Effect of multiple wetting and drying cycles on the macropore structure of granite residual soil

Abstract

Soil usually experiences multiple wetting and drying cycles due to changes in climate conditions, which directly lead to changes in the soil pore structure, especially macropores, thus affecting the mechanical properties and permeability. In this paper, the macropore structure of granite residual soil during three wetting and drying cycles is examined by X-ray computed tomography (CT). The pore spatial distribution (including connected pores and isolated pores), pore number, pore porosity and hydraulic conductivity are analysed and quantified. The results show that the variation in the 2D porosity distribution curve continuously decreases in size and reaches the smallest size in the third cycle. Interestingly, the variation in the connected pore porosity is greater than that of the porosity obtained based on all pores as well as isolated pores. The cycles make the soil shrink, the drying effect reduces the connectivity of the pore space, and the number of isolated pores increases; the wetting effect contributes to the recovery of the lost pores and connections, and the pore space and porosity increase after wetting. In addition, the hydraulic conductivity decreases due to the connected porosity, the pore/throat radius decreases, and the connected macropores play a decisive role in the saturated hydraulic conductivity.