Millville Lake Dam Improvements, Salem, New Hampshire

The Millville Lake Dam in Salem, New Hampshire was classified as a high hazard dam due to the potential to cause loss of human life and significant economic and environmental damage in the event of a failure. Due to seepage concerns and structural issues, the NHDES issued a letter of deficiency regarding the dam to the Town in 2016. SLR was contracted by the Town of Salem to evaluate existing conditions of the dam system and develop a comprehensive, sustainable, and resilient solution to address safety and structural integrity concerns. TF Moran completed survey and wetland delineation tasks.  

The challenge of this project was completing a detailed investigation to assess the root causes of deficiencies then develop an approach and design components to best address the deficiencies and safety issues with a sustainable, resilient, and cost-effective solution. A significant technical challenge was posed by the fact that the spillways were founded on soil and not on bedrock. SLR used an innovative approach to transmit stabilizing forces from rock anchors to the bedrock without stressing the soil below the spillway. The project improvements, which developed significantly improved safety and structural integrity of the dam, included:

• Concrete repairs (at several locations to remediate seepage problems);
• Incorporation of rock anchors Installed through the dam’s spillways (to significantly improve the structural integrity); 
• Incorporating permanent structural casing to act as a column to transmit rock anchor loads directly to bedrock without adding load to the soils below the spillway;
• Raising of the right cutoff wall and a portion of the left cutoff wall (preventing the potential overtopping of the dam);  
• Application of articulated concrete blocks (ACB) (for increased overtopping protection)
• Improved monitoring and maintenance measures (recommended)