Anticipating that future EPA regulations would mandate the closure of Coal Combustion Residual (CCR) ash ponds, the utility sought proposals for cleaning and pre-closing preparation of the ash pond serving one of its power plants. In addition to meeting current regulatory standards, the utility wanted to maximize available pond storage for future use. In 2011, Charah was awarded the contract based on its integrated proposal for cleaning the ash pond, as well as for its strategic plan for the short- and long-term management of the facility’s CCRs and pond closure. The 49-acre work area included a 25.5-acre ash pond and a 14-acre area of dry ash stockpile.
Rim Ditch Construction
Charah designed, engineered and constructed a new 3,200-foot rim ditch to manage ongoing sluice operations during ash pond cleaning. Charah also managed and maintained the new rim ditch using long-reach excavators for ash harvesting.
Advanced Modeling and Monitoring
To safeguard against liquefaction, Charah partnered with a specialized engineering consultancy to analyze geotechnical models and determine the dry stack’s stability under both static and dynamic forces. The basin was also required to provide storage volume for a Probable Maximum Precipitation (PMP) event throughout construction. To monitor water levels in and around the new dry stack, Charah strategically located piezometers to provide a constant data stream of measurements of the internal hydraulic conditions to ensure that there were no increases in the dry stack pore pressure, which could cause differential settlement.
- In addition to maintaining the TSS using an advanced rim ditch design, Charah made multiple improvements to the discharge system. They included:
- Placing a turbidity curtain across the polishing water pond.
- Installing an 8-foot high, 25-foot long rock-filter berm with a built-in weir at the pond outfall.
- Constructing a 300-foot vegetative buffer zone between the construction area and the lower polishing water pond.
Flexible CCR Management Strategy
The contract called for a basin with 200,000 cubic yards of capacity, but Charah engineered and provided a range of options for future expansion − up to 500,000 cubic yards, if needed.
- The project successfully allowed the facility to manage its CCRs and plan for future capacity and closure of the pond, with minimal investment.
- The project was finished without interruption of sluice discharge or plant operations.
- All environmental standards were met, and TSS, pH levels and ground erosion showed no downstream impact.
- Charah completed the project with no safety issues.