June 13, 2024

Tunnel Grouting Helps Reinforce Main Dam’s Water-Tight Barrier

Project managers and construction crews continue sparing no detail in making sure the water in Chimney Hollow Reservoir, once built, stays in all the right places. 

Grouting operations were recently completed around a key portion of the inlet/outlet tunnel, the infrastructure that one day will fill the reservoir and also release water to the project participants. The grouting took place around a specific 25-foot stretch of the tunnel where it crosses under the center line of the main dam. That particular portion of the nearly 2,000-foot-long tunnel was grouted in order to seal any cracks and voids in the geology between the tunnel and a grout curtain that extends down from the foundation of the main dam. 

The main dam grout curtain has been underway for about two and half years, with crews injecting grout down into the foundation to tighten up the bedrock below and create a barrier that will help prevent water from seeping underneath the dam. That grout curtain goes as deep as 220 feet in some areas below the dam’s foundation and runs along the center line of the entire 3,700-foot-long dam, including down to and around the inlet/outlet tunnel, which crosses about 60 feet below the foundation of the main dam.  

Chimney Hollow Tunnel Grouting

To make the geology water-tight between the inlet/outlet tunnel and grout curtain, crews conducted grouting operations from inside the tunnel between April 1 and May 7. As part of that effort, crews injected grout into 70 holes, each 2-3 inches in diameter, inside and near the center ring of the tunnel where it crosses under the main dam center line. 

Grout was injected 10-30 feet into the tunnel’s walls, with follow-up water tests indicating that operations were successful in sealing up the surrounding rock.  

The inlet/outlet tunnel’s recent grouting completion marks yet another 2024 milestone for that critical element of the project. Perhaps the biggest milestone took place back in March, when construction crews, nearly two years after tunneling had started at opposite ends, met in the middle. 

The next steps for the inlet/outlet tunnel include installation of a 72-inch steel conduit through the 1,250-foot upstream portion of the tunnel, and reinforced-concrete lining in the 716-foot downstream portion of the tunnel. The inlet/outlet tunnel is expected to be finished in late 2024.