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Forensic Examination of Severe Heaving of Embankment Constructed with Fluidized Bed Combustion Ash

Coal combustion by-products (CCB) are widely used as structural fill materials in embankment construction works. Fluidized bed combustion (FBC) units, which are used to burn coal, have an advantage over the conventionally used pulverized coal units because they can be used to burn even low-grade or high-sulfur content coal. FBC technology emerged from efforts to reduce the emission of pollutants. A roadway embankment was constructed in 1995 with 60% FBC ash, 35% stoker ash, and 5% Class-F fly ash as part of the South Russell Road extension work at Purdue University, West Lafayette, Indiana. The embankment length, maximum height, and crest width are approximately 200 m, 10 m, and 20 m, respectively. The side slopes are 3:1 (H:V). Heaving of the embankment was observed during and after construction, and the pavement exhibited severe damage at several locations. A forensic investigation of the possible causes and mechanisms for this swelling behavior was undertaken. X-ray diffraction and scanning electron microscopic studies were performed to compare the mineralogical phase composition of fresh samples and samples collected from the heaved embankment. The test results indicated that the formation of ettringite led to the swelling of the embankment, which caused cracking of the pavement. In addition, one-dimensional swelling tests on fresh FBC ash samples, as well as on samples collected from the embankment that underwent heaving, were conducted in a consolidometer to quantify the expansive behavior of this material.