Panama completes major 4.5 km metro tunnel beneath active shipping corridor

A 13.46 m diameter Mixshield tunnel boring machine has completed a 4.5 km drive beneath the Panama Canal, marking the first time a mechanised tunnel has been excavated below the interoceanic waterway.

The breakthrough represents a major milestone for Panama Metro Line 3, which will connect communities west of the canal with Panama City and provide an alternative to the heavily congested Bridge of the Americas. The wider monorail line is currently expected to be completed in 2028.

Tunnelling began from Farfan on the western side of the canal in September 2024. Over the following two years, the TBM, named Panama, excavated beneath the Pacific entrance to the canal before completing its final breakthrough on October 26, 2026.

The tunnel reaches depths of up to approximately 65 m below sea level, placing the excavation beneath one of the world's busiest shipping corridors. Maintaining stable excavation conditions while protecting both the tunnel and the canal above required careful control of face pressure and groundwater throughout the drive.

A Mixshield TBM was selected because of the variable geological and hydrogeological conditions along the alignment. The machine encountered sandstone, siltstone, tuff, clay, sand, breccias, mudstone, pyroclastic rocks, basalt and andesite during the 4.5 km excavation.

The TBM has a cutterhead drive power of 5,600 kW and can generate torque of approximately 26,600 kNm. At peak production, tunnelling crews achieved advances of up to 150 segment rings, or around 300 m, in a single month.

One important feature of the machine was an accessible cutterhead system that allowed excavation tools to be replaced from behind the cutterhead under atmospheric pressure. This reduced the need for personnel to enter pressurised areas at the excavation face, an important consideration when tunnelling at depth beneath high groundwater pressures.

As excavation progressed, precast concrete segments were installed behind the shield to form the permanent tunnel lining. The machine had to maintain excavation pressure and alignment while simultaneously removing spoil and constructing the segmental lining through changing ground conditions.

The tunnel now creates a continuous underground connection beneath the canal as part of Metro Line 3. Once operational, the new line is expected to serve around 250,000 commuters travelling between Panama City's western suburbs and the metropolitan centre.

The project is particularly significant because the Panama Canal has historically created a major physical divide between the capital and rapidly growing communities to the west. Existing road crossings have become increasingly congested, making additional high-capacity public transport infrastructure necessary.

Completing the mechanised drive beneath an active international shipping route demonstrates the increasing capability of large-diameter pressurised TBMs to negotiate deep underwater crossings while controlling ground and groundwater risks.

With the 4.5 km tunnel now excavated, construction can progress towards completion of the remaining railway systems, stations and associated infrastructure required to bring Metro Line 3 into operation.