Experimental Study On Mechanical Properties Of Unloading Damage Jointed Sandstone Under Cyclic Loading

In the construction of large-scale hydropower and transportation projects,a large number of unloading damaged engineering rock masses are formed after manual excavation,and many projects are built on the damaged engineering rock masses after excavation.Engineering rock mass itself may have different joint dips,and its mechanical properties are essentially different from those of "intact" rock mass.Tunnel excavation and support,repeated drainage of dams,water storage process and seismic load are the dynamic effects of cyclic loading and unloading often encountered by engineering rock mass.Therefore,the common sandstones in surrounding rock tunnels are selected to prefabricate different joint inclination angles,considering different degrees of unloading damage and different types of cyclic loading and unloading,and the main research results are as follows:1)Conventional uniaxial and triaxial compression and triaxial unloading tests are carried out on complete and different jointed sandstones.The differences and relations between jointed and intact rocks in deformation,strength parameters and failure law are analyzed.It is found that confining pressure and joint dip are important factors affecting the peak strength,deformation parameters and failure characteristics of sandstone.The peak strength of jointed rock sample is smaller than that of intact rock sample,and it is easier to be damaged under uniaxial and triaxial compression and unloading stress paths,and the degree of damage is more severe.2)The cyclic loading and unloading tests of intact and 45 degree jointed sandstones show that the hysteretic loops in front of the peak are narrow and long,and the hysteretic loops after the peak are wide and fat.There is a linear relationship between the apparent elastic modulus and the axial deformation.It is concluded that the increase of displacement rate has a linear acceleration effect on the increase of the acceleration of force loading,with a maximum of 28 times before the peak and 22 times after the peak.In engineering practice,the acceleration of force loading is predicted by the easily measured displacement rate.Under the same loading conditions,the hysteretic loop area,damping ratio and damping coefficient of jointed sandstone after peak are larger than those before peak,and the damage degradation degree after peak is higher,and the load-unload damping energy consumption is larger.3)Considering 45 degree jointed sandstone with weak strength in jointed sandstone,the cyclic fatigue failure tests with different upper limit stress ratios are carried out.It is concluded that the compression effect of low amplitude cyclic load on 45 degree jointed sandstone is greater than that of deterioration.The peak strength of rock sample is far greater than that of uniaxial peak strength.The peak strength is the largest when the upper limit stress is 0.75.The increase ratio of peak strength and peak strength to Y increases first and then decreases with the increase of the upper limit stress ratio.4)Based on the cascade cyclic loading and unloading tests of sandstones with different unloading damage joints,the dynamic parameters such as hysteretic circle area,damping ratio,damping coefficient,dynamic elastic modulus and dynamic deformation modulus of jointed sandstones with unloading damage,inclination angle of prefabricated joints and amplitude of cascade cyclic loading and unloading are studied,and the dynamic response and failure law of jointed sandstones with unloading damage are analyzed.