Deep Mixing Method for Stabilization of Coal Combustion Residual Material

The paper describes the activity performed nearby a large coal-fired power station (sited in the South-Eastern USA) for the stabilization of Coal Combustion Residual (CCR) material using the Deep Soil Mixing technology. For decades, the power station conveyed the CCR material via wet sluicing methods in dedicated ash-basin lying on a saprolite layer (impervious soil). In the late 1980’s, the plant converted with dry ash handling system, and the ash basin was converted to landfill for dry ashes. The landfill was positioned mostly on top and within the footprint of the existing ash-basin, which was permitted and constructed without liners, as standard practice for those times. For this reason, in recent times confinement works were required to stop leachate migration out of the landfill perimeter. The design foresaw the excavation to remove ashes along two segments of the landfill perimeter, which were not confined by the saprolite. Two retaining walls were designed to provide structural stability to the landfill during the excavation works. The two retaining walls were constructed by the Deep Mixing Method, installing soil-cement columns, partially overlapped and arranged in a grid manner, embedded 5 ft (1.5 m) in the saprolite. DMM columns (dia. 8-ft, 2.4 m) were installed to a max depth of 85-ft (26 m) to create cells and provide lateral support to the industrial landfill material. The DMM was designed to get a UCS of 200 psi (1.4 MPa) and a 1x10-7 m/s permeability. The material to be treated was mainly composed by CCR (i.e., bottom and fly ashes), for which very limited background experiences were available. In addition, the CCR was characterized by a certain degree of pozzolanic activity, extremely variable from place to place. Hence, an extensive pre-construction process was needed to define the installation parameters, involving a soil investigation campaign, two bench scale tests, and several stages of field validation. The paper describes the solutions developed during the Project pre-construction and construction phases, to address the variable behavior of the CCR when treated with the DMM. The paper deals also with the environmental impact of the project, which was reduced by (1) selecting a technology that mixes the soil in situ rather than replacing it, (2) using eco-friendly binders like cement type ASTM IL (CEM IIL) and ground granulated blast furnace slag, (3) taking advantage of the CCR pozzolanic activity, to reduce the binder content.