Research On Mechanical Properties And Acoustic Emission Characteristics Of Deep Marbles

In view of the problems such as different mechanical responses and frequent rock burst disasters in the construction of large-scale underground caverns for hydropower in China,the loading and unloading tests under different confining pressures were carried out for Jinping secondary auxiliary primer #1 construction tunnel 2400 m depth of deep marbles.By means of theoretical analysis,laboratory mechanical test and acoustic emission test technology,the macro and micro variation law of mechanical deformation characteristics and acoustic emission characteristics of deep marble were analyzed.By comparing the deformation and failure differences of deep marbles under different confining pressures,the mechanism of rock deformation and failure during deep underground engineering construction is fully revealed.Based on mechanics and AE theory,the deformation and failure damage evolution model of loading and unloading under different confining pressures of deep rock is established.The damage and fracture laws of deep rocks under different confining pressures are systematically investigated in order to provide theoretical support for engineering problems such as rock burst prevention during efficient excavation of deep underground engineeringThe main research results of this paper are as follows:(1)The mechanical deformation properties of deep marbles under five confining pressures of 0,25,50,80,100 MPa were obtained.It is pointed out that the strength and deformation characteristics of deep marbles have significant confining pressure effect.With the increase of confining pressure under loading and unloading,the peak strength of deep marbles increases gradually,and the rate of increase decreases gradually.Deep marble exhibits brittleplastic transformation under loading condition.When the confining pressure reaches 100 MPa,the strength of deep marble does not decrease significantly when it reaches the peak value,which is close to the ideal plastic state.Under unloading condition,deep marble has obvious brittle failure characteristics.(2)Deformation and failure of deep marbles in different loading and unloading states are dominated by dilatancy deformation,and dilatancy stress increases with the increase of confining pressure.However,under different loading and unloading conditions,the number of failure surfaces of deep marble is different,and the deformation and failure modes are different.Under uniaxial loading,there are multiple failure sections,while under triaxial loading,there are single failure sections,while under unloading,there are multiple failure sections.However,with the increase of confining pressure,the failure surface of each state becomes less and smoother.The failure mode of deep marble in uniaxial state is tension-dominated composite failure mode,in triaxial loading state is shear failure,and in unloading state is tension-shear failure with shear failure and tension-shear failure supplemented by tensile failure.With the increase of confining pressure,the shear failure becomes more significant.In the uniaxial state,the destructible body of deep marble has more debris,and the debris decreases with the increase of confining pressure.Under the same initial confining pressure,the fracture pattern of deep marble under unloading is more serious and the failure surface is more,which indicates that unloading confining pressure intensifies the rock failure.(3)Based on the loading and unloading strength and deformation characteristics of deep marbles,combined with the theoretical analysis method of rock mechanical strength,the applicability of strength criteria under loading and unloading condition of deep marbles is evaluated respectively.By comparing the predicted strength with the measured strength,it is pointed out that the linear Mohr-Coulomb criterion is more suitable for the state of low confining pressure,while the hyperbolic criterion is more suitable for the state of high confining pressure.The evaluation criterion has a high degree of coincidence and a small absolute error,which can accurately reflect the nonlinear characteristics of deep rock changes with confining pressure.(4)AE variation rules of loading and unloading of Jinping 2400 m deep marbles under different confining pressures were obtained.The analysis is carried out on three AE indicators including AE ring count,temporal and spatial distribution and RA value.It is found that the AE signal characteristics can well match the stage characteristics of the whole process of deformation and failure of deep marbles.The AE of deep marbles is uniformly distributed in the whole loading process under uniaxial loading.Under triaxial loading,with the increase of confining pressure,the occurrence of AE frequent period of deep marbles is later,and the cumulative ringing count is less,indicating that the larger the confining pressure is,the stronger the ability to suppress deformation is.Under triaxial loading,the pre-peak AE is in the quiescent period,and the quiescent period becomes longer with the increase of confining pressure.Compared with triaxial loading,the AE phenomenon of deep marbles is less under unloading,and the AE activity is more active after the peak under triaxial loading,while it is concentrated at the peak and the number is less under unloading test,which reflects the characteristics of sudden failure of deep marbles under unloading condition.(5)Based on the spatiotemporal distribution of AE and RA value,the evolution model of deep marbles crack was analyzed.The space-time distribution of AE shows that the mesofailure pattern of deep marbles is consistent with the macroscopic failure pattern.The AE RA results show that deep marbles are dominated by tensile and shear composite cracks under uniaxial condition.Tensile and shear composite cracks under triaxial loading are mainly shear cracks.Under the confining pressure of constant axial compression unloading,the crack of deep marbles develops into a tension-shear composite crack dominated by shear crack,and the crack failure is mainly dominated by shear crack at the peak.(6)According to the deformation and failure characteristics of deep marbles under loading and unloading,combined with the change rules of mechanics and acoustic emission,the stage characteristics of failure precursor information in the process of deep marbles deformation and failure were analyzed.In the third chapter,the plastic strain ratio is introduced to determine the precursors of crack damage and instability.The results show that with the increase of confining pressure,the crack development is more inhibited,and the precursors of crack damage and instability are more delayed.The initial point of dilatancy was determined according to the stress-strain curve,and it was pointed out that the dilatancy stress increased with the increase of confining pressure.In Chapter 5,the precursor points of peak failure are comprehensively determined according to the AE ringing count and the variation law of RA value.The stress value of the precursor points of peak failure increases with the increase of confining pressure.The precursory stress of crack damage and instability < dilatancy stress <premonitory stress of AE failure.(7)Based on the theory of rock damage mechanics,a damage evolution model was established based on plastic strain and AE cumulative ringing count respectively.The damage and fracture process of deep marbles under different loading and unloading conditions was systematically analyzed.Under the loading condition,the damage model based on plastic strain can better reflect the damage accumulation process of deep marbles.In the unloading state,the two damage models can reflect the sudden damage regularity of deep marbles.With the increase of confining pressure in the loading state,the damage quiescent period of deep marbles becomes longer,which indicates that the confining pressure has a stronger inhibition effect on deformation.When the confining pressure is larger,the increment of pre-peak damage is larger and the fracture is more sufficient.In the unloading state,with the increase of initial confining pressure,the more fully developed the damage in the unloading stage is,the smaller the damage increment in the post-peak stage is,and the more significant the sudden damage is.