Reduction of sliding resistance of unstable rock mass after an earthquake and heavy rainfall

is one of the main causes of failures. The instability and failure of unstable rock masses depend on the condition of the surface structure. As cracks open and the cohesive force of the bonding surface decreases

rock blocks become unstable

and then collapse occurs. If the conventional displacement-based monitoring method is adopted

it is generally difficult to achieve the goal of monitoring and early warning. In contrast

natural vibration frequency can effectively demonstrate the changes in physical and mechanical parameters of rock

and thus the safety assessments of unstable rock can be easily made. Through model experiments

the internal cohesive force and friction force of rock under constant sliding force are analyzed based on the natural vibration frequency. The friction force is compared to the maximum static friction force

in order to check whether or not the rock is prone to failure. It is shown that the calculated friction force can be used to effectively represent the safety of rock and the rock fails when the natural vibration frequency is below 6 Hz. The increase of the frictional force is up to 80% of stabilizing force

so that the friction increases rather than decreases in some cases. By monitoring the natural frequency

a quantitative assessment of the rock damage is made

and the static friction force index is identified

providing a new way for safety monitoring and damage evaluation of rocks based on natural vibration frequency.