Mechanism of zonal disintegration around deep underground excavations under triaxial stress – Insight from numerical test

• The existence of zonal disintegration is investigated by 3-dimensional numerical testing.
• The impact of tri-axial stress ratio on failure mode and failure mechanism of tunnel excavated in deep rock mass is discussed.
• The reason for different scale of zonal disintegration is analyzed.

The failure pattern and failure mechanism around deep underground excavations under tri-axial stress, in particular the zonal disintegration at different scales, were studied through three-dimensional numerical tests. It is found that the failure patterns of deep underground openings are obviously influenced by the triaxial stress. Zonal disintegration is a general failure mode of deep surrounding rock mass under high triaxial stress, where the alternate fracture zones and intact zones are formed by the intersection of the fully developed shear dominated fractures. The circular failure pattern of zonal disintegration is only a very specific failure pattern of zonal disintegration that is more likely to be formed when the horizontal stress in the direction of tunnel axis being the maximum principal stress. Sometimes the circular failure pattern of zonal disintegration is only the miss-judgment based on the limited borehole observations. Numerical results also denote that the heterogeneities of rock masses play a very important role on the scale of zonal disintegration.