Research on the mechanical behaviors of multi-fractured blocky rock masses

Deep rock masses have complex internal structures, which results in significant discreteness and blocky structures. With the increase in the depth of engineering construction and energy extraction, the unique pendulum-type waves emerge in deep blocky rock masses under the action of dynamic loads from mining and blasting, and they are characterized by low frequency, low velocity, large displacement amplitude and high kinetic energy, distinguishing them fundamentally from conventional seismic waves. Pendulum-type waves can induce alternating stress states of relative compression and separation within blocky rock masses, and may lead to rockburst disasters and even engineering-induced seismicity, thus posing great challenges to the safety of underground engineering such as tunnel construction and mining. In this paper, experimental research is conducted on the mechanical behaviors and typical characteristics of pendulum-type waves of multi-fractured blocky rock masses under static and dynamic loads. Firstly, the strength, deformation and failure mode were analysized based on uniaxial compression tests. The weak structural layers will significantly reduce the uniaxial compressive strength and enhance the ultimate deformation capacity of rock masses. Fractured rock masses have significant nonlinear deformation and may develop macroscopic fractures (vertical splitting failure, with the failure mode transitioning from brittle failure to ductile failure) at the stress level significantly lower than their uniaxial compressive strengths. Subsequently, based on the dynamic impact tests, the dynamic response, overall displacement, wave velocity and the mechanism of anomalously low friction were investigated, and the typical characteristics of pendulum-type waves, including the low frequency (177 Hz and 153 Hz in this experiment, which are much lower than the natural frequency of the rock itself), low velocity (about 900 m/s in this experiment, which is significantly lower than those of P-waves and S-waves), large displacement amplitude (it is more than two orders of magnitude larger than the deformation of an intact rock under an identical load) and high kinetic energy (The total kinetic energy accounts for 40% and 28% of the total energy in this experiment, which has its particularity and cannot be ignored) were quantitatively described. This study holds significant research importance for understanding the nonlinear waves in deep fractured rock masses and their dynamic behaviors, as well as for preventing and controlling engineering disasters in deep rock masses.