Abstract: Since various water-induced geological disasters occur in the red-bed mudstone, it is important to study disintegrating states of red-bed mudstone under dry and wet cycles. A series of wet and dry cycle tests was conducted on mudstone samples from Triassic Badong Formation. After each cycle, every group of screened particles was weighed, and morphological parameters of rock sample were also obtained using an image processing technique. With the combination of the fractal geometry theory and the gray relational analysis, the fractal characteristics of size distribution and the morphological characteristics of rock particles were gained. Thus, a new model was established to evaluate the stability of the red-bed mudstone. The results show that rock particles with a size larger than 10 mm disintegrated dramatically, which mainly occurred in former eight times cycles and tended to stop after 12 times cycles. Corresponding to the evolution of disintegration process, the fractal dimension of size distribution rapidly increased and stabilized at around 2.20 by the end of disintegration. Meanwhile, both the circularity of rock particles and the dimension of covering boxes showed correlations with disintegrating degree and eventually stabilized when the disintegration ended. From the gray relational analysis, the variation characteristics of circularity of the red-bed mudstone well reflect its disintegrating state. Therefore, the stability evaluation of the red-bed mudstone has a significant guidance in engineering construction.

Key words: red-bed mudstone, cyclic wetting and drying cyclic, disintegration, circularity, stability evaluation