Degradation of natural red mudstone subjected to long-term relative humidity cycles

This paper presents a multi-scale experimental investigation of the weathering degradation of red mudstone. Natural rocks were extracted from the surface ground to 120 m, in which three sets of samples were selected to consider the different initial rock fabrics. The long-term relative humidity (RH) cycles under two amplitudes were imposed on red mudstone to simulate the weathering process. After RH cycles, a series of uniaxial compression tests, Brazilian splitting tests and bender-extender element tests were carried out to examine the reduction in strength and stiffness. The objective of this study is to develop an extended stress-volume framework characterizing the degradation of natural red mudstone both at microscale and macroscale. Accompanied by the irreversible swelling of the rock specimen is the progressive degradation of strength, stiffness and Poisson's ratio. A unified exponential degradation model in terms of the irreversible volumetric strain was thus proposed to capture such a degradation pattern. The effect of the initial rock fabric was evident. The highest degradation rate and potential were identified in slightly weathered specimens. Significant slaking of aggregates and crack propagation were confirmed by scanning electron microscope (SEM) micrographs, which were considered as the main consequence of structure damage leading to degradation of mechanical properties. The structure damage during RH cycles denoted the hysteresis nature in the response to the cycling hydraulic reaction, in turn causing the increase in volumetric strain. Thus, the stress-volume relation rather than the suction relation was found in more reasonable agreement with the experimental results.