Comparative study on mechanical properties of hollow sandstones under hy-dro-mechanical coupling and isolation conditions

The hydraulic characteristics of rock are closely related to mine water inrush disaster.To reveal different water-rock interactions,a comparative study of the mechanical properties of rocks under two different conditions:hydro-mechanical coupling and hydro-mechanical isolation in the process of full stress-strain is conducted.The MTS815 rock mechanics test system is used to perform the conventional triaxial compression tests under different confining pressures and pore pressures.The confining pressure σ3 is divided into three series of 10,20 and 30 MPa.In each series of confining pressure tests,the pore pressure P is set to four levels,which are 20%,40%,60%,and 80% of the confining pressure σ3,respectively.The rock specimen under hydro-mechanical coupling is wrapped with a heat shrinkable tube,a water-proof device is added to the rock specimen under hydro-mechanical isolation.Under triaxial hydrostatic pressure state (σ1=σ2=σ3),the test specimen is axially loaded with a loading rate of 0.005 mm/s,until the test piece is broken.The results show that under hydro-mechanical coupling condition,peak deviatoric stress,initial expansion deviatoric stress,residual deviatoric stress and deformation modulus of specimens are positively related to confinement,however,negatively related to with pore pressure,while the Poisson’s ratio has the opposite relationship,specimens fail along single fracture plane with an angle ranging from 53° to 69°.Under hydro-mechanical isolation condition,the peak deviatoric stress,initial expansion deviatoric stress,and residual deviatoric stress of specimens have positive correlations with the confinement and pore pressure,the fracture angle of specimens does not change obviously.At the same confinement and pore pressure,the peak deviatoric stress,initial expansion deviatoric stress,and deformation modulus of specimens under hydro-mechanical coupling are lower than those under hydro-mechanical isolation condition,while the Poisson’s ratio has an opposite relationship.The differences of rock mechanical properties under two conditions are larger with the increase of pore pressure.The triaxial bearing capacity of the specimen can be significantly improved by applying appropriate stress on the inner wall of hollow sandstone under hydro-mechanical isolation condition.