Ultrasonic investigation of strongly weathered mudstone subjected to coupled drying-wetting cycles and vibration loading

In cut slopes composed of strongly weathered mudstone, coupled wetting-drying cycles and traffic-induced vibration readily trigger progressive structural damage and slumping. Conventional single tests-such as isolated dynamic triaxial tests or independent wetting-drying simulations-cannot accurately capture the underlying microstructural evolution. As demonstrated by recent research, such as the experimental investigation of acidic dry-wet cycles on coal-rock combinations, relying on uncoupled testing fails to reproduce synergistic damage mechanisms, highlighting a critical need for multi-field experimental frameworks. Therefore, here we report a new experimental framework in which ultrasonic responses measured during wetting-drying cycling alone and during coupled wetting-drying-vibration loading were compared, including wave velocity, first-arrival amplitude, time-domain features, frequency-domain features and spectral centroid, to identify the deterioration state of remoulded strongly weathered mudstone. Uniaxial compressive strength and elastic modulus were evaluated concurrently to clarify the evolution of hydraulic damage caused by repeated swelling and shrinkage of hydrophilic clay minerals and the associated transition of macroscopic mechanical behaviour towards plastic deformation. The arrival time of the received wave lagged markedly, and under coupled loading the waveform changed from a steep, prominent peak to a right-shifted attenuated response. During wetting-drying cycling alone, the decrease in first-arrival amplitude could be divided into an initial sharp-drop stage and a later slow-accumulation stage. When high-frequency vibration was superimposed, microcracks coalesced more rapidly, and both wave velocity and strength decayed faster. The migration of the frequency spectrum and its centroid towards lower frequencies can serve as a rapid and convenient indicator of different damage states in strongly weathered mudstone. Accelerated microcrack coalescence driven by high-frequency resonance and frequent displacement cycling is the main reason why acoustic attenuation under superimposed vibration is stronger than that under wetting-drying cycling alone, with frequency exerting a greater influence than amplitude. Using wave velocity, first-arrival amplitude and spectral centroid as ultrasonic parameters, we further constructed a dimensionless vibration-disturbance coefficient (IfA) based on the entropy-weight method to quantify vibration disturbance under different working conditions, and we suggest that it can be incorporated into a three-dimensional strength-surface model. By bridging non-destructive acoustic monitoring and macroscopic mechanical degradation under coupled environmental-dynamic loading, this research establishes a rapid, non-destructive, and cost-effective quantitative tool for real-time tracking of internal micro-fatigue damage and estimating residual peak strength in weathered rock masses. Ultimately, these findings provide essential theoretical insight and data-driven decision support for long-term stability assessment, risk mitigation, and early warning of transportation corridor slopes subjected to seasonal weathering and vehicle traffic.