Anderson Dam Tunnel Project - Diversion System

The Anderson Dam Seismic Retrofit Project (ADSRP), located near Morgan Hill, California, is being undertaken by the Santa Clara Valley Water District (District) to address several important deficiencies in the existing dam and appurtenant facilities. These include predicted seismic deformations of the embankment that could lead to overtopping and failure, inadequate spillway capacity to pass the probable maximum flood, and a risk of fault rupture that could sever the existing outlet conduit beneath the dam. To bring the reservoir and its appurtenant facilities to modern standards, the retrofit project will include removal and reconstruction of most of the dam embankment, replacement and extension of the existing spillway chute, and construction of a new permanent outlet works capable of remaining in service following the design fault offset. Because of the downstream hazard, the District has also initiated an interim risk reduction project to drain the reservoir and maintain it at the dead pool elevation until the dam retrofit can be completed. The risk reduction work is being conducted separately as the Anderson Dam Tunnel Project (ADTP), which includes construction of a new diversion tunnel through the right abutment of the dam.

The ADTP includes approximately 1,400 ft of conventionally mined tunnel, extending from the downstream toe of the dam to a location upstream of the dam axis. An underground operation chamber will be constructed at that location, from which a microtunnel boring machine (MTBM) will be launched for a 300-foot drive culminating with a wet-recovery lake tap into the reservoir.

Once these features are constructed, the main tunnel will be lined with reinforced concrete and steel pipe sections, connecting to a diversion outlet control structure constructed at the downstream portal. The outlet controls will consist of two 11-foot diameter fixed cone valves with sufficient capacity to prevent reservoir level rises during all but the most severe inflow events. The new diversion tunnel constructed during the ADTP overlaps with and will later be incorporated into the permanent new outlet works that will be constructed during the ADSRP. Coordinating the design between the two projects will help reduce the overall construction duration and facilitate an earlier return to service for this important water storage reservoir.

This paper will provide an overview of the project design, including the seismic shaking and fault rupture design criteria. Key design aspects and construction sequencing considerations for the outlet system will also be presented, including the tunnel lining design for both the interim diversion (ADTP) and permanent use (ADSRP). The paper will also provide an update on the ADTP tunnel construction status, including some of the design and construction challenges of open face tunnel excavation through the sheared and weak intermediate geomaterials present at the site.