Research On Mechanical Properties Of Marble And The Effects On Rock Burst Under Thermal-mechanical Coupling

Rock mechanics problems of deep buried tunnels in high temperature environment have become one of the research hotspots.High temperature,high ground stress and other extreme geological conditions bring frequent occurrence of rock burst in deep buried tunnel,the fundamental reason lies in the rock mechanical properties and deformation characteristics of tunnel surrounding rock under special coupling conditions.In order to study the mechanical properties of hard rock and rock burst effects under thermal mechanical coupling,a series of laboratory tests were carried out under the temperature with 20?~120? in this paper such as uniaxial compression test,tensile test,conventional three axis test,rockburst proneness test,X ray diffraction,analysis of slices from rock and so on,the basic mechanical properties of marble under test are obtained,and the failure mechanism of marble is analyzed from two aspects of macro and micro.On the basis of the experiment,the variation of rock burst tendency with temperature is obtained,combined with acoustic emission pattern,the possible rockburst effect is discussed.The main achievements are as follows:(1)It was found in the uniaxial compression test of marble under thermal stress that higher the temperature is,stronger the brittle characteristics of rock is.The uniaxial compressive strength decreases with the increase of temperature.It is considered that 40? is the intensity threshold temperature of Jinping marble,and the deterioration effect of temperature on the mechanical properties is obvious after the temperature of 40?.The average peak strain decreases first and then increases,and the elastic modulus and Poisson's ratio show the opposite law while elastic modulus changed rarely.(2)With the increase of temperature,the failure mode of marble after uniaxial compression tests gradually changed from tensile shear failure to tensile failure,which showed a certain degree of brittle characteristics.In addition to 90?,the overall level of the acoustic emission ringing rate increases with the increase of temperature,and reaches the maximum at the temperature of 120?.The trend of energy is the same as counting rate.At 20?,the cumulative count and cumulative energy are on the rise,but the damage accumulation has a certain stage at other temperatures.The cumulative parameters are suddenly increased when approaching failure stage,showing energy accumulation and sudden release process.(3)Conventional triaxial test showed that with the increase of confining pressure,deviatoric stress and principal strain curve was steep,yield stress,peak strength and residual strength of rock increases,the length of yield platform become longer,while peak residual decreases,reflecting transition characteristics of marble from brittle to ductile.The stress is still falling sharply,which is in the semi brittle stage.The strength and deformation parameters of marble increase with the increase of confining pressure.With the increase of temperature,the compressive strength decreases,the peak strain and elastic modulus fluctuate little,while the Poisson's ratio increases first and then decreases.Compared with the normal temperature,the deformation ability of the high temperature state is enhanced.(4)With the increase of temperature,the failure mode of the three axis of marble changed from shear failure to tensile failure(5MPa).When the confining pressure is greater,the failure mode is mainly shear,the higher the temperature,the more severe the damage,and the fracture angle reaches the maximum at 120?.The effect of temperature on the failure mode of marble is only when the confining pressure is low,when it is greater than 5MPa,the confining pressure effect is greater than temperature effect.The failure modes are mainly shear failure,and the temperature effect is reflected by the fracture angle.The higher the temperature is,the more the fracture angle is.(5)With the increase of temperature,the overall level of the acoustic emission ringing counts,energy and cumulative parameters of three axis test increase.The maximum values of the counting rate and energy of each confining pressure are all at 90? or 120?.The results shows that higher the temperature,the more prone to sudden severe damage.The "quiet period" of cumulative parameter curve appeared only at low temperature and low confining pressure,when the confining pressure is higher(25MPa,35MPa),there is no longer a "quiet period" even at room temperature,the effect of confining pressure is greater than temperature effect.(6)The main mineral composition of Jinping marble is calcite and dolomite which the calcite accounted for a larger proportion.Particle size ranged from 0.04-0.31 mm and it is better crystallinity,the diamond cleavage is obvious.The samples also contains a small amount of feldspar with sericitization and dolomite.The degree of crystallization of marble is good,which leads to the higher strength of the rock at room temperature,and the thermal motion of mineral particles is aggravated by high temperature,which leads to the decrease of friction coefficient.Therefore,the rock strength decreases with the increase of temperature.The reason that rocks show tensile failure at high temperature is width of grain boundary corrosion crack increases with the increase of temperature,and due to the random corrosion path,the microcrack may expand in many places,so the tensile failure appeared.(7)The marble rockburst proneness index(Wet)showed an increasing trend with the increase of temperature,at 20?~60?,it showed a moderate rock burst tendency,and it showed strong rock burst tendency at 90? and 120?.The change trend of brittleness coefficient R is used as the auxiliary index of Wet index,and it is concluded that R has a good correlation with Wet.It is found that the tendency of rock burst is slight and medium before 60?,and with the temperature rising,the tendency increases.(8)Acoustic emission characteristics of uniaxial experimental results are divided into three modes: swarm shocks(20?),pre-main shocks(60?),and main shock(120 ?).Unlike the other two,the failure process of the rock mass corresponding to the acoustic emission of the main shock does not have any precursory characteristics.The destruction is sudden,which indicates that the elastic strain energy is suddenly release after constant accumulating in the early stage.Compared with counting rate,hysteresis effect is shown on the energy characteristics of the three modes of acoustic emission which revealed that the fracture of rock is the characteristic of the evolution of small energy events to high energy events.(9)Based on the evolution law of Wet with temperature,the three types of acoustic emission patterns and their failure characteristics are analyzed.It is concluded that the rock burst tendency increases with the increase of temperature,while the acoustic emission and rock failure characteristics reflect that the damage is more sudden and more severe.There is a strong correlation between the three.The two kinds of rock burst effect models which may be produced in the field are summarized: spalling rock burst effect(below 60?)and the effect of ejection rock burst(above 60?).