China has reported a significant technical breakthrough during construction of the Jintang Subsea Tunnel, part of the Yongzhou high-speed railway, with the successful deployment of a domestically developed TBM saturated hyperbaric intervention system described as a “Deep-Sea Space Station”.

The development addresses one of the most demanding challenges in subsea tunnelling: cutter replacement under extreme water pressure conditions exceeding conventional compressed air limits.

Project Context

The Jintang Subsea Tunnel forms part of the Yongzhou Railway and is described as the world’s longest undersea high-speed railway tunnel currently under construction.

The project is being delivered by China Railway 14th Bureau Group.

The tunnel alignment passes through highly complex seabed geology, including:

• 24 transitions between hard and soft strata
• Rock strengths exceeding six times that of ordinary concrete
• Maximum hydrostatic pressures equivalent to 78 metres of seawater head

These conditions have led to severe cutter wear and frequent hyperbaric interventions.

TBM Configuration and Operational Demands

The tunnel is being excavated using the “Yongzhou” TBM, equipped with:

• A hyperbaric composite cutterhead
• 308 cutters in total
• A 30% increase in disc cutters compared with conventional configurations

The intensified rock-breaking requirements have resulted in high cutter consumption. To date, more than 2,900 cutters have been replaced.

As excavation progressed to greater depths beneath the seabed, conventional compressed air intervention methods reached operational limits.

Hyperbaric Constraints

In shield tunnelling, intervention at the cutterhead under pressure is required to maintain face stability. Compressed air is used to counteract water and soil pressure at the tunnel face.

Under traditional compressed air chamber methods:

• The generally accepted safe operating depth limit is approximately 60 metres
• Effective working time under high pressure is limited to roughly 40 minutes per day
• Decompression procedures can exceed four hours

At water depths of up to 75 metres, these limitations made conventional intervention increasingly impractical for the Jintang subsea section.

Deployment of the “Deep-Sea Space Station”

On 6 February, after 22 days of operation, China Railway 14th Bureau Group announced successful deployment of China’s first domestically developed TBM saturated hyperbaric intervention facility on the Ningbo side of the Jintang Subsea Tunnel.

The system successfully completed:

• Hyperbaric chamber intervention at 75 metres equivalent water depth
• Replacement of 46 cutters during its initial operation

This marks the reported first application in China of independently developed TBM saturated hyperbaric intervention technology for subsea tunnelling exceeding 60 metres water pressure.

Saturation Intervention Technology

The system was developed over a three-year period by:

• China Railway 14th Bureau Group
• Shanghai Salvage Bureau of the Ministry of Transport

The technology integrates established saturation diving principles from offshore marine engineering with shield tunnelling operations.

Under saturation conditions:

• Workers breathe a controlled mixed gas
• After initial pressurisation, inert gases in the body reach equilibrium
• Additional exposure time at the same pressure does not increase decompression duration
• Total decompression time remains fixed regardless of intervention duration

This approach significantly increases operational flexibility compared with conventional compressed air methods.

System Configuration

The “Deep-Sea Space Station” comprises multiple integrated modules:

• Living Chamber
• Transfer Chamber
• Control Chamber

These modules allow personnel to remain under saturation conditions for extended periods, enabling repeated cutter interventions without repeated long decompression cycles.

Significance for Subsea Tunnelling

Hyperbaric cutter replacement at depths beyond 60 metres has long been regarded as one of the most technically complex and high-risk procedures in shield tunnelling.

The successful 75-metre intervention on the Jintang Subsea Tunnel demonstrates:

• Application of saturation intervention in railway TBM works
• Expanded operational envelope for deep subsea tunnelling
• A potential new standard for high-pressure TBM maintenance environments

As subsea rail tunnels increase in length and depth globally, such systems may become critical in enabling efficient excavation under extreme hydrostatic pressures.

The Jintang Subsea Tunnel continues to advance under challenging geological and pressure conditions, with the new intervention system now integrated into ongoing TBM operations. For further information please click here 09/26.