Fly-Ash-Stabilized Gypsiferous Soil as an Embankment Material
A highway expansion project was proposed at the boundary of Texas and New Mexico State, where gypsum deposits are overlaid almost the entire construction area. In order to use the local gypsiferous soil as a suitable embankment material, soil stabilizatio
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FLY-ASH-STABILIZED GYPSIFEROUS SOIL AS AN EMBANKMENT MATERIAL lie Zhang, Ruben Solis Civil Engineering Department, New Mexico State University, P.D. Box 30001, MSC-3CE Las Cruces, NM 88003-8001, U.S.A. A highway expansion project was proposed at the boundary of Texas and New Mexico State, where gypsum deposits are overlaid almost the entire construction area. In order to use the local gypsiferous soil as a suitable embankment material, soil stabilization using fly ash as an admixture was proposed. Even though the application of fly ash as a soil stabilizer started around 1950s, there are not many case histories related to the stabilization of gypsiferous soils using fly ash. In this study, a series oflaboratory testing on gypsiferous soils before and after the addition of fly ash is presented. Analysis is done on the physical and mechanical properties of the stabilized soils as well as the chemical processes involved in the stabilization. INTRODUCTION
Gypsiferous soils have not been widely used as construction materials due to their unpredictable behavior when water exists. The main component of gypsiferous soils is gypsum (CaS04'2H20), which is a major rock forming mineral derived from karstic terrain. The term karstic terrain is normally applied when the dissolution of bedrock and development of underground drainage is involved. Therefore, gypsum is classified as a soluble and air-hardening material. Because of the property of changing its chemical structure at temperatures above 58°C or in situations where water exists, the behavior of gypsum is unpredictable. The hazard of subsidence, strata collapse and sinkhole formation is more evident in gypsiferous soils than in other soluble rocks. In addition, when gypsum exists in bedrocks, either as massive beds or veins, it can be associated with the sulphate-rich groundwater which is harmful to concrete, and precautions to prevent damage should be given (Forster et al. 1995). Because of the specific properties, gypsiferous soil as a structural fill or used in other engineering applications, such as embankments, pavement subgrades and bearing soils, is not recommended. According to Irfan and Ozkaya (1981), dissolution problems in embankments and slopes are associated with gypsum and anhydrite (CaS04), which may cause a shear strength reduction by a factor of 4 if 10% of gypsum or anhydrite is dissolved. Even though engineers have been trying to conduct soil stabilization in gypsiferous-soil-overlaid sites, references in this area are very limited and in some cases the construction cost could be very
high. However, soil stabilization in other problematic soils, such as soft clays, is widely used. Misra (1998), Cokca (2001), and Nalbantoglu (2004) applied fly ash in stabilizing clayey soils, soft subgrades and highway embankments. After soil stabilization, the engineering properties of these problematic soils were improved with increased maximum dry density, reduced swelling potential, and increased shear strength. Inspired by these successful soil stabilization cases
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