The stability of rock block on slope is an important evaluation index for the prevention of rockfall disaster. This study aims to provide an evaluation method other than stereographic projection and conventional static analyses. A physical model is manufactured to measure and analyze the microtremor characteristics of blocks on slopes designed with different conditions of joint roughness. The results show that when the rock block is stable, the frequency spectrum of microtremor signal is dispersed, and the predominant frequency is not prominent. When the rock block tends to be unstable due to the increase of slope angle, the frequency spectrum is concentrated, and the amplitude of predominant frequency increases in vertical direction and in the direction of potential movement. With smoother surface between slope and block, the frequency spectra of microtremor display a more concentrated figure, and the amplitude of predominant frequency also increase.

1 Introduction

Rockfall is one of the most difficult to predict slope instability, and it is also a slope disaster which is difficult to prevent and control effectively at present. The downhill movement may be carried out in the form of falling, slipping, rolling, or even bouncing once the rock mass is unstable and detached from the slope. It is difficult to predict accurately since motion trajectory is affected by many factors, such as slope, geotechnical material characteristics, vegetation, and rock block collision rupture in motion process. The stability of rock block on slope is related to internal factors such as rock strength and mechanical properties of discontinuous surface, and is influenced by external factors such as rainfall, earthquake or other vibration, so the occurrence of rock-fall disaster not only has the dispersion of space, but also has the sudden nature of time.

The stability analysis of rock block was paid attention to in the late 1960, and a series of analytical methods were proposed, including kinematic analysis method (Hoek 1973, Kogure & Matsukura 2012) and deformation analysis method (Zhu et al. 2016, Wu et al. 2016), and the simulation analysis methods of motion trajectory also moved forward in the 1980 (Azzoni et al. 1995, Chen et al. 2013). The occurrence position of rock-fall is the necessary information of motion trajectory analysis. Therefore, the identification of potential unstable rock block on slope is the first work of disaster prevention and control of rock-fall. Rock wedge or rock block is bounded by discontinuous surface and slope. The most direct and reliable method to assess their stability at present is field investigation through personal contact by geologists or engineers. In the near future, geometrical morphology can be obtained by non-contact mapping method, and the results may help derive the probability of instability through stereographic projection, hydrostatic analysis and empirical methods (Royán et al. 2014, Yang et al. 2014). It is also possible that vibration characteristics can be utilized as the signs of instability (Du et al. 2015, 2016, Chao et al. 2017).

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