Evolution of Time–Frequency Dynamic Parameters During the Instability of Falling-Type Unstable Rock Masses: An Experimental Study

Improving the accuracy of stable state identification and collapse early warning for unstable rock masses is an urgent challenge in slope engineering. In this study, a simplified dynamic model of falling-type unstable rock masses was established, and the dynamic response characteristics of unstable rock masses under different constraint conditions were investigated by combining modal analysis. Finally, physical model tests were carried out to explore the evolution of relevant time-domain and frequency-domain dynamic characteristic parameters during the entire process of falling-type unstable rock masses on slopes, ranging from a stable state, through the propagation of dominant structural planes, to final collapse. The results show that (1) the dominant frequency of the rock mass is independent of the magnitude and direction of excitation forces; (2) the coefficient of variation and waveform factor undergo significant changes during the critical failure stage; and (3) the acceleration amplitude ratio and natural frequency can synergistically and sensitively trace the progression of fracture development within the rock mass. An identification method for the stability stages of typical falling-type unstable rock masses was proposed, which integrates four time–frequency dynamic indicators. The stability state of unstable rock masses was divided into three phases: stable, fundamentally stable, and critical instability. This work provides a valuable reference for instability monitoring of falling-type unstable rock masses.