Effects of experimental frost–thaw cycles on sandstones with different weathering degrees: a case from the Bingling Temple Grottoes, China

Abstract

Numerous ancient sandstone grottoes remain in northwest and central China, and weathering issues have significantly influenced their preservation conditions. The dramatic naturally cyclic changes in water content and temperature in the environment have been considered to be the main drivers of the physical weathering that commonly occurs at these archaeological sites. Therefore, comparing and understanding the behaviors of sandstone with different weathering degrees under variable environmental conditions would be helpful for further study on predicting the type, location and extent of deterioration of sandstone relics in a small region (such as the surrounding rock of grottoes). This study examines Cretaceous sandstones with two weathering degrees from Bingling Temple Grottoes, China. Standard thin section photomicrographs provide petrographic and mineralogical data and show that the sandstones have identical lithologies. Three types of specifically designed frost weathering tests are then conducted on the samples. After every six weathering cycles, the weathering processes are suspended, and the corresponding parameters, such as dry weight loss, dry density, effective porosity, porosity, P wave velocity, surface hardness and drilling resistance, are measured. At the end of the weathering cycles, the sample variations in grain size distributions are compared, and statistical tests are performed to show the statistical significance of the results. The results indicate that similar deterioration patterns occur on the samples with two weathering degrees under the same weathering tests. The increase in effective porosity surpasses that of the porosity only when the weathering effect is large enough. Furthermore, sandstone with a high degree of weathering might be more susceptible to changes in the internal pores due to its greater initial interconnectivity. In an open system, physical weathering (frost–thaw and dry–wet cycles) would cause the superficial grain size distributions of different rocks to become relatively uniform. Finally, when exposed to the same weathering process, the decreases in the overall mechanical strength in the two sandstones do not differ significantly, but the loss of superficial strength may be different.