Consumers Energy Company
NTH Consultants, Ltd.
Thomas E. Fox, P.E., Environmental & Technical Services Manager
NTH’s Value Added: NTH was successful in designing an alternative means to improve slope stability, enabling a shortened project schedule, eliminating the need for Corps of Engineer permits by not altering waters of state and lowered overall project costs by 30 percent in comparison to buttressing, while assuring strict accordance with MDEQ regulatory requirements.
D.E. Karn Coal Ash Storage Facility
Perimeter Dike Integrity Assessment & Monitoring
The Karn/Weadock generating complex is Consumers Energy’s largest power production site and is located adjacent to Saginaw Bay and the Saginaw River. The facility consists of three separate plants: the 310-meagwatt coal-fueled Weadock plant; the 511-megawatt coal-fired Karn 1 and 2 plant; and the 1,276-megawatt natural gas-and oil-fueled Karn 3 and 4 plant. Both coal-fired plants have their own coal ash storage facility, surrounded by constructed earthen dikes of various heights and configurations.
Beginning in 2010, Consumers Energy retained NTH to evaluate the slope stability of existing perimeter dikes and to design a subsurface soil-bentonite wall to serve as a groundwater migration barrier around the landfill. In one area of the site, the existing perimeter dike had marginally stable factors of safety. Slope failure of the perimeter dike would release stored fly and bottom ash (coal combustion by-products) into the intake channel which is connected to the Saginaw River and eventually Lake Huron.
To facilitate potential slope improvements, NTH implemented an extensive subsurface condition evaluation including drilling 17 test borings, installing 11 temporary piezometers, and 13 staff gauges to determine surface water characteristics. Soil properties determined through laboratory testing and groundwater elevations were utilized to perform slope stability analyses.
NTH determined that re-sloping the perimeter dike using 2.5H:1V and installing rip-rap with a geotextile liner to prevent soil erosion, would increase the slope stability factor to an acceptable level. As an additional measure, NTH developed a geotechnical instrumentation monitoring program that would be in-place during construction activities (and beyond) as an early detection tool for slope failures.
Following MDEQ approval of the proposed design, NTH was retained by Consumer’s to conduct Quality Assurance (QA) services during construction of the slope improvements. NTH continues to conduct instrumentation monitoring at the site that demonstrates no soil displacement or other indications of slope instability have occurred.
NTH also conducted numerical groundwater modeling that to support eliminating the need for installation of a slurry wall groundwater barrier. The model demonstrated that installing a low-permeability soil cover on the ash fill and re-routing process and storm water flow patterns would virtually preclude groundwater migration outward from the site boundary. The demonstration was approved by the MDEQ, and the site’s operating license was amended to eliminate the required slurry wall.