Keywords: flowable fill, fly ash, contaminant leaching, controlled low strength material, CLSM, economic feasibility, logistics, utility cables, utility pipes, leachability, coal combustion ash, waste management, environment, environmental assessment
An economic feasibility model for ash use in flowable fill with integration of logistics and contaminant leaching factors
Encasement of utility cables and pipes in flowable fill monoliths formed in trenches requires the use of materials that satisfy both strength and contaminant leachability limits. Coal combustion ash is often readily available for use in flowable fill construction. The use of ash in desirably large quantities, calls for the determination and selection of the maximum ash mix fraction that meets strength requirements for excavatability and contaminant leachability. Initial economic feasibility assessments can then be performed in terms of the costs and revenues of installing flowable fill using the ash mix fraction selected. In this paper, the methodology is demonstrated by testing monoliths of flowable fill containing 0–20% (by weight) of coal combustion fly ash for Unconfined Compressive Strength (UCS) and leachability indices, Li values of selenium and arsenic. It was found that the monolith containing 20% by weight of ash satisfied the excavatable fill maximum strength requirements of 2070 N/m², and selenium and arsenic minimum Li values of 17.0 and 14.0, respectively. An economic feasibility estimation methodology is presented to relate ash sale revenues and flowable fill placement costs for various mix fractions of ash.