The free swell potential of expansive clays stabilized with the shallow bottom ash mixing method

Abstract

An expansive clay is considered a geological material and classified as problematic soil because its high free swell potential (FSP) after water absorption often damages light buildings or structures. Stabilizers such as cement and lime have been extensively used to improve the swelling characteristics of expansive clays. Bottom ash (BTA), a waste material obtained from coal lignite electric power plants, is considered a noncohesive coarse-grained soil without undesirable swelling characteristics. Thus, BTA can be applied as a suitable alternative stabilizer in cost-effectiveness and waste management to reduce the FSP of expansive clays. This paper investigates the FSP of expansive clays partially replaced with BTA by the shallow mixing method, which creates composite materials with a reduced FSP. FSP tests were performed on conventional and composite expansive clay samples using kaolin-bentonite mixtures with low to very high FSP levels to determine the optimal BTA content and study the effect of the stabilized thickness ratio (STR) on the FSP. X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques were applied to analyze the microstructural and mineral changes in the samples. Finally, FSP tests were conducted on natural expansive clay layers using six full-scale test sections to study the influence of STR on the field FSP. The main results show that when stabilized at the optimal BTA content, the montmorillonite reflection intensity and sodium (Na) amount of the stabilized expansive clay mixtures decreased, while the calcium (Ca) amount increased. This phenomenon was caused by the replacement of Na in montmorillonite with Ca in BTA, resulting in decreasing FSP values. A correlation was proposed between the FSP and STR of the composite samples based on the laboratory tests. However, such correlation overestimated the FSP values of the full-scale tests due to the effect of the overburden pressure caused by the actual thickness of the stabilized layer under field conditions. Hence, a modification factor related to increasing the overburden pressure induced by the upper stabilized layer was included in the correlation to derive an accurate prediction of the field FSP values.