Macroscopic and microscopic study of unsaturated shear strength behaviour of type-F fly ash

Abstract

Fly ash is a by-product of thermal power plant generated due to the combustion of coal. The silt size hollow spherical particles of fly ash have low specific gravity and high crushability. Fly ash is used as a structural fill in highway/railway embankments that remain unsaturated during most of the year, and its behavior under unsaturated conditions has not been explored yet. The current research explains a procedure to determine unsaturated shear strength parameters of type-F fly ash by using conventional shear strength parameters and matric suction. A series of filter paper tests and consolidated undrained (CU) triaxial tests were conducted to obtain the soil water characteristic curve (SWCC) and effective shear strength parameters (c' & ϕ') of fly ash respectively. Unconfined compression (UC) test series at varying degree of saturation was performed to determine unsaturated shear strength (USS) of fly ash as a function of matric suction. USS of fly ash was observed to increase with matric suction until it reached the residual zone of unsaturation, after which it was observed to reduce. The angle of internal friction with respect to matric suction (ϕb) increased and stiffness decreased with the increment in degree of saturation. Additionally, the effect of crushability of fly ash particles on matric suction evolution was also analyzed using Scanning Electron Microscope (SEM) images to evaluate its microscopic behavior. The reduction in matric suction with increasing crushing cycles could be attributed to the breakage of cenospheres and deformation of plerospheres.