The mechanism of excavation-induced rockbursts in ultra-thick soft-hard composite coal seams at shallow depths: an experimental study

Abstract Ultra-thick soft-hard (UTSH) composite coal seams, composed of alternating layers of soft and hard coal, frequently experience severe rockburst accidents during roadway excavation under shallow burial depths (< 500 m). However, research on the mechanisms underlying such rockbursts remains limited. This study simulates the failure process of surrounding rock in roadways excavated through UTSH composite coal seams under shallow depth conditions using true triaxial low axial pressure unloading tests. Through a comprehensive analysis of failure characteristics, mechanical properties, and acoustic emission (AE) parameters of soft coal, hard coal, and multi-layered soft-hard coal composite specimens (2–4 alternating layers), the mechanism of rockburst induced by excavation unloading in shallow depths UTSH composite coal seams is revealed. Results demonstrate that under low axial pressure unloading conditions, composite specimens exhibit higher crack quantities and lengths compared to the hard coal specimen, with energy accumulation ranging from 70.2 to 120.7 kJ/m3, significantly exceeding the 66.6 kJ/m3 observed in the hard coal specimen. AE parameters further confirm that composite specimens undergo more severe damage than the hard coal specimen. A three-stage failure mechanism is proposed: (1) low-stress shear failure in soft coal layers, (2) interface shear failure induced by concentrated stress, and (3) shear failure in hard coal layers caused by stress transfer. This sequential process explains rockburst occurrences in shallow depths of composite coal seams, and a mechanical criterion is established accordingly. The findings provide theoretical foundations for monitoring, early warning, and prevention of rockburst hazards in roadways excavated through UTSH composite coal seams.