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 nondestructive 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 χ^(2)distribution curve,which verifies the effectiveness of the assumption of χ^(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.

Stable expression and control parameters in the numerical simulation of unsaturated flow

The Richards’equation describes the flow phenomenon in unsaturated porous media and is essential to hydrology and environmental science.This study evaluated the numerical stability of two different forms of the Richards’equation.Sensitivity analyses were performed to investigate the control parameters of the equation.The results show that the h-form Richards’equation has better applicability for calculating variable saturation flows than theθ-form Richards’equation.For the h-form Richards’equation,the hydraulic conductivity of the soil in the low-suction range and the specific moisture capacity in the high-suction range primarily influenced the solution.In addition,sensitivity analyses indicated that the saturated hydraulic conductivity,initial condition,and air-entry pressure have a higher sensitivity to the simulation results than the saturated water content,rainfall intensity,and decline rate of hydraulic conductivity.Moreover,their correctness needs to be guaranteed first in numerical simulations.The research findings can provide a helpful reference for improving the reliability of numerical simulations of unsaturated flows.

Formation of groundwater-induced damage in Zhu Yuelian’s tomb and protection methods

Zhu Yuelian’s tomb has been exposed to groundwater for many years,and this exposure has caused many defects,including erosion,encrustation,patina,and spot marks related to groundwater,seriously affecting the artistic and ornamental value of cultural relics.In this study,high-density microelectrode spacing resistivity and ultrasonic transverse wave reflection were used to detect hidden damage in the burial chamber,and the results revealed hidden damage,corrosion,cracks,and pores in the surrounding geological body,concealed cracks in the sidewalls,and internal defects in the columns.An outdoor high-density resistivity test and hydrological observation experiment indicated that the tomb is located in a weakly permeable aquifer,and the groundwater head has been higher than the bottom of the tomb for many years.Natural groundwater flows from northeast to southwest in the aquifer,and the tomb forms an artificial concave cone,causing groundwater around the area to continually flow into the tomb.A chemical composition analysis of the groundwater and scale revealed that the groundwater within the tomb area is slightly corrosive,and the groundwater seepage along the fissures of the compacted layer induces damage.The threedimensional(3D)seepage numerical simulation results indicated that the combined use of drainage and impervious curtains can cause the water head in the curtain to drop significantly and control the water head to be lower than 8 m from the bottom floor of the tomb,meeting the requirements for seepage prevention and protection of the tomb without long-term maintenance.