Massive 442-foot tunneling machine building world’s longest undersea railway tunnel

Operators in China are steering a 442-foot (135-meter) tunnel boring machine from a ground control center roughly two miles from its excavation point beneath the sea, using cameras, sensors, and real-time alerts to monitor its progress.

The machine, named Yongzhou, is excavating part of the Jintang undersea tunnel connecting Ningbo and Zhoushan in Zhejiang Province.

It has advanced more than 13123.3 feet (4,000 meters) from the Ningbo side, completing about 83 percent of its planned excavation.

Now approaching a section about 305 feet (93 meters) beneath the seabed, the machine faces increasingly difficult ground conditions. Remote controls allow operators to guide excavation from the surface, while personnel remain onboard to supervise operations.

Steering from the surface

The remote-driving platform connects the ground control center with the machine’s underground operating systems. Cameras and sensors monitor cutterhead conditions, cutter wear, and excavation parameters, helping engineers identify problems as they develop.

The monitoring system also sends alerts directly to engineers’ smartphones, covering issues such as abnormal cutterhead torque, tunnel segment movement, and changes in oil and seal pressures.

“This tunnel boring machine’s (TBM) intelligent system is more sensitive than people,” said Dong Bing, chief mechanical engineer of the Yongzhou Railway project.

The team is also working toward greater automation. Assistant TBM operator Cheng Jie said future operators could initiate propulsion with a start command, with the machine then following preset parameters. That capability remains a development goal.

Cutting through difficult seabed rock

Built for the project’s demanding geology, Yongzhou has an excavation diameter of 47.8 feet (14.57 meters) and carries 308 cutters. It began operating in May 2024.

Nearly 70 percent of its route on the Ningbo side consists of unfavorable ground conditions. These include full-face hard rock, soft soil, and sections where the cutterhead encounters both materials simultaneously.

Rock strength reaches 240 megapascals, more than eight times that of ordinary concrete, according to the report.

The conditions have taken a toll on the cutting equipment. Around 4,000 cutters have been replaced so far.

Previously, excavation required roughly one cutter replacement per meter of advance. In tougher sections, a cutter may now need replacing after only a few dozen centimeters.

Dong said the machine is expected to reach the deepest section by the end of 2026.

Pressure chambers support cutter repairs

Maintaining the machine beneath the seabed requires workers to enter a high-pressure environment.

A saturation-pressure intervention system allows technicians to live in a pressurized chamber and work in shifts over periods of up to 28 days, reducing repeated compression and decompression.

During its first application in February, nine saturation divers worked in three groups over 22 days, replacing 46 cutters at approximately 75 meters beneath the seabed.

Yongzhou will eventually meet Dinghai, another boring machine advancing from Zhoushan. Both machines will then be dismantled inside the tunnel.

The Jintang tunnelstretches 10 miles (16.18 km), including a 6.96-mile (11.21 km) shield-driven section. The Yongzhou Railway is expected to open in 2028, bringing rail service to Zhoushan and cutting the Ningbo–Zhoushan high-speed rail journey to 26 minutes.