Through the theoretical analysis and numerical calculation

cracking of surrounding rocks in deep tunnel induced by excavation unloading is studied. The cracking mechanisms and their characteristics of quasi-static and transient unloading under high in-situ stress are discussed respectively. For a case of deep circular tunnel the effect of this transient process on cracking of surrounding rocks is investigated by adopting an initiation model of crack under biaxial compression

and revised stress intensity factor is taken as cracking criterion. The analysis results show that the stress of unloading is a significant cause of surrounding rock cracking; the tensile stress zone exists under transient unloading; and the tensile stress plays an important role in the surrounding rock cracking under high in-situ stress; the less the duration of excavation unloading is

the larger the tensile zone and cracking range will be. The cracking depth and range increase with increasing lateral pressure coefficient; and the cracking region is approximately in V-shape. This study may be of great significance for further understanding the failure mechanism of surrounding rocks.