Abstract: To reveal the occurrence mechanism of ultra-low friction-typed rockburst, deep coal-rock is used as the study material. Firstly, a self-developed ultra-low friction experimental device was used to reproduce the ultra-low friction-typed rockburst under laboratory conditions. A high-speed camera and an acoustic-emission system were used to capture the dynamic fracture evolution of the coal seam and the associated roadway squeezing induced by unstable ultra-low-friction slip. Based on laboratory tests and engineering examples, we established a model of ultra-low friction-typed rockburst and derived an energy index using an energy-based criterion., and the energy index of the ultra-low friction-typed rockburst is given based on energy criterion. Using the coal–rock interfacial friction force, vertical displacement difference, and fractured-seam length as key indices, we evaluated how stiffness, damping, perturbation amplitude, perturbation frequency, and elastic-modulus ratio affect the ultra-low friction response. The results show that increasing the stiffness or damping coefficient of the structural plane reduces the possibility of ultra-low friction sliding or mitigates its intensity. High disturbance amplitude is more prone to induce ultra-low friction sliding of the coal seam. Compared to other disturbance frequencies, the disturbance frequency between 14 Hz and 16 Hz most likely trigger ultra-low friction sliding. When the coal–rock interfacial friction forces are comparable, a higher coal elastic-modulus ratio more readily induces ultra-low friction sliding in the fractured coal seam. These findings provide a basis for predicting and preventing rockburst hazards.

Key words: coal-rock interface, ultra-low friction effect, coal seam fracture, rockburst, energy index, friction force