Since the Coal Combustion Residuals (CCR) regulations became effective in October 2015, many power generators have begun closing surface impoundments, and many others are in the planning stage. The approaches being employed or considered include closure in-place, clean closure or some combination of the two. The method selected is most frequently based upon site-specific factors such as the pond’s location and stability, political/social pressures, future land use, schedule constraints, costs, and constructability.
Due to the challenges of dealing with some of the unique characteristics of CCR impoundments, constructability can potentially have the largest single effect on successful project execution. CCR’s by their nature can be difficult to manage and present even potentially dangerous construction conditions.
Material handling during construction needs to address the following characteristics of CCR impoundments:
Impoundments are frequently large. A large pond can cover a footprint of tens to hundreds of acres and contain millions of cubic yards of CCR materials. Regardless of the closure method selected, by virtue of the size of the impoundments typically involved, closure will entail the handling of large volumes of CCR or fill. This requirement can strain the traffic capacities of narrow haul roads or the availability of near-by borrow resources. The impoundment can also contain large volumes of water that must be drained and discharged prior to construction.
CCRs are weak. CCRs that have been hydraulically deposited in impoundments are typically saturated and unconsolidated. As a result they are in a weak condition that makes them incapable of supporting excavators and other heavy equipment needed for construction. Undrained CCRs will not maintain excavation slopes, and are susceptible to liquefaction when subjected to vibration. This puts heavy equipment (and operators) at risk of immobilization, instability or engulfment when operating on materials that are insufficiently drained.
CCR materials are slow draining. The hydraulic conductivity of CCR materials generally ranges between 0.03 to 0.3 ft/day. Dewatering of the CCR materials can only accelerated by shortening the length of the drainage path or increasing the hydraulic gradient.
Approach to CCR materials handling
There are several options available for handling the solids or de-watering of CCR materials. The suitability of the each approach will be driven by site-specific conditions and end goals. Advance CCR dewatering is required if the solids are to be left and covered in place, or if they are mechanically removed. Where sufficient surface water cover exists, hydraulic dredging allows the removal of CCR solids without the need to dewater.
CCR dewatering. Because the CCR is slow to yield water, drainage activities should begin well in advance of any excavation work. Frequently, the impoundment contains large volumes of free water, where the rate of dewatering may only be limited by permit discharge limits. To expedite de-watering of existing solids, perimeter and internal drainage ditches along with strategically-placed sumps are needed. Where feasible, passive de-watering by existing outfalls can be the most cost-effective approach. A project construction schedule should allow sufficient time for dewatering. It is also helpful to consider a schedule that coincides with drier construction seasons to take advantage of atmospheric drying.
Mechanical excavation. If CCR solids are to be relocated to a solid waste disposal, or if the impoundment is closed in-place, sufficient dewatering will be required to support conventional construction equipment and to allow safe excavation. This can have schedule impacts. CCR materials that are not adequately dewatered can liquefy when subjected to the vibration of digging or hauling, thereby losing all strength.
Hydraulic dredging. Hydraulic dredging of CCR materials can be an attractive alternative when prior dewatering is less viable. The dredged materials are discharged to a constructed dredge cell where they are allowed to drain and dewater. This approach can provide attractive production rates and a shortened schedule. Often this method of removal can be further accelerated when used in conjunction with polymer additions and geotube structures.
In the end, successful closure projects must adequately consider and plan for the unique characteristics of CCR materials. Accounting for site-specific conditions and particular owner objectives is essential to cost effective, efficient and safe construction execution.