection and high-quality development in the Yellow River Basin.

The new progress represents a major breakthrough in overcoming the project's most challenging section, characterized by the strictest control requirements, longest-duration risks, and most variable water and soil pressures.

Sunday's tunneling breakthrough was achieved using the Shanhe tunnel boring machine (TBM), which has a diameter of 17.5 meters, successfully navigating under the southern embankment of the Yellow River, CMG reported.

The Shanhe TBM is the largest diameter slurry balance shield tunneling machine in use globally, reaching world-leading standards. In May 2024, this state-of-the-art machine was successfully launched in Jinan for the Huanggang Road tunnel project.

TBMs can excavate tunnels using a circular cross-section that drills through soil and rock strata, seamlessly laying concrete segments to construct tunnels, subway tubes and sewer lines.

The tunnel traverses highly sensitive areas, including embankments on both sides of the Yellow River, with a maximum overburden depth of 49.2 meters and water-soil pressure reaching 6.3 bar, equivalent to 6.3 kilograms of force per square centimeter. Such extreme conditions demand precise settlement control and pose significant engineering challenges, the CMG report said.

Also, the riverbed's silty clay soil, interspersed with calcareous nodules, risks clogging the cutterhead with mud cakes and obstructing slurry pump pipes, the CMG report said. 

The project team addressed these issues by increasing the cutterhead's opening ratio and installing a high-flow central flushing system to prevent mud caking. They also optimized the cutter configuration to handle the irregularly distributed calcareous nodules effectively, CMG reported, citing Zhou Xiang, a senior engineer with the project.

Additionally, the team adopted innovative techniques to boost waste slurry treatment capacity, processing nearly 600 cubic meters per hour - a 50-percent efficiency boost - while reducing filter cake moisture to below 30 percent, Zhou noted.

The Shanhe TBM has safely traversed the primary water source protection area, sedimentation ponds, and embankments on both sides of the Yellow River, completing 83.6 percent of the total shield-driven section, according to CMG. 

The progress of this project is yet another testament to China's infrastructure prowess and a prime example of the nation's continuous advancements in high-end manufacturing, Wang Peng, an associate research fellow at the Beijing Academy of Social Sciences, told the Global Times on Sunday. "These project and technological breakthroughs will provide robust support for unlocking greater development potential in cities across China and accelerating economic growth."

The tunnel is expected to achieve full breakthrough by the end of 2025 and begin operations in September 2026, the Xinhua News Agency reported in February.

Upon completion in September 2026, the tunnel will enhance connectivity between Jinan's main urban area and the city's New and Old Growth Driver Conversion Pilot Zone, accelerating regional development and bolstering ecological conservation and high-quality growth in the Yellow River Basin, according to CMG.