Experimental Study of Mechanical Behavior of Interlayer Staggered Zone under Cyclic Loading and Unloading Condition

The unloading and reloading of geostress during the excavation of large underground caverns tends to bring about large deformation and failure problems of rock masses with interlayer staggered zones (ISZs), posing significant threats to the safety and stability of the underground cavern. Hence, for interlayer staggered zones with complex mechanical response behaviors, a series of conventional triaxial cyclic loading tests under different confining pressures were carried out. It can be seen from these results that the envelop curves of stress–strain relations were of obvious memory, but the peak deviator stress, internal friction angle, and cohesion were lower than those in monotonic loading. The phenomenon of volume expansion occurred in the whole process of cyclic loading and unloading, and the unloading dilatation seemed to be more sensitive to relatively low confining pressure. The volume expansion reached its highest value when the unloading stress level (i.e., the ratio of the stress ratio in the process of the cyclic test to the stress ratio when the ISZ comes to the failure point) increased to more or less 0.6∼0.8. Moreover, the changing trend of the average rebound modulus was humplike with the increase of unloading stress level, and the ratio of average rebound modulus to monotonic initial elastic modulus declined as a power function. Additionally, the validity of the unloading rebound modulus calculation by the Duncan-Chang model is proved. The analysis of energy consumption in plastic hysteresis loops indicates that the plastic hysteresis energy of the specimen in each cycle rose with the stress level’s increase. Further microscopic failure mechanism analysis shows that the previous research results are synthetically affected by factors including particle motion and breakage, initiation and propagation of original and new fractures, yield, and damage of interlayer staggered zones. This research provides a theoretical premise for exploring the cyclic unloading and reloading stress path influence on the failure of surrounding rock with ISZs, as well as for more reasonable mechanical parameter selection in deep underground excavations.