Experimental And Theoretical Study On Macroscopic And Mesoscopic Failure Mechanism Of Rock Mass Under Loading And Unloading Conditions

With the deyelopment of water conservancy and hydroelectric engineering,resource development and rail traffic,a lot of excavation engineering problems exist,such as steep rock slopes and underground tunnels.The design,construction,management,and stability of excavation engineering directly depend on the rock's strength,deformation,and failure.Excavation of underground engineering is a complex loading and unloading process.And the mechanical characteristics,deformation behaviour and constitutive model are different under loading and unloading conditions.So,it is not correct to evaluate the failure mechanism of rock mass for the underground engineering by method of loading rock mass mechanics.In light of characteristics of underground engineering,the loading and unloading experiments were carried out in this thesis. Deformation and failure mechanism of rock under loading and unloading conditions were studied. The stability analysis method of surrounding rocks and the mechanism of rock burst under unloading condition were discussed.The main work can be summarized as follows:(1) The deformation features,mechanical parameters and failure characteristics at different unloading velocity were obtained based on the loading and unloading experimental results.It showed that the evolution of volumetric strain was determined by axial strain under loading condition.However,it was determined by lateral strain under unloading condition.Elastic modulus decreased and Poisson ratio increased during the unloading stage.The rebound value of axial strain increased,while the lateral strain jump decreased with the initial confining pressure increasing at the same unloading velocity.However,they decreased with unloading velocity increasing under the same initial high confining pressure.The failure modes of rock specimens can be classified into three types:primary shear failure,conjugate shear failure and splitting-shear failure,which are related with confining pressure and unloading velocity.The cohesion of rock decreased but the internal friction angle increased in the process of unloading.Based on the experimental results,a constitutive model was established by using statistical analysis method.The model agrees well with the experimental result.(2) The results of loading and unloading experiments showed the evolution of failure modes, deformation behaviors and mechanical parameters of natural jointed rock specimens.It showed that the failure modes of the specimens can be classified into two types,one is shearing-failure across the jointed plane,the other is sliding-failure along the jointed plane.Shearing-failure across the jointed plane occurred when the included angle is less than 40°,otherwise sliding-failure along the jointed plane occurred under loading condition.Sliding-failure along the jointed plane occurred when the included angle is less than 35°.otherwise the regulars of failure modes are not obvious under unloading condition.Peak strength and residual strength of shearing-failure along the jointed plane are obviously lower than those of shearing-failure across the jointed plane.Axial strains tended to develop accelerate and the strains at peak strength increased obviously in the beginning of unloading stage for the specimens with sliding-failure along the jointed plane.Elastic modulus decreased and Poisson ratio increased during the unloading stage.(3) Based on experimental results,the numerical simulations of failure process for intact specimens and jointed rock specimens under loading and unloading conditions by discrete element procedure PFC2D were performed.And the cracks generation,expansion and piercing process were studied. The failure processes of intact rock specimens under loading and unloading conditions can be divided into four phases,i.e.linear elastic phase,initiation and expansion phase of crack, expansion along the shear zone phase of crack,crack piercing along the shear zone phase.The failure processes of jointed specimens under loading and unloading conditions can be divided into three phases,i.e.linear elastic phase,initiation and expansion phase of cracks along joints and crack piercing along the joints phase.Compared with intact specimens,the length of time from loading to failure is shorter for jointed specimens in the same stress path,and peak strength is lower,peak strain increase.Those results show that jointed specimens is more easily failure than intact specimens. Compared with loading condition,the calculation of numerical experiments for unloading damage is shorter,the peak strength is lower,the peak strain decreases,partial crack shear zone caused by the tension cracks after rock specimens failure exists,which show that the.sudden brittle rock failure can be caused easily by unloading.(4) The concept of safety factor of tunnel is brought forward based on the strength reduction finite element method for elastic-plastic analysis individually.And the safety factor and the plastic zone are studied under the two conditions of considering and not considering the excavation unloading. Based on the constitutive model of unloading rock,elastic-plastic analysis was performed on the tunnel under the equal pressure.The formulas of stress and displacement of the above zones are established.Analysis on the stability of surrounding rocks were done by the strength reduction finite element method under loading and unloading conditions,which can determine the failure surface and the safety factor of tunnel,and evaluate the stability of tunnel.The safety factor not considering excavation unloading is bigger than that of considering excavation unloading.Stability analysis not considering unloading excavation is biased towards unsafe.(5) Rock burst was studied by using triaxial unloading tests from qualitative and quantitative aspects.The equilibrium equation was obtained based on the conservation of energy principle and the whole unloading stress-strain curve.A fold catastrophe model of rock dynamic destabilization is generalized by adopting a general form of material constitutive equation.The equations of deformation jump and energy release of the material are obtained.The stress difference-strength ratio method was put forward,and the rock burst intensity of the same size and different cave shape was calculated without considering the impact of excavation unloading and considering the impact of excavation unloading by the finite element method.Result shows that the grade of rock burst predicted by the stress difference-strength ratio method is lower than by stress-strength ratio method. The rock burst grade is lower one grade for not considering the excavation unloading.The finite element method simulation considering unloading excavation is better to reflect the actual stress distribution in the tunnel surrounding rock,and also reveals depth,width and intensity of rock burst exactly.