Deformation Characteristics and Damage Evolution Mechanisms of Dry and Saturated Argillaceous Siltstone Under Cyclic Loading

Argillaceous siltstone is a common hydrophilic rock in geotechnical engineering, and its mechanical properties undergo significant changes under the combined influence of water and cyclic loading. Therefore, systematically investigating the mechanical behavior of argillaceous siltstone under cyclic loading in different moisture conditions is of great significance for guiding engineering design and improving the accuracy of disaster prediction. In this study, triaxial cyclic loading tests were conducted on dry and saturated specimens of argillaceous siltstone from Shiyan, Hubei Province, under different frequencies and confining pressures. A comparative analysis was performed on the strength and deformation characteristics of dry and saturated specimens, as well as the influence of frequency and confining pressure on their mechanical properties. The evolution of damage in argillaceous siltstone under cyclic loading was elucidated. The results indicate that: (1) Compared to dry specimens, the strength of saturated specimens significantly decreases, and the movement of the hysteresis loop towards larger strains is more pronounced; (2) Deformation of dry specimens shows minimal variation with increasing frequency, while an increase in frequency significantly accelerates the degradation of saturated specimens; (3) Lower confining pressures can restrain crack development in saturated specimens, but excessively high confining pressures can accelerate their failure; (4) After saturation, damage variables are noticeably influenced by frequency, showing a trend of initially decreasing and then increasing with changes in confining pressure. 1. INTRODUCTION In China, red-colored soft rock formations, commonly referred to as the "red layer" in engineering contexts, are widely distributed. Among these formations, argillaceous siltstone stands out as a crucial component. Characterized by minerals such as montmorillonite and kaolinite, argillaceous siltstone exhibits hydrophilic properties, resulting in significant mechanical changes upon immersion in water. This type of rock is particularly prevalent in the southern regions of China, notably in the southwest and south-central areas, where heavy precipitation is common, rendering water presence inevitable in various construction projects.