Experiment research about mechanical characteristic of gneissic granite while confining pressure is unloaded

Unloading confining pressure tests are conducted before different pre-peak confining pressure of gneissic granite obtained at the foundation of Xiaowan hydrodynamic station,the results show that the deformation modulus changes little,but poisson's ratio increases greatly compared with the loading conditions under the same confining pressure.Samples with different sizes will show different behaviors under the same unloading confining pressure,and the sample with a larger size will show tensile failure;The peak-stress of the sample will increase linearly with the confining pressure increasing,and size effect is obvious,namely,the larger the size of sample,the less the influence of the confining pressure change.