Effects of dry-wet cycles on three-dimensional pore structure and permeability characteristics of granite residual soil using X-ray micro computed tomography

Due to seasonal climate alterations, the microstructure and permeability of granite residual soil are easily affected by multiple dry-wet cycles. The X-ray micro computed tomography (micro-CT) acted as a non-destructive tool for characterizing the microstructure of soil samples exposed to a range of damage levels induced by dry-wet cycles. Subsequently, the variations of pore distribution and permeability due to drywet cycling effects were revealed based on three-dimensional (3D) pore distribution analysis and seepage simulations. According to the results, granite residual soils could be separated into four different components, namely, pores, clay, quartz, and hematite, from micro-CT images. The reconstructed 3D pore models dynamically demonstrated the expanding and connecting patterns of pore structures during drywet cycles. The values of porosity and connectivity are positively correlated with the number of dry-wet cycles, which were expressed by exponential and linear functions, respectively. The pore volume probability distribution curves of granite residual soil coincide with the chi(2) distribution curve, which verifies the effectiveness of the assumption of chi(2) distribution probability. The pore volume distribution curves suggest that the pores in soils were divided into four types based on their volumes, i.e. micropores, mesopores, macropores, and cracks. From a quantitative and visual perspective, considerable small pores are gradually transformed into cracks with a large volume and a high connectivity. Under the action of dry-wet cycles, the number of seepage flow streamlines which contribute to water permeation in seepage simulation increases distinctly, as well as the permeability and hydraulic conductivity. The calculated hydraulic conductivity is comparable with measured ones with an acceptable error margin in general, verifying the accuracy of seepage simulations based on micro-CT results. (C) 2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. All rights reserved.

Automated summary

This study investigated the microstructure and permeability changes of granite residual soil during dry-wet cycles using X-ray micro computed tomography (micro-CT). Three-dimensional pore distribution analysis and seepage simulations were carried out to reveal the variations in pore distribution and permeability due to dry-wet cycling effects. The results showed that the porosity and connectivity were positively correlated with the number of dry-wet cycles. The pore volume distribution curves of the soil coincided with the chi(2) distribution curve. Additionally, under the influence of dry-wet cycles, small pores in the soil gradually transformed into cracks with larger volume and higher connectivity. The permeability and hydraulic conductivity increased significantly, and the seepage flow streamlines contributing to water permeation increased. The accuracy of seepage simulations based on micro-CT results was verified by comparing the calculated hydraulic conductivity with measured values.