Experimental study of influence of excavation unloading on mechanical properties of sandstone

The phenomenon of unloading occurs widely in human project activities and the geological process. The research of rock mass mechanical properties under unloading conditions has been one of the hot topics of engineering all the time. Based on the results of three-dimensional unloading tests, the stress-strain characteristics, the destructions and the law of mechanical parameters variation of sandstone have been discussed and compared with the law of loading tests. It is shown that: the deformation modulus of rock mass can be reduced by 5%-42% at continued unloading condition; the higher the initial confining pressure, the greater the deformation modulus reduced; the relationship between them could be fitted by the exponential function; the peak strain increased linearly with the confining pressure when the rock mass damaged under unloading condition, and the residual strain had no significant relationship with the confining pressure; compared with loading destructions, cohesion c of rock mass damaged under unloading condition reduced by 4%, and internal friction angle ? increased by 12%; the speed ratio of unloading has great influence on the strength and deformation parameters of sandstone; the damage characteristics of rock samples indicate that strong expansion along the direction of unloading and circumferential cracks are presented in process of unloading; different from compression and shear failure under loading conditions, unloading damage is characterized by strong tensile and shear fracture; the higher the initial confining pressure, the stronger the degree of unloading and the higher the degree of fragmentation.