Fracture evolution characteristics of sandstone containing double fissures and a single circular hole under uniaxial compression

The uniaxial compression experiments on the sandstone samples containing double fissures and a single circular hole were carried out by using electro-hydraulic servo universal testing machine to investigate the effect of rock bridge angle β and fissure angle α on mechanical properties and evolution characteristics of cracks.The results show that the peak strength,peak strain and elastic modulus of defected specimens decrease comparing with those for intact sample,and show a decreased trend firstly and then increase with β changing from 0° to 90°.The peak strength and elastic modulus achieve the minimum value as the rock bridge angle is 60°,while the peak strain reaches the minimum value with the rock bridge angle of 45°.The crack initiation of tested rock samples occurs firstly in stress concentration areas at tips of prefabricated fissures under uniaxial compression,and then propagates constantly and coalescences with the prefabricated hole.Some secondary cracks initiate and propagate as well until buckling failure happens.The rock bridge angle has a great influence on crack initiation,coalescence,final failure mode,crack initiation stress and transfixion stress.The peak strength varies significantly,while the elastic modulus and peak strain change slightly,and the failure modes are also different due to the influence of fissure angle.

Strength failure and crack coalescence behavior of sandstone containing single pre-cut fissure under coupled stress, fluid flow and changing chemical environment

In order to study the strength failure and crack coalescence characteristics of cracked rocks, uniaxial compression experiments were conducted on cylindrical sandstone specimens, sampled from Longyou Grottoes of Zhejiang Province, China, with a single pre-cut crack soaking in different chemical solutions. Based on the results of uniaxial compressive test under different chemical solutions and velocities of flow, the effect of strength and deformation characteristics and main modes of crack coalescence for cracked rocks under chemical corrosion were analyzed. The results show that the pH value and velocity of the chemical solutions both have great influence on the sandstone sample's uniaxial compressive strength and deformation characteristics. Cracked sandstone samples are tension-destructed under uniaxial compression, and the crack propagation directions are consistent with the loading direction. The phenomena of crack initiation, propagation and coalescence of sandstone are well observed. Four different crack types are identified based on the crack propagation mechanism by analyzing the ultimate failure modes of sandstone containing a single pre-cut fissure. The failure process of specimen in air is similar with the specimen under chemical solutions, however, the initial time of crack occuring in specimen under chemical solutions is generally earlier than that in the natural specimen, and the crack propagation and coalescence process of specimen under chemical solutions are longer than those of the natural specimen due to softening of structure of rock caused by hydro-chemical action. Immersion velocity of flow and chemical solutions does not have influence on the ultimate modes of crack coalescence.

Numerical simulation and damage analysis of fissure field evolution law in a single coal seam mining

Based on the different deformation and failure laws of coal and rock materials under tensile or compressive loads,the damage variable was defined to divide three phases from the damage and rupture point of view in order to reveal the fissured field evolution characteristics and spatial distribution of coal and rock in the single coal seam mining with low permeability and rich methane.According to the corresponding damage constitutive equations,the secondary development of the finite element program was completed.The fissures field evolution law of a coal mine with single coal seam mining was calculated and analyzed by this new program and the distribution areas and failure degree of the surrounding coal and rock structures with damage,fissure or rupture are given on the condition of mining.This paper provides a scientific basis for quantitative research and evaluation of the safe simultaneous production of coal and gas.

Experimental and numerical study on loading rate effects of rock-like material specimens containing two unparallel fissures

A series of laboratory experiments and PFC numerical simulations for rock-like material specimens containing two unparallel fissures were carried out.On the basis of experimental and numerical results,the stress-strain curves,mechanical properties,AE events,cracking behavior and energy characteristics were analyzed to reveal the macro-mechanical behavior and meso-mechanism of pre-fissured specimens under different loading rates.Investigated results show that:1)When the loading rate is relatively low,the stress-strain curves show a brittle response.When the loading rate is relatively high,the curve shows a more ductile response.Both of the peak strength and elastic mudulus increase with the increase of loading rate,which can be expressed as power functions.2)Four crack types are identified,i.e.,tensile crack,shear crack,far-field crack and surface spalling.Moreover,the tensile crack,far-field crack and surface spalling are under tensile mechanism,while the shear crack is under shear mechanism.3)The drops of the stress-strain curves all correspond to the crack initiation or coalescence,which is also linked to a sudden increasing in the accumulated micro-crack curve.4)Both of the maximum bond force and energy have the similar trend with the increase of loading rate to peak strength,which indicates that the trend of peak strength can be explained by the meso-mechanics and energy.

