EXPERIMENTAL STUDY ON CRACKING CHARACTERISTICS OF CONCRETION SANDSTONE WITH FREEZING-THAWING CYCLES

In order to study the formation mechanism of sandstone pore weathering in Yungang Grottoes, three kinds of typical regions of sandstone concretion, sandstone matrix and sandstone matrix interface were observed by laser confocal microscopy. The crack propagation law of the concretion sandstone is analyzed, and the freezing and thawing failure model of the concretion sandstone is established. The results show that with the increase of freeze-thaw cycle, the cracks at the interface between concretion and rock matrix, at the concretion center and at the sandstone matrix appeared successively. The crack width of each region increases with the increase of freeze-thaw cycle, but there are differences in the development process of the three positions, and the cracks at the interface extend along the interface. The new cracks are produced in the concretion center, and the early cracks are extended, producing crack network. Sandstone matrix crack was developed slowly. Concretion changed the physical and mechanical properties, and significantly enhanced the heterogeneity of sandstone. The model of freeze-thaw damage of concretion sandstone revealed that the crack was generally developed at the interface. The heterogeneity of sandstone is the internal reason for the formation of pore weathering in Yungang Grottoes.