Anderson Dam Tunnel Project

Santa Clara County, California, United States

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The Anderson Dam is a 240-foot-high zoned rockfill embankment built in 1950 in an area of high seismic activity.

The dam retains a 90,000-acre-foot reservoir located on Coyote Creek, which travels through the cities of Morgan Hill and San Jose and discharges into the San Francisco Bay. As a major urban center, San Jose’s proximity to the dam underscores the critical nature of dam safety. A failure of the dam would cause significant flooding and inundation in several heavily populated areas.

As the lead designer, AECOM is helping our client the Santa Clara Valley Water District (Valley Water) implement a retrofit project to strengthen Anderson Dam so it can safely withstand a large earthquake. This project will allow the reservoir to be safely put back into service, securing a reliable water supply for the residents of Santa Clara County. As an interim risk reduction measure to protect the community prior to the retrofit project, the reservoir was drained and limited to 3 percent capacity.

Drainage issues

In an evaluation conducted for the Valley Water, we determined that a major earthquake could damage the outlet pipe and impede release of water from the reservoir during a dam safety emergency.

Light at the end of the tunnel

To address this risk, we designed a new, larger capacity outlet tunnel with a seismic lining system capable of withstanding fault rupture and seismic shaking. The Anderson Dam Tunnel Project broke ground in 2021 and when complete in early 2026, will allow Valley Water to safely maintain a lowered reservoir until the dam retrofit project is completed in about 2032.   

The new tunnel includes approximately 1,400 feet of conventionally mined tunnel about 20 feet in diameter and 350 feet of 8-foot diameter tunnel constructed using a micro tunnel boring machine.

Micro Tunnel Boring Machine (MTBM)

Unlike traditional tunnel boring machines that propel themselves, the MTBM was pushed from the rear using a series of steel pipes with the MTBM at the front excavating rock and clearing the path. The process was particularly challenging due to the MTBM being launched from within the tunnel — a novel approach — and mined out into the lake before being retrieved underwater via a ‘wet recovery’ process. Any failure could have resulted in a catastrophic influx of water, so extensive safety measures, including steel bulkheads, were implemented. The wet recovery operation involved divers and a heavy lift crane working from barges in the reservoir.   

Seismic lining

As part of the dam retrofit project, the new outlet tunnel will be converted into a system capable of withstanding fault displacement up to 4 feet along the alignment. In order to develop the design, an extensive program of three-dimensional soil-structure interaction modelling was performed. The seismic lining design consists of independent reinforced concrete segments with weak gaps in-between to allow flexibility along cross-tunnel movement. Special sliding supports and compressible supports will also be installed to allow the permanent carrier pipe to deform elastically within the tunnel to accommodate the ground movements and remain in service.

The next phase

Mining and initial support of the tunnel was completed in November 2024, and construction of the final reinforced concrete articulated lining is currently underway. When this is complete, the tunnel will allow Valley Water to reliably maintain the reservoir at a restricted level while continuing to plan and design the full retrofit of the dam.

Once the Anderson Dam Tunnel Project is completed, the Anderson Dam Seismic Retrofit Project will strengthen the dam and replace the outlet pipe to reduce fault rupture risks, improve flood management, and rebuild the spillway to meet current safety regulations.