EXPERIMENTAL STUDY OF SANDSTONE MECHANICAL PROPERTIES BY UNLOADING TRIAXIAL TESTS

Based on the results of triaxial unloading tests,the stress-strain characteristics and failure characteristics of sandstone have been discussed.The results show that the brittle characteristic of sandstone failure under unloading conditions is obvious.Compared to loading failure,unloading failure is more sudden and severe with a higher degree of rock fragmentation.Axial deformation increases continuously with decreasing confining pressure during unloading;and it increases slowly at the beginning of unloading.When the unloading reaches a certain value,deformation suddenly increases;a little change in confining pressure can cause large unloading deformation.Considering unloading degree as an important parameter,an elastic-brittle-plastic mechanical model has been established according to the characteristics of rock mass stress-strain curves under unloading conditions,which assuming that rock mass conform to Griffith criterion in unloading damage segment and Hoek-Brown criterion in unloading failure segment,unloading yield function varies linearly between Griffith and Hoek-Brown criteria.