深埋大理岩三轴压缩渐进破坏裂纹扩展特征

Crack Propagation Characteristics of Deep Marble Under Triaxial Compression

为深入研究深埋大理岩渐进变形破坏过程中的裂纹扩展特征,基于常规三轴室内试验完成数值模拟参数标定,利用PFC3D颗粒流模型对深埋大理岩开展25、50、80 MPa 3种不同围压下的裂纹扩展数值模拟试验,根据大理岩加载过程中微裂纹演化状态参数定义3种特征应力,并据此开展深埋大理岩渐进破坏过程中的宏、细观破坏特征及其对应裂纹扩展特征的规律研究。结果表明:1)室内试验与数值模拟的应力-应变曲线吻合较好,峰值应力相差较小,破坏形式与室内试验一致,故数值模拟参数标定合理。2)依据总裂纹、张拉裂纹和剪切裂纹扩展数量演化曲线斜率变化规律,将深埋大理岩渐进破坏过程划分为弹性压缩阶段、裂纹稳定扩展阶段、裂纹加速扩展阶段和峰后残余阶段4个阶段,并根据裂纹数量定义3个特征应力点。3)随着围压的增加,深埋大理岩达到起裂应力σci时,裂纹从两端开始萌发;加载至损伤应力σcd点时,裂纹向中间扩展;达到峰值应力σc点时,在宏观破坏面附近扩展、增生;加载至峰值点后70%峰值应力点时,最终形成以剪切破坏为主的贯通宏观破坏面。4)随着围压的增加,裂纹出现的范围更广,贯通性减弱,峰值应力σc点处产生的裂纹对宏观破坏产生的影响更为剧烈,峰后残余阶段张拉裂纹发育更明显。5)加载前期,围压的增长对裂纹数量增长促进作用更强;加载后期促进作用减弱;最终裂纹扩展形成的实际宏观破坏角度大约为210°。

Objective To conduct in-depth research on crack propagation characteristics during the progressive deformation and failure process of deeply bur-ied marble,numerical simulation parameters were calibrated based on conventional triaxial indoor tests.The aim is to fully replicate the macro-scopic mechanical properties and crack failure characteristics of deeply buried marble during deformation and failure processes.Methods Utilizing the PFC3D particle flow model,numerical simulation experiments were conducted on crack propagation in deeply buried marble under three different confining pressures:25 MPa,50 MPa,and 80 MPa.Three characteristic stresses were defined from the perspective of progressive failure of deeply buried marble,based on the evolution state parameters of microcracks during the marble loading process.Sub-sequently,the macro and micro failure characteristics of progressive failure of deeply buried marble and their corresponding crack propagation characteristics were investigated.Results and Discussions 1)Optimal microscopic parameters for indoor and numerical simulation tests under three different confining pressures are presented.Stress-strain curves obtained from this set of mesoscopic parameters under various confining pressures were compared with those derived from indoor tests.Results indicate a close agreement between stress-strain curves obtained from indoor tests and numerical simulations,with negligible differences in peak stress—all relative errors falling within 10%.Thus,the calibration of numerical simulation parameters is deemed reasonable.The failure modes of rock samples from indoor tests,numerical simulations,and acoustic emission under three different con-fining pressures are illustrated.Failure modes derived from numerical simulations closely mirror those observed in indoor test samples,along with the spatial-temporal distribution of acoustic emission points,predominantly indicating shear failure supplemented by tensile failure,forming a continuous shear failure pattern.Hence,it is concluded that the set of microscopic parameters of the parallel bonding model in PFC aptly de-scribes the mechanical properties and failure characteristics of marble samples,demonstrating PFC’s efficacy in simulating hard rocks.2)By ana-lyzing the variation law of the slope of the evolution curve of total crack,tension crack,and shear crack propagation quantity,the progressive fail-ure process of deeply buried marble is categorized into four stages:elastic compression stage,crack stable propagation stage,crack accelerated propagation stage,and post-peak residual stage.Three distinct characteristic stress points are defined based on the number of cracks at the initi-ation stress pointσci,damage stressσcd,and peak stressσc.3)Loading the specimen to the initiation stressσci point initiates crack formation,with resulting crack points located at both ends of the sample.Further loading to the damage stressσcd prompts sporadic scattering of shear cracks throughout the entire specimen.Upon reaching the peak strength peak stressσc,cracks propagate and proliferate near the macroscopic failure sur-face,with tensile cracks also emerging.Upon loading the sample to 70%of the peak stress after the peak,both shear and tensile cracks undergo rapid growth,ultimately leading to sample failure.Shear and tensile cracks are distributed throughout the entire sample,resulting in a relatively consistent macroscopic failure surface dominated by shear failure.The distribution range and quantity of shear failure are more extensive,playing a predominant role in marble failure.Conversely,the distribution range and quantity of tensile failure are relatively small,serving as an auxiliary factor in marble failure.In conventional triaxial compression tests,the combined action of axial pressure and confining pressure generates shear stress parallel to microcracks and in the opposite direction within the rock.As loading progresses,propagating shear cracks and gradual relative slip in the rock can easily induce shear failure.With increasing confining pressure,cracks emerge more rapidly during the elastic compression stage,and the number of cracks during failure also increases.As confining pressure rises,crack width widens,continuity weakens,and cracks generated at peak stressσc points exert a more severe impact on macroscopic failure.Tensile crack development becomes more pronounced in the residual stage post-peak.4)The distribution diagram illustrating the number of cracks at different stress levels within deeply buried marble under varying confining pressures is presented,highlighting shear failure as the primary mode.Microcrack generation at peak stress points under high confining pressure predominantly influences macroscopic failure.The proportion of tensile cracks notably increases in the residual stage post-peak,with shear and tensile cracks developing concurrently.Increasing confining pressure promotes tensile crack development,with a stronger ef-fect on crack growth numbers during early loading stages.Conversely,the impact of confining pressure on crack growth numbers diminishes after reaching the peak,as illustrated.The actual macroscopic failure angle formed by crack propagation measures approximately 210°,corresponding to 60°in elevation view.