China Deploys Mixed Gas System for Jintang Undersea High Speed Rail Tunnel

A New Breathing System Beneath Zhejiang's Waters

China has put its first domestically developed ternary mixed gas shield tunneling system into service at the Jintang undersea tunnel in Zhejiang Province. The equipment supplies a controlled mixture of helium, nitrogen and oxygen so workers can maintain tunneling machinery at pressures beyond the traditional limits of compressed air operations.

The system is operating on the Zhoushan side of the project, where construction crews are responsible for a shield tunneling section measuring 6,270 meters. Their work forms part of a railway tunnel measuring 16.18 kilometers that will connect Ningbo and Zhoushan and become the world's longest undersea high speed railway passage when complete.

The tunnel's deepest section lies 78 meters below sea level. Combined water and soil pressure there reaches 0.85 megapascals, compared with the recognized safety pressure limit of 0.5 megapascals for conventional compressed air techniques. A megapascal is a unit used to measure pressure, or how much force acts on a given area.

The new equipment supports operations between 0.5 and 1 megapascal. Its upper operating limit therefore exceeds the maximum pressure identified for the Jintang tunnel, giving engineers room to manage the project's demanding conditions.

For workers, the central challenge is entering a pressurized excavation chamber to inspect or replace cutting tools. Engineers have adapted breathing technology used in deep ocean diving to help them remain alert and manage the physical risks of working in that environment.

Why the Tunnel Needs Workers at the Cutting Face

The Jintang tunnel crosses a busy waterway between Ningbo and the archipelago city of Zhoushan. It is a critical part of a larger transportation network intended to connect the islands more closely with the mainland, where geographic separation has long complicated journeys and logistics for commuters and freight.

The Zhoushan construction section requires months of continuous boring through changing geology beneath the seabed. Shield machines, also known as tunnel boring machines or TBMs, excavate circular passages and install supporting structures as they advance, helping prevent the surrounding ground from collapsing into the tunnel.

A rotating cutter head at the front of each machine breaks through rock and sediment. Its cutting tools wear down during excavation, so crews must inspect them and replace worn components. The machine can perform the excavation, but maintaining the equipment still requires people to work close to the cutting face.

Workers carrying out those tasks enter a chamber kept under pressure to hold back water and soil. Maintenance in that enclosed space is among the most dangerous jobs in civil engineering because crews must manage both the machinery and the effects of pressure on their bodies.

The Jintang project combines a long excavation route with demanding conditions below the sea. Keeping its cutting equipment in working order is essential to continuing the 6,270 meter drive on the Zhoushan side.

Where Compressed Air Reaches Its Limit

Construction teams have traditionally used compressed air to keep water out of excavation chambers while workers enter to service cutter heads. That approach works within established pressure limits, but the health risks increase sharply as pressure rises beyond them.

Conventional compressed air techniques have a recognized safety pressure limit of 0.5 megapascals. Beyond that threshold, decompression sickness and other physiological dangers make human entry prohibitively hazardous under standard protocols.

The Jintang tunnel reaches 0.85 megapascals at its deepest section. Using those figures, its maximum pressure is 0.35 megapascals, or 70 percent, above the traditional compressed air limit. That gap explains why an alternative breathing system is central to the work.

The pressure can also be pictured as roughly 30 kilograms of weight pressing on an area no larger than a one yuan coin. Workers need carefully managed breathing gases and pressure procedures to function in such conditions.

Water and soil pressure together create the load around the excavation chamber. Keeping the chamber pressurized protects the working space, while managing the gases workers breathe protects the people inside it. Both tasks have to be handled at the same time.

What Engineers Borrowed From Deep Ocean Diving

The construction team turned to deep ocean diving technology to address the pressure problem. Commercial divers have long used mixed breathing gases and saturation diving methods to work at depths where ordinary compressed air would be dangerous.

The same basic effects of pressure apply to a person maintaining an offshore oil platform and a person replacing tools on a tunnel cutter head. Engineers adapted those marine principles into a system designed specifically for shield tunneling.

Ternary means that the breathing mixture contains three gases: helium, nitrogen and oxygen. Each gas must be present in a suitable proportion for the pressure at which the worker is operating.

Helium is the key addition. It has low density and diffuses rapidly through body tissues. Replacing part of the nitrogen with helium helps workers stay mentally alert under pressure, while controlling the oxygen proportion helps limit the risk of oxygen toxicity.

Ordinary air contains a large proportion of nitrogen. At high pressure, more nitrogen dissolves into the body and can produce narcosis, a condition that clouds judgment and slows reactions. Oxygen is essential for breathing, but it can also become toxic when its partial pressure is too high for too long.

Partial pressure means the share of the total pressure contributed by one gas in a mixture. Engineers therefore have to consider both the proportion of oxygen and the pressure around the worker. A mixture suitable at one pressure may need adjustment at another.

Two Main Modules and 113 Supporting Units

The Jintang system combines two main modules: one mixes the gases and the other supplies them. They are supported by 113 specialized submodules, including gas distribution hubs, supply hubs and individual breathing apparatus.

This arrangement allows precise control of the mixture delivered to each worker in the pressurized chamber. The equipment is more extensive than a collection of gas cylinders because it must coordinate gas preparation, distribution and breathing support throughout an operation.

Gas blending algorithms adjust the proportions of helium, nitrogen and oxygen in real time to match the pressure at the cutter face. The aim is to provide an appropriate breathing mixture as conditions change along the tunnel route.

