ABSTRACT INTRODUCTION

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1 Title: Using Fly Ash to Construct New Ash Pond Containment Dikes at the Wabash River Generating Station Authors: Mr. Keith Brinkman, Mechanical Engineer, Duke Energy Mr. Terry Larson, PE, Project Manager, Burns & McDonnell Date: 2006 Presented at: Coal-Gen 2006 ABSTRACT The existing fly ash ponds at Duke Energy s (formerly Cinergy) Wabash River Generating Station are nearing their useful capacity and site constraints limited the available options for new on-site ash disposal facilities. Ash had to be prepared to optimum moisture and marketed for outside uses to provide sufficient capacity until another ash disposal facility could be constructed. Other site constraints had to be considered in the design of the new facility, including maximizing the use of existing ash materials in the construction of the new facility. Therefore existing ash materials were utilized in the construction of dike structures that will be used for containing ash produced in the future. This paper outlines some of the site constraints and other unique challenges that had to be considered in the design of the new ash pond disposal facility as well as an in depth look at why and how existing ash materials were incorporated into the new disposal facility design. Design considerations to utilize the fly ash in the dike construction include seismic stability, dealing with very moist conditions, and ash placement concerns with respect to removing it from the existing pond, drying to acceptable moisture content, and placing the ash in the dike. Quality assurance testing of the in-place ash also had some unique challenges that will be addressed in this paper. INTRODUCTION The Wabash River Generating Station is located north of Terre Haute, Ind., along the Wabash River and has six coalfired units with a combined generating output of 668 megawatts (MW). Unit 1 s boiler was retired in the 1990s and the steam turbine and generator were repowered using steam from a new coal gasification plant sited northwest of the Wabash River Station. Slag produced from the coal gasification facility is hauled offsite for other uses. Units 2 thru 6 burn a local bituminous coal that produces a Type F fly ash. Fly ash and bottom ash from these five units are sluiced to a common ash disposal pond facility (see Figure 1) located south of the plant. The existing ash pond facility was initially constructed in the mid-1960s. The ash pond was expanded in the 1980s when a new primary disposal cell along with a secondary settling cell were added. The existing pond facilities were nearing their useful capacity and the plant needed to look at construction of a new ash disposal facility and/or identify offsite beneficial uses for the ash material. Page 1 of 5

2 Figure 1: Existing ash disposal pond POND DESIGN PHASE Several design challenges and obstacles had to be overcome in the design of the new ash disposal pond facility. The biggest challenge was keeping the existing pond in-service until the new ash disposal facility could be designed and constructed. As mentioned earlier, the existing pond was already nearing full capacity when the decision to construct a new disposal facility was made. In order to maintain sufficient capacity in the existing pond until a new facility could be constructed, offsite beneficial uses of the ash had to be identified and implemented. Ways to maximize use of available pond storage capacity also had to be incorporated in the overall ash management plan. Cinergy found an ideal beneficial use for the ash. The ash was used as fill material to help reclaim an existing coal strip mine. In order for this beneficial use to occur, the ash had to be dewatered to near optimum moisture before it was loaded onto trucks and hauled to the mine. Cinergy also found other uses for the ash, such as use in highway embankment for bridge approaches. To maximize use of existing pond storage capacity, Cinergy purchased a hydraulic dredge (see Figure 2) to redistribute ash within the pond to fill in dead storage areas in order to maintain sufficient free water surface area and create additional ash storage capacity within the active pond cell for settlement of ash. A dragline was also used to maintain the initial settlement chamber where the ash sluice pipes discharge into the pond. The ash removed by dragline was dewatered by temporarily stacking it above the pond water surface. Then it was loaded onto trucks for beneficial use purposes as discussed previously. Several alternatives were considered for future disposal of ash produced including the following: Raising the earthen dikes of the existing ash pond to provide additional capacity. This alternative was not pursued due to limited space available to raise and widen the existing pond dikes. Constructing internal containment berms within the existing pond dike system. The berms would be constructed out of ponded ash and synthetic geogrid material. However, the cost of stabilizing the in-place ash to provide a suitable base and the cost of installing the geogrid made this alternative economically unattractive. The alternative selected was to construct a new 130 acre HDPE-lined ash disposal pond facility in an open field located immediately south of the existing ash pond facility. Siting the new facility adjacent to the existing pond was beneficial because the existing plant ash conveyance system is operating at maximum capacity and upgrading the existing system Page 2 of 5

3 would be cost prohibitive. Ash will be conveyed to the new ash pond facility by placing a hydraulic dredge in the initial settlement chamber of the existing pond and hydraulically conveying the dredged ash 5,000 feet to the new pond expansion area. Therefore, the existing ash pond will remain in service to receive ash. Effluent from the new pond expansion overflows back through the existing secondary settling cell where it discharges through the existing NPDES permitted outfall. Figure 2: Hydraulic dredge used For moving ash The initial pond design included use of earthen dikes to form the new pond disposal facility. However, the mine reclamation work that used the majority of ash being produced was nearing completion before the new pond could be constructed. Therefore, an additional beneficial use had to be identified. The decision was made to use previously ponded ash as the core material for the new dike system. The new ash pond expansion had to be approved through the Indiana Department of Natural Resources (IDNR). A seismic stability analysis demonstrated the facility would be stable during a seismic event. The stability analysis demonstrated that the ash was structurally suitable for dike construction. CONSTRUCTION PHASE The ash used in dike construction had to be placed in a manner that it was properly contained in the event of a flood condition. The ash dike core is encapsulated by 5 feet of soil so that it is not exposed to the elements. During construction, the outer soil layer was placed sequentially ahead of the inner ash material so that the ash would be protected in the event of a flood event (see Figures 3 and 4). Page 3 of 5

4 Figure 3: Placing ash in core of new pond dike Figure 4: Completed pond dike The construction phase of the project also included several challenges. The biggest challenge was dewatering the ponded ash to an acceptable moisture content range before placement. The ash removal and drying process in the pond had to be conducted sufficiently in advance of the dike construction process to give the ash ample time to dewater and dry before placement. Also, the ash source location impacted the required dewatering time and ash handling requirements since the ash tends to segregate by sediment size across the pond. The coarser ash tends to settle near the ash sluice line discharge, while finer material carries across the pond. The coarser ash will dewater faster and is easier to handle, so this material was typically used for dike construction when available. Another challenge was in-place measurement of ash moisture content during the construction quality control process. Because of the relatively high carbon content of the ash, the nuclear density gage systems used to measure in-place Page 4 of 5

5 density would not accurately measure in-place moisture content of the ash. Consequently, in-place fly ash moisture contents had to be obtained by oven drying methods. After construction of the pond system was in progress, a decision was made to install a center dike in the pond to reduce the HDPE composite lined portion of the pond to 80 acres (see Figure 5). The remaining 60-acre area will remain unlined until future decisions are finalized relative to plant combustion waste disposal needs. SUMMARY The Type F ash produced at the Wabash River Generating Station is an excellent material for use in dike construction and other general fill applications. The biggest challenge to utilizing ash from an ash pond system for fill applications is dewatering the ash to optimum moisture. The ash readily dewaters if it can be stacked above the pond water surface and allowed to sit for several days before using. The coarser ash materials located closer to the ash sluice line discharge point are typically easier to dewater and handle, though the finer material can be used if a longer dewatering time is allotted. Figure 5: Complete pond extension Page 5 of 5