Sept. 14, 2023

Project Dams Vary at the Core While Serving Similar Purpose

Within the Chimney Hollow Reservoir Project, two unique dams contribute to the project’s success. At their core, each dam is constructed differently due to the fluctuating geology of the valley. The 350-foot asphalt core main dam encompasses a complex scope of work, while two and a half miles south lies the 40-foot-tall clay core saddle dam, which completes the 90,000 acre-foot reservoir capacity and adds 30 percent additional storage to the project. 

The dams require a stable surface below their cores to strengthen the structures. For the main dam, this is known as the plinth, whereas at the saddle dam it is referred to as the grout cap. Both the plinth and grout cap tie the above-ground features of each dam to the foundations below. They act as a sturdy surface to conduct grouting, which involves pumping Portland cement grout into the foundational holes to seal any voids in the bedrock to reduce seepage. With the plinth and grout cap held tight against the rock by steel anchors to resist upward pressures, this allows crews to create a water-tight bond between the foundations and the cores. 

Saddle dam grout cap and main dam plinth

Although the purpose of the plinth and the grout cap are identical, each is constructed slightly differently due to the individual characteristics of each dam site. The saddle dam’s grout cap is flush with the valley’s slope and embedded into the foundation against dental concrete (used to fill holes, grooves or areas of vertical surfaces) unlike the plinth at the main dam, which is slightly raised against the slope. Both dams have double-row grout curtains, which are thin, vertical rows of grout placed below the foundations. To ensure each is properly sealed, the main dam grout curtain is as deep as 230 feet below the foundation while the saddle dam grout curtain extends approximately 70 feet below.  Additionally, the main dam has up to four additional rows of shallow grout holes, known as blanket grouting, to help seal off the near-surface rock and help resist consolidation in the foundation. 

The grout cap is 8 feet wide, while the plinth varies between 25, 30 and 35 feet wide. Below the plinth, the valley’s slope varies in elevation, which results in fluctuating water pressures across the main dam. This requires the plinth to be wider in areas with greater pressures and force. On the south end of the reservoir, the valley does not drastically change in elevation below the grout cap. This allows the grout cap to maintain the same width and number of grouting rows since the pressure does not fluctuate. 

The saddle dam grout cap was recently completed in August, while crews continue to push forward on the much larger construction of the main dam plinth.