Rockburst processes of a tunnel at different confining pressures are simulated by using of FLAC.To simulate the tunnel excavation

a FISH function is written and used to delete the elements in the tunnel.As rock elements exhibit linear strain-softening behavior after the occurrence of failure and then ideal plastic behavior

the failure criterion is a composite Mohr-Coulomb criterion with tension cut-off.Results show that an annular shear strain concentration zone

which is closer to the tunnel surface

is generated at lower confining pressures.The tunnel is still stable after rockbursts and no shear bands are observed.When the confining pressure increases to a certain value

dog-ear or V-shaped rockburst notches are generated in the surrounding rock and the surrounding rock is stable.However

when the confining pressure further increases

many long shear bands are generated in the surrounding rock and the entire section of the tunnel is destroyed

suggesting that violent rockbursts take place.With increase of the confining pressure

V-shaped notches become wider and deeper and the symmetry of shear band patterns becomes weaker.At higher confining pressures

shear band patterns are similar to plastic slip lines in plastic mechanics.