Sulfate-induced heave, resulting from the chemical treatment of sulfate rich soils, has been known to cause significant damage to pavements and other structures particularly in the south-western United States. This research addressed the problem of sulfate-induced heave in coal mine spoils, formed as a result of shallow strip coal mining, after treatment with calcium-based stabilizers. These spoils occur in areas of Indiana in which substantial infrastructure development is taking place and will continue to occur in the next 10-15 years. Samples of coal mine spoils were obtained from different sites in Indiana. While the majority of the samples exhibited sulfate contents below values generally considered to be a concern, selected samples exhibited elevated sulfate contents, in the thousands of ppm. One of these samples, from the Hawthorn coal mine area in Sullivan County, Indiana, with sulfate content exceed 10,000 ppm, the critical threshold value above which, according to the literature, significant heave may be expected, was selected for extensive testing. The sample, extremely heterogeneous in nature, was crushed and screened to eliminate the coarser aggregates. The study investigated the swelling behavior of this material treated with the following calcium-based stabilizers: hydrated lime, quick lime and Portland cement, at dosages ranging from 1.5% to 9% by dry mass of the soil. The experimental program involved compaction tests performed using the Harvard miniature apparatus, and subsequent swelling tests on specimens compacted at optimum water content. The swelling tests, conducted over a period of one month were performed using the sand bath method in a custom setup housed in an environmental chamber. All specimens of the spoils treated with the stabilizers were found to exhibit swelling greater than that measured on the untreated spoil. Swelling of the treated spoil, was initially delayed and approached a constant value after approximately one month. The highest swelling strain (in excess of 10%) was measured on the specimens treated with 3% hydrated lime. Also for the other stabilizers, a lower dosage typically resulted in higher final swelling strains. Tests were also conducted to examine the effect of the surcharge stress on the swelling behavior and explore the effectiveness of the following two mitigation techniques: pre-compaction mellowing and double lime treatment. A pre-compaction mellowing period of 2-3 days was observed to lead to swelling strains comparable to those of the untreated soil. No advantages could be identified in using double lime treatment. The experimental program also included tests for the chemical and mineralogical characterization of the spoil-stabilizer mixtures and the specimen pore fluid at the end of the swelling stage. In particular, X-ray diffraction and thermo-gravimetric analyses provided evidence of the presence of ettringite in all soil-stabilizer specimens. The crystallization pressure exerted from inside the soil because of the formation of this new phase can be considered responsible for the observed swelling. In a few cases, including that corresponding to the highest swelling strain, the detection of ettringite was problematic. This seems to result from the formation, with low additions of calcium, of a poorly-crystallized ettringite. Several mitigation strategies were investigated: double lime treatment, pre-compaction mellowing

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sulfate induced heave, coal mine spoils, lime treatment, cement, SPR-2636

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Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

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