Deep Mixing Method for Stabilization of Coal Combustion Residual Material-Paper

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The paper describes the use of the Deep Mixing Method (DMM) to stabilize coal combustion residuals (CCR) at a power-station landfill in the southeastern United States. Two retaining walls, built from overlapping soil-cement columns arranged in cells, were designed to support the landfill during excavation and help contain leachate. The project installed 4,613 columns across approximately 28,000 m².<br /><br />CCR conditions varied substantially: sedimented ash was weak and wet, while compacted ash was dense and dry. Its fly ash also showed inconsistent pozzolanic activity, causing field strengths—especially at the West Wall—to develop faster and higher than laboratory tests had predicted. This complicated installation of overlapping columns. The team responded through extensive laboratory and field trials, adjusting binder content, cement-to-slag ratios, and grout additives. Pre-cutting helped penetrate dense surface ash, while extra mixing at column bases improved treatment in the plastic saprolite layer.<br /><br />During construction, installation parameters were continually adapted to local CCR behavior. Real-time equipment monitoring, verticality measurements, wet-grab sampling, and verification coring were used to assess quality. Testing confirmed that the specified strength and permeability requirements were met; only four additional columns were needed to address detected deficiencies.<br /><br />The project demonstrates that DMM can effectively stabilize CCR in place, minimizing excavation, waste handling, and spoil generation. Environmental impacts were further reduced by exploiting the ash’s pozzolanic activity to lower binder demand and using limestone-blended cement and ground granulated blast-furnace slag. More than half of the binder content came from recycled material.

Keywords

Deep Mixing Method

coal combustion residuals

soil-cement columns

retaining walls

landfill stabilization

leachate containment

fly ash

pozzolanic activity

binder optimization

environmental remediation