Study On The Model Of Rock Block Stability Evaluation Based On Natural Vibration Frequency

The stability of rock block is one of the basic study areas in geological engineering and geotechnical engineering. In recent years, the fast development of economy in China has accelerated the construction of transportation, water conservancy and energy engineering with more and more artificial slopes and modified natural slopes. The disaster triggered by rock mass failure has been increased which caused irreparable loss and also threat the security of the highway, railway and other transportation system. Therefore, the study on perilous rock collapse is very significant especially the analysis on stability for geological disaster prevention and mitigation. The stability of rock block mainly depends on its adhesion to slope. Based to the theory of structural dynamics, the changes in cohesive force will cause changes in the vibration characteristics of rock mass. This paper mainly focuses on the dynamic model between natural vibration frequency and dangerous rock stability from three aspects:fast identification of unstable rocks, safety evaluation, and monitoring and early warning. Based on the analysis of the test data between non-contact remote laser vibrometry and the traditional sensor, the rock block fast safety evaluation method based on dynamic characteristic parameter was established and monitoring and early warning research methods were discussed.The paper studied relationship between natural vibration frequency and dangerous rock block stability from the aspects of theoretical research, experimental model and field application. The main conclusions are as follows:(1) Based on the principle of vibration mechanics, the theoretical relationship model between natural vibration frequency and cohesive force was established. Based on the result of the model, the changes in natural vibration frequency can reflect changes of the bonding strength of unstable rock block, and thus frequency analysis can response dangerous rock damage on slope in time.(2) Through the research on the theoretical model, the changes in anti-slide force index at different time were analyzed and the process of slope rock block failure can be divided into three stages:strong stable stage, weak stable stage and failure stage.(3) During the experiment, it was found that the natural vibration frequency decreased from the original 12.67Hz to 4.67Hz gradually with the decrease of the bonding force of potential slip surface. The natural vibration frequency was a good indication for the binding force index. With decrease of the adhesion in rock block potential slip surface, the natural vibration frequency initially changed and declined significantly in 200-230s; and the displacement index changed in the 220-230s correspondingly. So the best warning time based on natural vibration frequency was 20 seconds earlier than that based on displacement.(4) In the early warning experiment of friction, the friction force in the strong stable phase rarely changed. However, in the weak stability stage, it gradually increased and occupied more than 80% of the whole anti-slide force. In the end, the failure occurred when the friction force reached maximum static friction. During the experiment, the criterion of failure was whether the friction force reached the maximum friction, and its early warning time was 30 seconds earlier than displacement warning.(5) Based on the result of monitoring natural frequency, the cohesive force index of the limit equilibrium model was adjusted dynamically, and safety coefficient formula of the limit equilibrium model was revised. The experimental results showed that the safety factor calculated by the dynamic parameters of cohesive force was consistent with the experimental results.(6) Based on the model simulating the natural vibration frequency and the cohesive force, the formulas calculating safety factor in four three-dimensional limit equilibrium models were revised and improved. The calculation results showed that, after the disturbance of heavy rain, the three-dimensional safety factor of the slope dangerous rock reduced from 1.3 to 1.1 which was compatible with the actual observation results.(7) Based on the research on the application, the Laser Doppler Vibrometry (LDV) can possibly treat natural vibration frequency as an indicator for field monitoring. Compared with traditional sensors, LDV has advantages in measurement accuracy and damage identification.(8) The advantages of LDV include non-contact, high sensitivity and fuzzy recognition which can possibly enable the LDV to quickly measure natural vibration frequency of dangerous rocks. Based on the previous study and three-dimensional limit equilibrium models, a monitoring system and early warning method that mainly based on vibration monitoring and supplemented by stress, strain and environmental monitoring was developed to rapidly evaluate the safety factor of the unstable rocks.