Study On Long-term Stability Of Mine Slope Based On Quantification Of Creep Damage Of Mud Shale Interlayer

There are a large number of Permian limestone strata in southwest China,such as Yunnan and Sichuan provinces,which are important building materials for a large number of infrastructure construction.However,mud shale with weak strength and obvious rheological characteristics exists in this kind of stratum,which belongs to the"weak link"of mine slope.Under the continuous action of the stress level higher than the creep damage threshold of mud shale,its internal primary joints and cracks expand,and the rock mass strength parameters after creep damage are much lower than the instantaneous failure peak strength.Based on the key project of National Natural Science Foundation of China（U1802243）,this paper takes mud shale as the research object,and investigates its geological and topographic conditions.Through laboratory physical and mechanical tests,the basic physical and mechanical properties of shale,damage degradation mechanism under different stress levels,instantaneous and aging damage law of strength parameters,and the relationship between microscopic damage and morphology were studied.Using FLAC^3D numerical simulation software and combining strength reduction method with rheological calculation,the slope stability analysis method considering the timeliness of rock mass strength parameters is studied.The main research achievements of this paper are as follows:（1）The creep characteristics of shale are obvious when it is lower than the damage stress threshold,and the decay creep is completed within 24h of loading.Viscoplastic failure occurs when the damage stress is higher than the damage stress threshold,and the time of accelerated creep stage is very short,but the strain produced is very large.When the shear stress is 0.39MPa,the accelerated creep duration only accounts for 1.1%of the total time,but the creep deformation accounts for 22.72%of the total deformation.The creep failure strength is 57.69%of the instantaneous peak strength,but the creep deformation is much larger than that of the instantaneous failure.The initial bonding force and internal friction angle are 0.0167MPa and 24.2°.（2）Compared with the instantaneous shear failure,the internal damage of rock during creep failure is more serious.Creep damage is the result of continuous adjustment of micro-particles in rock mass,coordinated deformation to form a new equilibrium,and then rupture and rebalance,cyclic evolution and damage accumulation.Under the continuous stress above the damage threshold,the rock will produce transient and aging damage.The instantaneous damage of shear rock is quantified by the decreasing degree of instantaneous shear elastic modulus with increasing stress.Based on the strength tests of mud shale at four normal stress levels of 8 groups of samples with different creep damage degrees,the quantitative law of aging damage of strength parameters of mud shale is obtained（3）Based on the quantitative creep damage law of strength and mechanical parameters of mud shale higher than the damage stress threshold,a viscoelastic-plastic shear creep model of rock,D-t-NVPB,was established with stress and time thresholds,and the solution method of each parameter of the model was described.The rationality and applicability of the model are proved theoretically through creep characteristics and parameter sensitivity analysis.Combined with the results of laboratory shear creep test of shale,the feasibility of the model is proved by practice,and the secondary development and application are carried out.（4）The creep damage law of strength mechanical parameters of mud shale is introduced into the traditional element,which reflects the creep mechanism of viscoplastic failure of rock due to strength deterioration under continuous stress.The modified Kelvin body improves the fitting accuracy in the decay creep stage.Introducing time threshold conditions t_s|ε=ε_s,has been clear about the constant creep rate greater than zero is the difference in stress threshold by the injury（τ₀-τ_s）caused by physical meaning,and using（τ₀-τ_s）as a function of t quantitative characterization of its influence degree.The piecewise-whole fitting method is more likely to be successful.（5）In FLAC^3D,based on the theory of aging using fish language to compile strength parameters for the decreasing function of time with the rheological time step calculation,combining the rheological analysis and strength subtraction,puts forward a quantitative consider timing parameters calculation method of the long-term stability of slope,and the reduction first,then the rheological,rheological and then reduction calculation,this paper compares and analyzes.The study shows that considering the creep damage characteristics of rock mass strength in stability analysis is more consistent with the failure mechanism of slope.（6）Taking the 640-910m creep of Mohuangtang new landslide west of Huangshan limestone mine as the research object,a 2-2’numerical model of typical engineering geological profile was constructed.The D-t-NVPB model parameters of shale were retrieved by using uniform design-GA-BP neural network model using displacement back analysis method.According to the actual mining order of slope,starting from the original topographic line,a simulation scheme of 8 steps and 9 steps of excavation are designed,and the long-term stability prediction analysis is carried out.In the next three years,the horizontal displacement at the foot of the slope is the largest,which is 6.118cm.The shear strain rate mainly develops along the shale interlayer,with the maximum value of 3.5416E-03 mainly concentrated near the slope foot and extending towards the deep and outcropping at the back edge of the shale interlayer.The creep rate gradually approaches zero,indicating that the rock mass has not entered the damage state and the slope is temporarily in a stable state.In order to prevent the damage and weakening of interbedded rock mass from affecting the overall stability of the slope,it is recommended to conduct concrete grouting anti-seepage treatment at the outcropping area of mud and shale behind edge,and adopt anchor bolt,anchor cable or anti-slide pile at the foot of slope.