| The occurrence of karst geological disasters is closely related to runoff.The impact of runoff will produce a large number of karst crevices(pipes)in the limestone.The formation of these karst-fissures will greatly reduce the stability of karst areas.The surface and groundwater system are well developed in the Three Gorges Reservoir area because of the Yangtze River,thus forming a complex network of karst-fissures,as a result karst geological disasters are more likely to occur.Therefore,in order to deeply understand the mechanism of limestone karst geological disasters,it is necessary to further study the damage and weakening mechanism of karst-fissure limestone.This article relies on the National Natural Science Foundation of China's“Research on the Macroscopic and Mesoscopic Damage and Degradation Mechanisms and Constitutive Models of Jointed Rock Masses under Dry-Wet Cycles(51978106)”and the results of on-site hydrogeological investigations divides the limestone crevices into different shapes which are"fissure","elliptical hole","mushroom-like hole",and"dumbbell-like hole".After that,laboratory tests and theoretical analysis are used on the karst-fissure limestone,combined with nonlinear dynamics,damage mechanics,and fracture mechanics,etc.,to study the macro-and micro-crack evolution mechanism,mechanical characteristics,acoustic characteristics,optical characteristics,energy characteristics,nonlinear characteristics,nonlinear macro-micro-coupling damage variables and damage constitutives of dissolved limestone under uniaxial compression model.The main content and conclusions of the paper are as follows:(1)The mechanical properties of karst limestone are obtained through uniaxial compression tests.The elastic modulus and peak strength gradually decrease with the increase of the volume of the karst-fissure,and the specific performance is:intact limestone>"fissure"limestone>"elliptical hole"limestone>"mushroom-like hole"limestone>"like"Dumbbell hole"limestone.(2)Digital image correlation method is used to reveal the crack evolution process of karst-fissure limestone.At the initial stage of loading,the maximum principal strain field of the rock sample appears spotty.Before macro cracks appear,stress concentration points will be generated around the cavernous caves and obvious form strain zones appear;the properties of intial crack propagation of karst-fissure limestone caves are all basically tension;the shape of pores in the karst-fissure limestone under uniaxial compression will change,and the specific manifestations are:the length of the"fissure"is elongated and the"fissure"rotates clockwise;the long axis and the short axis of the"elliptical hole"rotate away from each other and elongation;the long axis and short axis of the"mushroom umbrella"of the"mushroom-like hole"move and stretch away from each other,the vertical axis of the"stalk"is compressed,the horizontal axis expands,and the entire hole rotates slightly counterclockwise;the long axis and short axis of the"umbrella"of"dumbbell-like hole"move and stretch away from each other,the middle vertical axis is compressed,the"bacterial base"is compressed,the"stalk"expands horizontally,and the entire hole rotates slightly clockwise.(3)The acoustic emission data processing method is used to study the meso-evolution characteristics of the karst-fissure limestone in the process of failure.1)According to the intensity characteristics of acoustic emission,the loading process of karst limestone can be divided into four stages:the initial nonlinear stage,the quasi-linear stage,the nonlinear stepped power law stage,and the peak active stage.2)The cluster analysis of micro-cracks based on GMM and RA-AF shows that the failure mode of intact limestone and"elliptic hole"limestone is mainly tensile fracture,and the failure mode of"fissure"limestone is mainly shearing,"mushroom-like hole"limestone is a mixed tension and shear failure that is first pulled and then cut,and"dumbbell-like hole"limestone is a mixed tension and shear failure that is first cut and then pulled.(4)The hole shape of karst-fissure limestone under uniaxial compression has a certain control effect on the failure of the rock sample.It is manifested as:the existence of“fissure”will enhance the shear properties of rock samples,“elliptical hole”will reduce the shear properties,but will significantly increase the tensile properties;“mushroom-like hole”will divide the failure of the rock sample into two parts,that are first controlled by the lower"stalk"and then by the upper"umbrella";although the"dumbbell-like hole"limestone has more stress concentration points,it will greatly increase due to the lower"bacterial base"inhibitory effect and enhances the life of the rock sample,at the same time,the entire destruction process of the rock sample is first controlled by the upper"mycelium"and then by the entire hole.(5)Based on energy dissipation theory and nonlinear theory this paper studys the law of macroscopic energy evolution.The energy density curves of karst-fissure limestone and intact limestone can be divided into five stages according to the energy damage theory:1)the initial closing stage of microcracks or micropores;2)the stable closing stage of microcracks or micropores;3)microcracks or micropores completely closed stage;4)local crack formation stage;5)Macroscopic main fracture formation stage.According to the nonlinear characteristics of the average growth rate of elastic strain energy density and the characteristics of stress changes,the failure process of intact limestone and karst-fissure limestone under uniaxial compression can be divided into five stages,namely:1)nonlinear oscillation stage;2)nonlinear stable weak stage;3)the nonlinear elastic stable stage;4)the period-doubling bifurcation stage before the arrival of chaos;5)the chaotic stage.It is further divided into 7 stages(including sub-stages)according to the characteristics of acoustic emission.The corresponding boundary stresses are the initial equilibrium stress,the intial crack closing stress,the new crack initiation stress,the periodic crack propagation stress,the chaotic initiation stress,and the crack damage stress and peak stress.The energy storage limit of the rock sample gradually decreases with the increase of the defect volume in the rock sample.Both karst-fissure limestone and intact limestone have strong impact characteristics under uniaxial compression:"mushroom-like holes"have strong impact,which illustrates that the formation of"stalk"will greatly enhance the stress concentration effect in the rock sample.The formation of"dumbbell-like hole"and"bacterial base"can inhibit the impact characteristics,which makes the impact characteristics of rock samples fall sharply..(6)By defining the average crack growth strength function F(?_i)to reflect the growth characteristics of microcracks in the rock sample,according to the chaotic feature:the max Lyapunnov exponent and Kolmogorov information entropy,the internal instability degree of the microcracks in the rock sample during the growth of the microcracks can be obtained.The results show that:1)"fissure"limestone>"elliptical hole"limestone>"mushroom-like hole"limestone>"dumbbell-like hole"limestone;2)"elliptical hole"limestone has the least obvious chaotic characteristics,"fissure"and“dumbbell-like hole”limestone have the largest information entropy and the highest degree of chaos.(7)Based on damage mechanics and nonlinear dynamics,the meso-damage variables of the karst-fissure limestone compaction stage are derived,and the meso-damage variables of the karst-fissure limestone damage section are derived based on the acoustic emission theory,micro-element strength theory and nonlinear dynamics.Damage mechanics and fracture mechanics are used to derive the macroscopic damage variables of karst-fissure limestone.The Lemaitre strain equivalence hypothesis is used to couple the meso-damage variables and the macro-damage variables,and the nonlinear macro-and meso-coupling damage variables and damage constitutive model of fractured limestone under uniaxial compression are obtained.The experimental data is used to fit the unknown parameters in the formula.The verification shows that the damage constitutive model in this paper can better reflect the stress and strain changes in the rock mass compression process. |
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