True triaxial test study on mechanical properties of deep rock mass in different stress paths

To study the deformation and failure characteristics of deep rock under the influence of excavation disturbance during the construction of deep underground engineering,the true triaxial compression test,constant axial pressure single side unloading test,augmented axial pressure single side unloading test and reduced axial pressure single side unloading test were conducted using the true-triaxial testing system for characteristic of rock disturbance and unloading. The deformation and failure characteristics of deep rock under different stress paths and the evolution law of mechanical parameters are analyzed by load test. The results show that:① Under triaxial compression,rock deformation is characterized by axial compression and dilatation along the direction of minimum principal stress σ3. The failure mode is shear failure. The failure reason is that the external load increases continuously and exceeds the bearing capacity of rock. Under unloading conditions,the deformation shows a strong expansion along the unloading direction. The failure reason is that the load on unloading surface decreases continuously,so that the bearing capacity of rock decreases continuously,and ultimately less than the external load. The failure mode is tension-shear composite failure. ②Under different unloading paths,the change of maximum principal stress σ1 has a significant impact on rock deformation and failure. Under the augmented axial compression path,the stress of unloading and unloading surface increases σ1 at the same time,which intensifies the release of rock strain energy and causes more severe damage. Under the unloading path,the decrease of σ1 reduces the influence of stress change on rock deformation and failure. The process of rock failure and deformation is relatively smooth. The main reason for rock failure is that unloading surface stress reduces rock bearing capacity faster than external load reduction rate. ③ Under triaxial compression,with the increase of σ1 ,rock deformation modulus E decreases,Poisson’ s ratio μ increases,and the change rate decreases with the increase of σ3. Under unloading conditions,E decreases with unloading,while μ increases continuously,and the deterioration rate of deformation parameters increases rapidly near failure. ④ Under unloading conditions,rock bearing capacity decreases with the decrease of stress on unloading surface. While unloading under augmented axial compression path,increasing σ1 promotes the initiation and expansion of cracks in rock,and the decrease of rock bearing strength is the largest. Under unloading axial compression path,the decrease of σ1 inhibits the expansion of cracks to a certain extent. The decrease of bearing strength of stone is relatively small.