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In order to study the failure of surrounding rock under high in situ stress in deep underground engineering projects,disturbed by excavation unloading,we carried out triaxial unloading experiments using thickwalled cylinder specimens on a TATW-2000 rock servo-controlled triaxial testing machine in a laboratory.The specimens were made of limestone material,taken from Tongshan county.Xuzhou city,Jiangsu province,China.In our experiments,rock deformation and failure behavior was studied through loading and unloading of inner hole pressure of thick-walled cylinder specimens.At first,the axial stress,confining pressure and inner pressure were increased simultaneously to a specified designed state of stress.Then,keeping the axial stress and confining pressure stable,the pressure on the inner hole was decreased until the specimen was fractured.When the inner pressure was released completely but the specimen did not fracture,the confining pressure was decreased subsequently until complete failure occurred.Our experimental results suggest that traces of major circular ringlike fractures with a number of radial cracks often appear in thick cylinder walls.This type of ringlike failure phenomenon,similar to intermittent zonal fracturing characteristics of deep exploitation,has,so far,not been published.Our experimental results show that rock deformation and failure behavior of thick-walled limestone cylinders vary under different stress paths between loading and unloading.Tensile failure and orderiy failure surfaces occur under unloading conditions while irregular damaged rock blocks are produced during loading failure.This type of triaxial unloading experiment provides for new research methodology and approach for thorough investigations on intermittent zonal fracturing in deeo underground excavations.