On the 22nd, inside the Jintang Subsea Tunnel of the Yongzhou Railway, the world's longest underwater high-speed rail tunnel, the "Dinghai" shield machine successfully completed the replacement of the 67th cutter, marking the successful conclusion of the world's first "ternary mixed gas" shield cutter replacement operation. China has pioneered a new technical route in the field of ultra-high pressure pressurized shield construction.

Ultra-high pressure chamber entry operations are recognized as a world-class challenge in the shield tunneling industry. The safe pressure limit for traditional compressed air chamber entry operations is 0.5 MPa, while the maximum burial depth of the Jintang Subsea Tunnel is 78 meters below sea level, with a maximum water and soil pressure of 0.85 MPa, equivalent to a one-yuan coin bearing a weight of about thirty kilograms. Facing extreme working conditions, China Railway 11th Bureau, responsible for the 6,270-meter shield construction on the Zhoushan side, drew on deep-sea diving technology to successfully develop helium-nitrogen-oxygen "ternary mixed gas" pressurized shield operation equipment suitable for shield construction, and carried out shield cutter replacement operations in the Jintang Subsea Tunnel.

The "ternary mixed gas" is composed of helium, nitrogen, and oxygen in specific proportions. Helium, with its low density, fast diffusion, and high safety, effectively prevents risks such as "nitrogen narcosis" and "oxygen toxicity" for workers in ultra-high pressure environments, building a safe breathing barrier for operations in deep-sea ultra-high pressure conditions.

This cutter replacement operation lasted 75 days. The construction team carried out work at six locations 64 meters below sea level, completing a total of 313 pressurized chamber entries and 936 safe personnel entries, successfully replacing 67 cutters of various types. This set multiple world records in the shield tunneling engineering field, including the deepest operating depth, highest pressure, highest frequency of ultra-high pressure cutter replacements, and longest continuous operation period.

To date, the overall shield tunneling progress of the Jintang Subsea Tunnel has reached 10,005 meters, with 89.3% of the engineering construction completed. According to construction plans, subsequent work will continue to rely on the "ternary mixed gas" pressurized shield operation equipment to ensure the safety of pressurized operations and guarantee the timely completion of the tunnel.