CS27A - Concurrent Session 27A: Anderson Dam Tunnel Project - Diversion System

The Anderson Dam Seismic Retrofit Project (ADSRP) is being conducted 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. An interim risk reduction project to drain the reservoir and maintain it at the dead pool elevation includes the Anderson Dam Tunnel Project (ADTP) that will construct a new diversion tunnel and a microtunnel lake tap through the right abutment of the dam.

The ADTP includes approximately 1,400 ft of conventionally mined tunnel that begins about 400 feet downstream of the downstream toe of the dam and terminates at about 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 tapping into the reservoir underwater.

Once these features are constructed, the tunnel will be lined and a diversion outlet control structure will be constructed at the downstream portal. The outlet controls will consist of large valves capable of draining the reservoir quickly and preventing significant water level rises during all but the most severe inflow events. The design of the ADTP and ADSRP are coordinated so that the permanent outlet tunnel could be adapted to serve as part of the temporary diversion system. This will help reduce the overall construction duration and facilitate an earlier return to service for this important water storage reservoir. This paper will describe the seismic shaking and fault rupture hazards at the project site, along with the design aspects and construction sequencing for the outlet system, including the tunnel lining design for both interim diversion and permanent use. The paper will also provide an update on the ADTP tunnel construction status and will discuss some of the design and construction challenges of open face tunnel excavation through sheared intermediate geomaterials.