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Abstract: In order to explore the influence of the relative positional relationship between the dynamic load and the long axis of the elliptical cavern on the characteristics of impact rockburst, a cuboid sample with an elliptical hole is prepared. With two positional relationships of disturbance direction parallel or perpendicular to the long axis of the elliptical cavern, the elliptical cavern impact rockburst experiments are carried out. With the aid of video recording device, the characteristics of the impact rockburst of the elliptical cavern are discussed in terms of damage process of rockburst ejection, ejection speed, failure mode and fragment size. The experimental results show that the rockburst processes of the two samples with different positional relationships are the same, and the rockburst pits are all V-shaped. The initial failure stress, rockburst stress, rockburst ejection velocity, rockburst pit size, fragment mass and proportion of coarse-grained fragments of the samples whose disturbance direction is perpendicular to the long axis of the ellipse are smaller than those of the samples whose disturbance direction is parallel to the long axis. The fragment characteristics of the samples whose disturbance direction is perpendicular to the long axis of the ellipse are mainly presented as thin stripes. The samples whose disturbance direction is parallel to the long axis of the ellipse can significantly improve the bearing capacity of the cavern structure. However, the rockburst phenomenon is more severe and its debris characteristics are mainly of thick plate.

Impact rockburst is induced by the combined static and dynamic loads after excavation.There are two types of impact rockburst experiments，one is induced by changing the static load and another is induced by changing the amplitude of dynamic load.The sandstone fragments after these two experiments were collected.Meanwhile，the sandstone fragments generated from the similar compression experiment under static load were collected too.And the mass， length，width and thickness of each fragment has been measured respectively.The quantities and mass distribution of fragments with coarse，medium，fine and micro grains in different size ranges，and size distribution were further analyzed.The crushing level and shape characteristic of fragments were obtained.Then the fractal dimensions were calculated.The results show that the degree of crushing level of fragments from impact rockburst is higher and with obvious slice shape；the ways of combined static and dynamic loads have a little effect on the crushing level and scale characteristic of fragments from impact rockburst experiments；energy dissipation from impact rockburst experiments is higher than that of similar compression experiment.

Herein, a finite discrete element method was used to simulate the rockburst phenomenon of elliptical caverns with different axis ratios. Two situations were employed, namely when the disturbance direction is perpendicular and parallel to the ellipse. Based on the peak stress, maximum velocity, stress nephogram, and image fractal characteristics, the influence of axis ratio and direction of the disturbance on rockburst were analyzed. The results show that the samples with different axis ratios experienced the same process of quiet period, slab cracking period, and rockburst. The rockburst pit had V shape, and the failure modes of rockburst primarily included shear cracks, horizontal tension cracks, and vertical tension cracks. With the rise in axis ratio, the peak stress and maximum speed increased. Furthermore, the pressure area on the left and right sides of the sample cavern decreased when the disturbance direction was parallel to the short axis of the ellipse, while it increased for the sample with a disturbance direction perpendicular to the short axis. The fractal dimension value of the crack was gradually amplified with disturbance. The fractal dimension value of the sample whose disturbance direction was perpendicular to the minor axis of the ellipse was lower, and it was more difficult to damage. [ABSTRACT FROM AUTHOR]