| Reconstruction & development
Left: Anderson Reservoir is managed by the Santa Clara Valley Water District in California. In February 2020 FERC ordered that the water level in the reservoir be lowered to 3% capacity
water level in the reservoir, sending most of the water to treatment plants for delivery to water retailers. By mid-December 2020, the water level in the reservoir was at 3% of capacity. This is the lowest that can be achieved to allow proper functioning of the existing outlet pipe.
Retrofit Project After receiving the Federal Energy Regulatory
Commission order, Valley Water divided the Anderson Dam Seismic Retrofit Project into two stages. The first stage, known as the Anderson Dam Tunnel Project (ADTP), includes construction of a new 518m long diversion tunnel. The second stage of the work, known as the Anderson Dam Seismic Retrofit Project (ADSRP), consists of removing and reconstructing the spillway and the dam embankment and building a high-level tunnel as part of the outlet works. Valley Water began construction on the ADTP in 2021; work is estimated to take approximately two to three years. The new diversion tunnel will allow Valley Water to draw down the reservoir reliably and quickly, providing greater control over the water levels in the reservoir and increased public safety. The new outlet tunnel will comprise of three sections, including: ● A 122m long, 2.4m diameter ‘lake tap’ pipe upstream of the dam.
● A 30.5m micro tunnel boring machine launch chamber, connected to a 282m long, 5.8m diameter reinforced concrete-lined pipe within a 7.3m diameter tunnel.
● A 114m long, 4m diameter steel pipeline within a 5.6m diameter horseshoe tunnel.
The existing outlet pipe runs beneath the dam where it crosses the fault lines. This outlet pipe was not designed to withstand movement caused by earthquakes and must be replaced. The new tunnel
will form a low-level outlet that can release and regulate water flows, and in the event of a major earthquake, allow Valley Water to quickly drawdown the reservoir.
Most of the tunnel will be excavated by roadheader, with the small length mined by MTBM. Lining will comprise shotcrete for initial support followed by a cast-in-place concrete secondary lining. Construction on the ADSRP will begin after the first stage is completed and the required regulatory permits are received. Construction of the ADSRP is estimated to take seven to eight years and is dependent on the permit requirements and the field conditions. Valley Water is treating the ADSRP first and foremost as a public safety project. It is working with numerous state and federal agencies to obtain the necessary permits to move forward with the project as quickly as possible. ●
More information For more project information, including meeting materials, frequently asked questions, reports and other project-related documents, please visit the project webpage at
http://Delivr.com/2uanu
Anderson Dam Tunnel Project This $8.8 million project for Santa Clara Valley Water consists of a single microtunneling drive with the installation of 372 lf of 96-in. diameter steel casing pipe. The MTS 2350 MTBM will be launched from a chamber tunneled under the Anderson Dam and recovered in the Anderson Reservoir. The tunnel drive will be a mixture of soft and hard materials. The Anderson Dam Tunnel Project MBTM lake tap will be launched from inside of a 24-ft horseshoe tunnel mined under the Anderson Dam. The 1,600 lf conventional tunnel will incorporate steel horseshoes and a reinforced concrete lining. A reaction frame and portal launch seal will be used to tunnel from the chamber to a lake tap landing pad deep below the surface of the Anderson Reservoir. The MTBM will be recovered using specialty divers, barges and cranes. Planned Mobilization for the MBTM lake tap is scheduled for November 2022 with a Completion Date of May 2023. AECOM is the engineer. Flatiron West is the general contractor with Vadnais Trenchless Services Inc as tunneling subcontractor.
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