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Tunnel boring machine manufactured in Germany will be taken disassembled to California for an underground project in Silicon Valley, with a structure designed to gather tracks, platforms, operational systems, and safety areas in a single tunnel under San José.
A German tunnel boring machine worth US$ 76 million will be used to open about 8 kilometers of tunnel under San José, California, during the BART expansion in Silicon Valley.
Custom-made by Herrenknecht, the equipment will have nearly 54 feet in diameter, equivalent to about 16.4 meters, and will be transported disassembled from Germany to the United States.
The machine is part of the BART Silicon Valley Phase II project, conducted by the Santa Clara Valley Transportation Authority, the VTA, responsible for the railway expansion in Santa Clara County.
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With this section, the regional railway system is expected to advance from Berryessa/North San José to Santa Clara, passing through downtown San José and connection areas with other transportation services.
According to the VTA, the complete project will be about 6 miles, or 9.8 kilometers, with four stations, three underground platforms, one surface-level station, and a new train maintenance and storage facility.
The agency reports that the extension is designed to serve 55,000 passengers on weekdays when it becomes operational.
Single tunnel will concentrate tracks and platforms
The choice of such a large tunnel boring machine is related to the excavation method defined by the VTA for the BART Silicon Valley Phase II.
Instead of two smaller tunnels, the agency adopted a single large-diameter tunnel, known as single-bore, capable of accommodating two independent tracks, platforms, trains, operational equipment, and safety structures.
This model concentrates part of the railway infrastructure in a single continuous underground structure.
According to the VTA, the solution also allows streets and avenues to remain operational during deep excavation, reducing the need for prolonged opening of sections on the surface.
The transportation authority reports that the tunnel will have an internal diameter of 48 feet, while the external diameter will be just under 52 feet.
The tunnel boring machine will have a larger size because it needs to excavate enough space for the structural lining installed as the machine progresses underground.
Machine will be assembled, tested, and disassembled
Herrenknecht, a German company specialized in custom excavation equipment, will be responsible for designing and manufacturing the machine used in the underground section.
Before transportation to California, the tunnel boring machine will be assembled, tested, and disassembled in Germany, a step planned to verify the equipment’s functionality before shipment.
The shipment will depart from Kehl, Germany, heading to California, where the parts will be taken to the construction site for reassembly.
After arriving in the United States, the equipment will be reassembled at the West Portal, an area located at the future Newhall Yard, between Santa Clara and San José.
The VTA estimates that the reassembly on site will take approximately six months before excavation begins.
When in operation, the machine is expected to advance 30 to 40 feet per day towards the Berryessa/North San José BART station.
Route will pass under San José and Santa Clara
The tunnel will start south of the future Santa Clara station, within the Newhall Yard, and will follow under the I-880 and Caltrain tracks.
From this point, the route will continue southeast, passing through areas near Diridon Station and following under Santa Clara Street to the future Downtown San José station.
Subsequently, the underground structure will curve towards the future 28th Street/Little Portugal station.
After this section, the tunnel will continue under the US 101 until it emerges at the East Portal, near Las Plumas Avenue and Marburg Way, on the east side of the highway.
The underground excavation is expected to last three to four years, according to VTA’s estimate for the opening of the five-mile section.
This activity will be part of the tunnel and track works package of the second phase of the rail expansion.
How the tunnel boring machine makes its way underground
During excavation, the tunnel boring machine uses a rotating cutting head installed at the front to break through soil, rock, and other materials encountered along the route.
The removed material is carried through an internal transportation system, while concrete segments are installed to form the tunnel’s final lining.
The VTA compares the machine’s operation to that of a mechanical mole, as the equipment creates a passage at depth without requiring continuous opening of the surface above the route.
This explanation is used by the agency to describe the role of tunnel boring machines in underground works conducted in densely populated urban areas.
The tunnel lining will consist of large concrete segments, installed in rings as excavation progresses.
Each ring will have nine pieces, and the operational forecast is to install up to six rings per day, according to the structure prepared at the West Portal.
The thickness of this lining considers the pressure of the water and soil around, as well as safety requirements in a region prone to earthquakes.
For this reason, the machine was designed to operate in the geological conditions of the South Bay, with the presence of sand, gravel, silt, clay, and areas of high groundwater level.
West Portal will concentrate the operation of the work
The West Portal will be the assembly, launch, and support point for the tunnel boring machine during the excavation phase.
The area will receive internal access, operational parking, spaces for teams, support structures, excavation of the launch shaft, and facilities for the production and storage of lining segments.
From the same location, the work should remove the excavated material and send the concrete pieces that will form the walls of the structure into the tunnel.
A grout plant is also planned, a material applied to seal the lining segments as the tunnel boring machine advances underground.
Another planned installation is the temporary storage compartment for the removed soil, known in underground works as muck.
This material can be transported for reuse or disposal, according to the destination defined in the construction process.
The VTA also informs that it will adopt measures to reduce impacts around the construction site.
Among them is the installation of an acoustic curtain on the west side of the area, in addition to coordinating truck routes and work phases with Santa Clara and San José.
Expansion will have a maintenance yard in Santa Clara
Besides the tunnel, the future Newhall Yard will receive a train maintenance and storage structure.
The complex should include a workshop, car wash, technical offices, operational yard, and control tower, according to the configuration planned for the end of the line in Santa Clara.
According to the VTA, this facility will allow the maintenance and storage of BART vehicles in the region served by the expansion itself.
Without the local yard, trains would need to return to the East Bay at the end of the daily operation, which would increase the operational movement of the system.
The United States Federal Transit Administration informs that the project will be designed and built by the VTA, will remain under the ownership of the agency itself, and will be operated and maintained by BART.
The federal profile of the project, updated on April 13, 2026, describes the corridor as a 6.1-mile extension, with five miles of underground tunnels, 48 heavy rail vehicles, two intermediate ventilation and exit structures, as well as a storage and maintenance yard.
VTA states that the tunnel boring machine is among the largest machines of its kind ever built, considering the planned diameter for the equipment.
In Silicon Valley, the second phase of BART combines a machine manufactured in Germany with an underground intervention that, once completed, will largely remain out of sight for those traveling the streets of San José.
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