The operating range of 0.5 to 1 megapascal covers the Jintang tunnel's maximum of 0.85 megapascals. The difference between the project maximum and the system's upper limit is 0.15 megapascals, providing an operating margin and scope for use in projects with greater pressure demands.

Each worker's supply depends on several connected parts functioning together. Gas must be mixed correctly, routed through the distribution equipment and delivered through the individual breathing apparatus at the required pressure.

Keeping Workers Alert and Managing Oxygen

Nitrogen narcosis is one of the most dangerous effects of working under high pressure. Nitrogen dissolves into neural tissues and can cause symptoms similar to alcohol intoxication, including euphoria, disorientation and slower reactions.

A worker experiencing narcosis may struggle to judge a situation or carry out a familiar maintenance task correctly. In an enclosed chamber beside heavy machinery, impaired judgment can endanger both that worker and colleagues.

Helium does not produce the narcotic effect associated with nitrogen. Its use in the mixture helps preserve alertness, which is why it is used in deep ocean diving and has now been incorporated into this tunneling system.

Oxygen presents a different problem. Breathing it at elevated partial pressures for prolonged periods can damage the lungs and central nervous system. The system controls the oxygen fraction and dilutes it with helium and nitrogen to supply enough oxygen for physical work while managing exposure.

The breathing mixture has to meet several needs at once: support vigorous labor, reduce nitrogen narcosis and keep oxygen exposure within the conditions planned for the operation. Precise gas control is therefore a central part of worker protection.

Leaving the Chamber Takes Careful Decompression

Workers finishing a shift must pass through a series of pressure stages so dissolved gases can leave their bodies gradually. Moving too quickly from high pressure to lower pressure can cause bubbles to form in the blood.

Those bubbles can trigger decompression sickness, a painful and potentially fatal condition. The danger makes the exit procedure part of the job itself, rather than a simple step through a chamber door.

The mixed gas system supports controlled decompression schedules tailored to the gas load accumulated during each shift. Breathing a helium mixture still requires careful pressure management when workers leave the chamber.

The use of diving principles extends beyond the choice of gases. Entry, work and decompression all need to be coordinated so that workers can perform maintenance and return from the pressurized environment under controlled conditions.

Equipment Built for Saltwater, Vibration and Pressure

The system's components must withstand a corrosive saltwater environment, continuous vibration from the tunnel boring machine and the mechanical stresses of operating under high pressure. Those demands affect the equipment supplying the workers as well as the machine excavating the tunnel.

Engineers have designed redundant safety systems so that failure in a single submodule does not compromise life support for the entire crew. Redundancy means providing backup capacity or alternative functions when a component fails.

Remote monitoring allows teams at the surface to follow gas flow rates, pressure levels and individual workers' vital signs without entering the chamber themselves. Those measurements help them track both the gas supply and the condition of the people using it.

The combination of controlled mixing, backup systems and monitoring supports work in a space where access is difficult and mistakes can have serious consequences. Reliable equipment is especially important during cutter maintenance, when excavation depends on crews completing their tasks.

What the Deployment Means for Construction

The Jintang system addresses a major bottleneck in undersea tunneling: how to maintain cutting equipment when chamber pressure exceeds the range of conventional compressed air work. On the Zhoushan side, it is intended to help the 6,270 meter section proceed on schedule while protecting workers.

The tunnel will become the longest undersea high speed railway passage when completed. Its 16.18 kilometer route is intended to shorten journeys between the mainland and the archipelago and support closer economic ties and tourism across Zhejiang.

China already undertakes some of the world's most ambitious underground transit projects. The deployment adds a domestically developed system for a particularly demanding part of that work and draws together experience from marine operations and civil engineering.

Coastal cities such as Tokyo and New York face pressure to expand transport networks beneath waterways. Routes with geological conditions similar to those between Ningbo and Zhoushan could benefit from methods that allow maintenance at higher pressures.

The ability to support work at pressures up to 1 megapascal expands the range of conditions engineers can consider when planning deeper, more direct alignments. Such routes could reduce journey times and environmental disruption where the surrounding conditions allow.

Underground mining, subsea cable installation and scientific drilling are other activities involving environments under high pressure where mixed gas life support could be useful. Those are potential applications of the approach; the deployment described here is at the Jintang tunnel.

The project also shows how technology developed for one kind of demanding work can be adapted to another. Offshore divers and railway construction crews perform different tasks, but both need breathing systems that account for the effects of pressure on the human body.

The Bottom Line

• China has activated its first domestically developed ternary mixed gas shield tunneling system on the Zhoushan side of the Jintang tunnel project.
• The tunnel will connect Ningbo and Zhoushan in Zhejiang and measure 16.18 kilometers, becoming the world's longest undersea high speed railway passage when complete.
• Crews on the Zhoushan side are responsible for a shield tunneling section measuring 6,270 meters.
• The deepest section is 78 meters below sea level, with combined water and soil pressure reaching 0.85 megapascals.
• The system operates between 0.5 and 1 megapascal, compared with the traditional compressed air safety limit of 0.5 megapascals.
• Two main modules and 113 specialized submodules mix and supply helium, nitrogen and oxygen for work inside the pressurized chamber.
• Worker protection includes gas control, staged decompression, redundant equipment and remote monitoring of pressure, gas flow and vital signs.