Surface In-situ Stabilization of Saturated Ash for CCR Impoundment Closures-Article

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The article describes using in-situ stabilization (ISS) to create a safe working platform on saturated coal ash during closure of a coal combustion residuals (CCR) impoundment. Saturated ash can lose strength under disturbance and may liquefy, making heavy equipment operation hazardous. Excavating it was impractical at the case-study site, where saturated ash extended as deep as 40 feet.<br /><br />ISS treats the upper 3–5 feet of ash by mixing in Portland cement. In a 280-acre impoundment project, an 8-acre work area was divided into treatment cells of about 1,800 square feet, each receiving approximately 25–26 tons of cement. Long-reach excavators mixed the cement and ash for at least an hour, working from the perimeter inward. Laboratory tests showed that after 2–3 days of curing, the treated material could provide adequate bearing capacity for heavy equipment, with limited settlement.<br /><br />During the project, 54,000 cubic yards of saturated ash were treated over 30 working days. Equipment could generally operate on treated cells within 2–3 days, enabling dry ash placement, compaction, and capping to proceed alongside stabilization. Capping of the treated area was completed within seven days after ISS work ended.<br /><br />Safety required close supervision and continual monitoring. Cracks and mudwaves sometimes appeared as excavators worked near newly treated cells; neither reached the project’s stop-work thresholds, but both were treated as warning signs requiring communication and observation. The project also found that deeper saturated ash warranted longer curing periods. Water cannons effectively controlled dust during cement handling and mixing.<br /><br />The case study concludes that surface ISS can provide a practical, timely working platform for CCR closure, allowing construction and capping activities to proceed safely without removing deep saturated ash.

Keywords

in-situ stabilization

saturated coal ash

CCR impoundment

Portland cement

working platform

liquefaction risk

heavy equipment

curing period

capping operations

coal combustion residuals