The mechanism of zonal disintegration is studied from the viewpoint of spatial strain localization.Using a three-dimensional heterogeneous strain-softening model

we investigate distributions of shear strain increments at different planes and monitored lines of the surrounding rock of a horseshoe（U-shaped）tunnel under progressive excavation and different ratios of axial pressure to he lateral pressure.We find that the fractured zones are annular in the surrounding rock of the U-shaped tunnel;prior to and beyond the heading of the tunnel

the zonal disintegration can be found;prior to the heading of the tunnel

the zonal disintegration becomes less apparent until it disappears with an increase of the distance from the heading;higher ratio of the axial pressure to the lateral pressure leads to more apparent zonal disintegration;at different planes orthogonal to the tunnel axis

the zonal disintegration is different

which is related to the different shear band inclination angles and their developing degrees;conjugate shear bands lead to the complex zonal disintegration.A full development of spatial cone-shaped shear planes originated from the tunnel surface results in the zonal disintegration.