Strain arch-bottom rockbursts in deep D-shaped tunnels under high horizontal principal stress

Arch-bottom rockbursts pose a significant threat to the safety and long-term stability of deep tunnels, yet the triggering mechanisms remain poorly understood. To investigate this problem in deep tunnels, true triaxial simulation experiments were conducted using a self-developed servo-controlled testing system equipped with visualization monitoring. Granite from the project site was prepared into 100 mm × 100 mm × 100 mm specimens containing a prefabricated D-shaped hole to simulate the tunnel geometry. The experiments successfully reproduced the arch-bottom rockburst phenomenon, which was in good agreement with field cases. A comparison with the vertical maximum stress adjustment loading path suggested that horizontal maximum principal stress adjustment is the key in reproducing arch-bottom rockburst. Analysis demonstrated that the location of rockburst zones in surrounding rock is strongly influenced by the direction of stress adjustment. The evolution of arch-bottom rockbursts in deep D-shaped tunnels follows a staged process from stability to instability: a calm period, buckling ejection, crack development, uplift ejection, and ultimately formation of a V-shaped pit. High-precision characterization of the rockburst pit morphology was achieved using three-dimensional (3D) laser scanning technology, showing that the arch-bottom rockburst pit exhibits a typical irregular deep V-shape, with a large destruction range, deep pit, and high intensity. Moreover, arch-bottom rockbursts exhibit significant time-delay effects. This study provides mechanistic insights into the initiation and staged evolution of arch-bottom rockbursts, offering experimental evidence for assessing and mitigating risks in deep D-shaped tunnels.