Vibration response of the interfaces in multi-layer combined coal and rock mass under impact load

Abstract

The stress wave generated by impact or dynamic load would have significant reflection and transmission at the rock-coal or rock-rock interfaces in the process of propagation, resulting in dynamic effects such as dynamic tensile, stress superposition and mutation. These dynamic effects would lead to obvious vibration at the interfaces which is a key factor leading to dynamic damage and failure of coal and rock mass. In the process of underground engineering excavation, the dynamic damage of a series of layered rock masses is the important factor causing geological disasters. Based on the two-dimensional similar material simulation experiment, the coal and rock mass combined by five layers of fine sandstone, medium sandstone, coal, coarse sandstone and mudstone is taken as the research object, single and multi-point excitation (synchronous / step-by-step) were used to test the time-history vibration curves of rock-coal and rock-rock interfaces under impact load. It was concluded that the change of extreme value of vibration amplitude presented two stages: first increase and then attenuation. Most of them needed 2.25 cycles to reach the peak value, and the dynamic attenuation of amplitude conformed to the law of exponential. Based on Fast Fourier transform (FFT), the spectrum structures of amplitude-frequency of interfaces vibration were studied, and the two predominant frequencies were 48.9Hz~53.7Hz and 92.4Hz respectively. Based on Hilbert-Huang transform and energy equation, 5~7 vibration modes (IMF) were obtained respectively by decomposing the time-history curves. The three modes IMF1、IMF2、IMF3 contained high energy and were effective vibration modes. IMF2 accounted for the highest proportion and was the main vibration mode whose predominant frequencies were concentrated in 45.6Hz~50.2Hz. Therefore, IMF2 played a decisive role in the whole vibration process and had an important impact on the dynamic response, damage and failure of coal and rock mass. In real condition, the actual predominant frequencies could be converted according to the size and mechanical properties of coal and rock mass, and the vibration response characteristics of the interfaces between coal and rock mass under impact load were preliminarily revealed.