Deep Mixing Method for Stabilization of Coal Combustion Residual Material-PPT Presentation

PPT Presentation

Pdf Summary

The project used deep mixing to stabilize coal combustion residuals (CCR) at a large U.S. coal-fired power station. Two retaining walls were built to support a landfill during excavation for new ash basins. The walls comprised overlapping, 2.4 m-diameter soil-cement columns arranged in cells, extending to a maximum depth of 26 m and embedded 1.5 m into residual soil. Design targets were an unconfined compressive strength of 1.4 MPa and permeability of 1 × 10⁻⁷ m/s.<br /><br />Because CCR properties and pozzolanic activity varied substantially across the site, the team conducted subsurface investigations, laboratory mix tests, and field validation before production. Tests showed that sedimented CCR required more binder than compacted CCR or residual soil, and that ordinary Portland cement generally produced higher strengths than limestone cement.<br /><br />Field trials revealed differing site conditions. At the West Wall, rapid early strength gain made installation of overlapping columns difficult, so binder dosage was reduced and bentonite and admixture were added to delay setting. The East Wall’s CCR had weaker pozzolanic activity, so those additions were not needed. Water injection before mixing improved penetration through dense, dry compacted CCR, while additional grout mixing at the column bases improved treatment of plastic saprolite.<br /><br />The production works installed 4,613 columns across approximately 28,000 m². Installation data systems tracked construction parameters, column overlap, and verticality. Verification included 235 continuous-core boreholes, 1,290 strength tests on cores, 3,750 wet-grab strength tests, and 1,017 permeability tests.<br /><br />The presentation reports that required performance was achieved throughout the project, with installation parameters adjusted to accommodate changing CCR behavior. In-situ treatment also reduced the need to replace material and enabled lower binder use by leveraging CCR’s pozzolanic properties and using supplementary cementitious materials.

Keywords

deep mixing

coal combustion residuals

soil-cement columns

retaining walls

landfill support

pozzolanic activity

binder dosage

compressive strength

permeability

field validation