Abstract: Extremely alternating extreme climatic events will aggravate weathering of rock, reduce rock strength, and affect the stability of geotechnical engineering structures. In this paper, the mechanical properties and microstructural changes of the weak interlayer were studied at room temperature and under dry and wet cycles (air drying at 60 ℃) by direct shear tests and scanning electron microscope. Significant variables of microstructure influencing the intensity were obtained through the correlation analysis and stepwise regression. Moreover, a regression equation was constructed to reflect the correlation between them. The results showed that the shear strength and cohesion of the weak sandwich in the red sandstone decreased drastically from the first to the fifth cycle, whereas the change was slower after five times. The cycle times had little influence on the change of the internal friction angle. As the number of dry and wet cycles increased, four microstructure parameters were decreased, including the average area, average diameter, average perimeter circumference and orientation probabilistic entropy, which showed that the positive correlations with the cohesion. However, three microscopic parameters, i.e., the number of particles, the average shape coefficient and the fractal dimension of the morphological distribution showed negative correlations with the cohesion. The results indicate that there are significant correlations between macroscopic mechanical properties and changes of microstructural parameters during wet and dry cycles. The weakening of weak sandwich is caused by the swelling and shrinkage, fragmentation and structural failure of red sandstone due to wet and dry cycles.

Key words: dry and wet cycles, red sandstone, weak interlayer, microstructure, shear strength