Experimental Study of Single Structure Surface Dangerous Rock Mass Dynamic Characteristics Based on Constant Micromotion

The dynamic parameters of a dangerous rock mass reflect the degree of damage of the structure of its surface. There is still an urgent problem to identify the dynamic parameters of a dangerous rock mass based on the characteristics of the constant micromotion at its site. To address this problem, a method is proposed to identify the dynamic characteristics of a dangerous rock mass undergoing excitation caused by constant micromotion: (1) the vibration of a dangerous rock mass undergoing excitation from constant micromotion is classified as forced undamped structural vibration with a single degree of freedom. The ratio of the amplitude of the spectrum of the dangerous rock mass to the amplitude of the spectrum of the bedrock is defined as the relative amplitude spectrum. The first-order natural frequency is identified from the relative amplitude spectrum. (2) Bedrock is the source of excitation of a dangerous rock mass. When a mechanical wave propagates to a dangerous rock mass, it crosses the porous surface of media with structural damage, and mechanical wave scattering occurs. The frequency domain of the mechanical wave changes. The center frequency shifts to a low frequency. By means of laboratory model tests, the changes in the dynamic parameters of models of a cantilevered dangerous rock mass and a sliding dangerous rock mass with structural surface damage are analyzed. It is concluded that (1) based on the theory of vibration mechanics, the first-order natural frequencies of dangerous rock masses can be obtained from their relative amplitude spectra. The first-order natural frequencies of dangerous rock masses undergoing constant micromotion are measurable. (2) The damage of the structural surface of a dangerous rock mass with macroscopic fractures can be identified by its first-order natural frequency. The center frequency cannot reflect the development of fractures. The damage of the structure of the surface of a dangerous rock mass with microscopic fractures can be identified by the change in the center frequency in its high-frequency band. The first-order natural frequency cannot reflect the development of fractures. (3) There are limitations in using single vibration mechanics theory or elastic wave scattering theory to analyze the damage of the structure of the surface of a dangerous rock mass; it is more effective to integrate both methods.