干湿循环条件下红砂岩软弱夹层微结构与剪切强度的关联性

极旱极涝频繁交替的极端气候事件会加剧岩土风化，降低强度，影响岩土工程结构稳定。采用直剪试验和扫描电镜技术，研究了对顺层红砂岩边坡稳定性起控制作用的软弱夹层在常温状态、干湿循环条件下（60℃风干）的力学特性及微观结构变化规律，通过相关分析、逐步回归方法得到了影响强度的微结构显著变量，构建了反映二者关联性的回归方程。研究表明：红砂岩软弱夹层剪切强度及黏聚力在1～5次循环过程中急剧降低，5次后变化趋缓，循环次数对内摩擦角变化影响微小；干湿循环次数增加，平均面积、平均直径、平均周长、定向概率熵等4个微结构参数减小，并与黏聚力呈现正相关关系；颗粒个数、平均形状系数、形态分布分形维数3个微观参数与黏聚力呈现负相关关系；分析认为，干湿循环过程中宏观力学特性蜕化及微观结构参数变化之间显著相关，干湿循环造成的岩土颗粒胀缩、碎裂、结构破坏是导致红砂岩软弱夹层剪切强度蜕化的主要原因。

Relationship between microstructure and shear strength of weak interlayer of red sandstone under dry and wet cycles

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.