Investigating weathering features of sandstones in the Yungang Grottoes based on SEM images and micro-scale flow model

The quantitative relationship among the weathering, microscale structure, and microscale flow properties is a fundamental issue for the efficient preservation of the cultural relics in the grottoes. Sandstone weathering may affect the efficient preservation of the cultural relics in the grotto rocks. In this study, the microscopic images of the sandstones of various grottoes in the Yungang Grottoes are obtained by Scanning Electronic Microscopy (SEM), and the digital characteristic parameters of the particles and pores in the rock are obtained by using the image enhancement and image segmentations. A flow model reflecting the characteristics of the pore-throat connection is also established in the microscopic scale. The local hydraulic conductivity coefficients of the rocks at various grottoes are then obtained. The relationship among the hydraulic conductivity coefficients, digital characteristic parameters, and weathering levels is further explored. The results show that the weathering much affects the microstructure of the grotto sandstones; the corresponding great-small order of the weathering for the average long axeses of pores are the complete or high weathering, the moderate weathering, and the slight weathering or fresh sandstones, respectively, with the axeses of greater than 40 μm, 25 to 35 μm, and 15 to 25 μm. The higher levels of weathering will result in the greater hydraulic conductivities ranging from 1×10⁻⁹ to 1×10⁻⁴ cm/s; the hydraulic conductivity is closely related to the size and connectivity of the pore-throats; and the increases in the pore-throat radius ratios will result in the increase in the hydraulic conductivities.