Use of CFB fly ash for soft subgrade stabilization of highway construction in China

摘要

Circulating Fluidized Bed (CFB) fly ash is the solid residue by-product produced by coalburning electric plants do to strict air emissions requirements in China. It is usually disposed of as a waste in utility disposal sites. Direct use of these materials in highway construction projects consuming large volumes of materials, not only provides a promising solution to the disposal problem, but also an economic alternative to the use of traditional materials. This paper represents the results of a study aimed at using CFB fly ash for soft subgrade stabilization of a highway in China. Unconfined compressive strength (qu), California Bearing ratio(CBR) and Resilient Modulus (Mr) tests were conducted on the soil alone and the soil-fly ash -lime mixture to assess how CFB fly ash improves the bearing resistance, stiffness and shear strength of the soil. Representative samples of CFB fly ash were collected from one utility power plant in China. Mixtures of CFB fly ash with different mixture ratios (i.e. 10, 20, and 30% fly ash content based on soil solids) were prepared for testing. Both strength and modulus-based approaches were applied to estimate the optimum mix design and to determine the thickness of stabilized layer. The mixtures were subjected to unconfined compression test with no delay and/or within 2 h after mixing to develop water content-strength relationship. Test results indicated that CFB fly ash mixtures compare favorably with untreated soft soils. The average of CBR increased from 4 to 60.8. The average of Mr increased from 7.976 MPa to 40.986 MPa, and qu increased from 376 kPa to 637 kPa, on average. The test data showed that the effectiveness of stabilization decreases when the water content of the soil increases.