Significant Project Features:

• Installation of construction access (1/2 mile), clear and grub project limits and perimeter erosion control (installation and maintenance)
• Abandonment and upgrade of various monitoring wells positioned around the ash pond
• Pump House demolition
• Excavation and off-site disposal of shallow ash deposits located outside of the pond limits
• In-Situ Stabilization (ISS) of approximately 48,000 CY CRR and miscellaneous debris.
• Import and placement of 35,000 tons of material for subgrade establishment.
• Installation of geomembrane and geocomposite drainage layer in conjunction with other shallow drainage features
• Northern berm armoring and improvements
• Placement of clean cover consisting of vegetative soil and topsoil
• Establishment of pollinator habitat and site restoration

Project Background

GSI was contracted by an energy provider to complete the closure of a former ash pond located in the mid-west where the primary means of addressing CCR materials was by way of ISS. The site was in operation for over 50 years and utilized multiple man-made impoundments across the site to store ash.

Leading up to field implementation, GSI performed a two-part mix design program in conjunction with an in-field test pit program to further evaluate the CCR materials and required reagent addition to achieve both strength and permeability targets.

Upon mobilization, GSI completed various site improvements needed to advance the overall scope of work. These tasks included; access road improvements, clearing and grubbing, regrading of pond limits, shallow ash excavation and pump house demolition. Transitioning into the soil mixing scope, GSI completed the stabilization of approximately 48,000 cubic yards of CCR, slag, coal, and general fill materials achieving an average unconfined compressive strength of 275 psi and a geometric mean in the evaluation of permeability of 1.86×10-7 cm/sec. The ISS extended from the prepared work platform to the designated bottom of ISS which ranged in depth from 0 to 23 feet.

Following the completion of ISS, GSI began the import of approximately 35,000 tons of subgrade material used to shape the site and establish the grades needed to promote positive drainage outside of the pond footprint. Locally sourced materials were placed in lifts and tested for compaction. Positioned overtop of the subgrade fill, GSI installed a geotextile separation layer, single-sided texture 40mil HDPE geomembrane and 6oz. double-sided non-woven geocomposite drainage layer. Additional drainage features consisting of shallow, gravel-filled swales and perforated piping were installed to provide a controlled and direct drainage pathway.

To protect the northern berm, which is constructed of mostly hardened slag and exposed to harsh elements such as wave action and ice heave, GSI installed a slope armoring system using articulated concrete panels. These panels, laid on top of a crushed stone base, locked together and were anchored along both the leading and trailing face.

To complete the clean cover installation, GSI imported and placed two layers of soil. The first was 18 inches of vegetative cover soil which was placed and compacted to 90% modified and the second was 6-inches of topsoil. Fertilizer was applied and a specific pollinator habitat seed mixture was drilled into the soils and watered until appropriate coverage was achieved.

Project Challenges

• Sequencing of work while maintaining production across the ash pond. Having equipment advance into the center of the pond by working off of previously stabilized CCR material.
• Incorporating heavy slag deposits and other debris encountered during ISS production.