Dec. 6, 2023

Milestones on the Horizon for Inlet/Outlet Tunnel  

Progress is steadily being made on the 2,000-foot-long tunnel that will allow water to be transferred in and out of Chimney Hollow Reservoir. The tunnel is comprised of three sections: the downstream (dry) portion, upstream (wet) portion, and the valve chamber that sits between the two.   

Downstream Tunnel

The 26-foot diameter circular downstream portion of the tunnel required about seven months of excavation, which runs to the valve chamber at the center of the main dam. To date, crews have tied three-quarters of the required rebar. Two placements of the reinforced concrete liner are also complete. The concrete liner is installed using an Everest Carrier Form, which is a piece of equipment that places concrete along the tunnel’s inner walls. The biggest challenge for crews has been installing rebar in confined spaces on the tunnel’s circular profile, which is necessary to reinforce the concrete liner. The second challenge is keeping the formwork intact, which sees nearly 2 million pounds of uplift pressure during concrete placements. The downstream tunnel concrete lining is anticipated to be completed in March 2024. 

Liner inside downstream tunnel
Crews place rebar along the tunnel's inner walls, necessary to reinforce the concrete liner.
Upstream Tunnel

The 11-foot diameter horseshoe shaped upstream portion of the tunnel is in the excavation phase. While the plan was to mine this portion with a roadheader, the composition of rock has required a combination of excavation techniques. Crews have encountered soft and hard rock that randomly changes throughout the process, leading to some delay and the need for a mix of roadheader and drill and blast operations. Crews have about 400 feet to go before holing through to the downstream portion of the tunnel, which is anticipated for early 2024. Once excavation is complete, crews will begin pipe installation.   

Valve Chamber

The valve chamber, a tunnel section between the upstream and downstream tunnels, will house a butterfly valve. This will allow the downstream tunnel to be isolated for maintenance. Installation of the concrete liner is complete, and the waterproof membrane will be complete upon tie into the upstream tunnel. Any seepage that goes through the concrete will be caught by the membrane, like a rain jacket, to protect all of the valve house equipment.