Tunnel Boring Machine Begins Second Drive

After a successful first tunnel drive, Salus, the Tunnel Boring Machine, is ready for her next journey – tackling twice the distance of the first drive with ease and precision as part of Silicon Valley Clean Water’s (SVCW) Gravity Pipeline Project. Based on monitoring and data derived from the first drive, you won’t be able to tell that she will be hard at work beneath the surface on her 2.5-mile voyage (approximately 12,500 feet), launching from the Tunnel Boring Machine (TBM) launch shaft located at Holly Street and Shoreway Road, as she heads to the TBM Receiving Shaft located at the Front of Plant at SVCW’s wastewater treatment site.

Salus completed her first tunnel drive and broke through into the shaft of Inner Bair Island in March 2020. She came in on time and on target to the “Tunnel Eye.” She traveled nearly 1 mile and stayed well within the 0.50 inches of the contract-mandated ground surface settlement, mostly staying within half that at 0.25 inches, along the centerline of the tunnel over the length of the entire first drive. There was no vibration felt by stakeholders along the drive, including the hotel on Skyway Road at Monte Vista, near the tunneling alignment under the San Carlos Airport! SVCW and its tunnel contractor partner, Barnard-Bessac Joint Venture (BBJV), continuously monitored the performance of Salus during the first tunneling drive and conducted daily Tunnel Task Force meetings to review progress and to anticipate and resolve potential issues.

The subsurface conditions for the second drive are similar to the first, and SVCW anticipates similar performance from Salus in safety, production and non-settlement. The depth from the ground surface to the bottom of the tunnel will range between approximately 50 feet at the launch shaft to approximately 65 feet at the TBM Receiving Shaft located at SVCW’s wastewater treatment plant. As was the case for Salus’ first drive, there should be no indication at the ground surface of the tunneling operation; residents should not experience noise or vibration from the tunneling operation.

As was done for the first drive, to initiate the launch of Salus for her second drive, the BBJV team will construct concrete excavation support rings in the starter tunnel immediately behind Salus. Once the first rings are constructed, Salus will begin mining towards the SVCW wastewater treatment plant. First steps of the launching phase require slow progress as the TBM operations team re-familiarizes with the properties of the soil through which Salus will be mining. Operations crews will monitor Salus’ performance during these early stages, and continuously monitor her performance throughout the entire second tunnel drive.

We are wishing Salus a safe and happy journey!

To view more about SVCW’s Regional Environmental Sewer Conveyance Upgrade Program (RESCU), please visit https://svcw-rescu.org/. Updates on Salus’ drive will be posted every two weeks, and you can also follow us on Facebook at @SiliconValleyCleanWater

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