China launches first ternary mixed gas shield tunneling system

China has successfully introduced its first domestically developed ternary mixed gas shield tunneling system. This innovative technology is currently operational in the 16.18-kilometer Jintang undersea tunnel, which holds the record as the world’s longest undersea high-speed rail tunnel.

The tunnel connects Ningbo and Zhoushan in East China’s Zhejiang Province. According to a report by Science and Technology Daily, the new equipment precisely mixes helium, nitrogen, and oxygen to create safe, pressurized conditions. This environment allows workers to perform critical cutter inspections and replacements on the shield machine.

The mega-project must navigate an ultra-high-pressure construction environment. The deepest part of the Jintang tunnel is buried 78 meters below sea level, where water and soil pressure can reach 0.85 megapascals. This is roughly equivalent to 30 kilograms of weight pressing down on a coin the size of a one-yuan piece.

Traditional compressed-air methods are generally limited to a safety pressure of 0.5 megapascals. To overcome this barrier, the construction team for the 6,270-meter section on the Zhoushan side adapted deep-sea diving technology to develop the specialized gas system.

The mixture uses helium because of its low weight, rapid diffusion, and high safety. These properties help reduce the risks of nitrogen narcosis and oxygen toxicity, ensuring workers can breathe safely in extreme, deep-sea-like conditions.

The system includes two primary modules for gas mixing and supply, supported by 113 submodules, including distribution hubs and breathing devices. It supports precise operations at pressures between 0.5 and 1 megapascals, perfectly matching the requirements for ultra-high-pressure undersea work.