Fracture behavior of granite specimen containing a single fissure under uniaxial compression by grain-based model 3D

The understanding of the mechanical behavior of granite containing fissures is of great significance for geothermal energy exploitation and high-level nuclear waste disposal repository.To consider the grain interlocking and rotational resistance behavior in granite specimens,the grain-based model 3D(GBM3D)was constructed to investigate the fracture behavior of granite specimens containing a single fissure.The numerical result indicates that this GBM3D can be used to investigate the mechanical behavior of granite constituted by different minerals,and it can provide more information to understand the mechanical behavior of specimens under loading process.The inter-granular cracks initiate in the first place,but the initiation of intra-granular cracks mainly concentrates after the peak strength.The tensile cracks between different minerals are more than that between the same minerals,and the intra-granular cracks in muscovite initiate firstly,and the number of them is more than that in other minerals.The wing crack is mainly constituted by the inter-granular cracks,and the shape of it is different in different depths,whereas the anti-wing is similar in different depths.

Three-dimensional failure behavior and cracking mechanism of rectangular solid sandstone containing a single fissure under triaxial compression

In actual rock engineering,fissures play an important role in determining the mechanical parameters of rock mass,whereas it is very difficult to construct fissures in cylindrical specimens.Therefore,the pre-fissured rectangular rock specimens were constructed innovatively.Moreover,a series of triaxial compression experimental results on the failure mechanical behavior of rectangular solid sandstone specimens containing a single fissure were reported.The lateral strain in different directions was monitored and the experimental results show that elastic modulus and axial strain increase non-linearly with confining pressure,and the average Poisson’s ratio parallel to fissure(μ2)is larger than that vertical to fissure(μ3).The cohesion,Hoek-Brown parameters of peak strength show similar trends with that of crack damage threshold to the fissure angle(α),and the parameters of the peak strength are larger than those of crack damage threshold.However,the internal friction angles of the peak strength and crack damage threshold are almost equal.Based on the geometries and properties of cracks,ten typical crack types are identified.Cracks vertical to pre-existing fissures occur in specimens under uniaxial compression,whereas cracks parallel to pre-existing fissures occur under triaxial compression.Finally,X-ray micro-computed tomography(CT)observations are conducted to analyze the internal damage mechanism of sandstone specimens with respect to various fissure angles.Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of sandstone specimens.This research elucidates the fundamental nature of rock failure under triaxial compression.

三轴压缩下单裂隙位置对复合岩样力学变形与破坏模式的影响

在地下富存裂隙的层状复合岩体中,复杂围压环境和裂隙分布对复合岩体力学性质和损伤破坏具有显著影响。选取由类灰岩和类砂岩组成的含预制单裂隙的复合岩样作为研究对象,通过常规三轴压缩试验,分析不同裂隙位置和围压条件下岩样的力学变形和破坏模式。结果表明:(1)完整复合岩样的强度受砂岩控制,而变形受灰岩限制。(2)随着围压的增加,复合岩样破坏特征由裂隙位置主导转变为由围压主导;复合岩样的体缩受围压的影响,而体胀受裂隙位置的影响。(3)在单轴压缩条件下,裂隙对岩样力学性质的削弱程度最大;当裂隙位于灰岩中时,损伤应力受围压影响最显著,岩样的强度和弹性模量随着裂隙位置的改变(从灰岩、接触面到砂岩)及围压的增加呈现出增加的趋势。(4)灰岩中的裂隙易产生拉伸裂纹,砂岩中的裂隙易产生剪切裂纹;随着围压的增加,复合岩样由拉伸破坏转变为剪切破坏,破坏模式由围压主导。该研究成果对复合岩体工程的安全加固设计具有一定的指导意义。

三轴压缩下裂隙岩体破坏模式及能量演化研究

为研究不同围压条件下含不同长度单裂隙岩体的裂纹扩展特征和能量演化规律,基于室内三轴压缩试验结果标定细观参数,开展了PFC^(2D)颗粒流数值模拟试验。结果表明:拉伸裂纹先于剪切裂纹产生,两者呈指数增长,裂隙长度减小和围压增大使拉伸裂纹和剪切裂纹快速增长时间滞后;最终破坏时,随裂隙长度增加,拉伸裂纹和剪切裂纹减少。应力集中于裂隙两端,裂纹周围存在应力集中现象。相同围压下,裂隙长度增加,岩样破坏时块体数减少。岩体破坏本质为能量储存、耗散与释放的过程,在加载过程中,岩体能量转化被分为4个阶段。裂隙长度增加削弱岩样储存应变能的能力,总能量减少,围压增强岩样储存应变能的能力。岩样破坏时,耗散能大于应变能,随裂隙增长,耗散能减少。

作用在裂隙中的渗透力分析

裂隙岩体中流体对岩体的作用力 ,是研究岩体稳定性的重要问题。本文认为流体作用于裂隙壁面上的力包括两部分 ,即垂直于裂隙壁面的流体静水压力 (导致裂隙垂向变形 )和平行于裂隙壁面的拖曳力 (导致裂隙切向变形 ) ,此拖曳力为面力。文中以单裂隙水流的立方定律为基础 ,运用流体力学的动量方程 ,推导出了单一平直光滑无充填裂隙、有充填的裂隙及水流和充填物一起运动情况下 ,裂隙壁面承受的切向拖曳力公式。该公式对于分析流体对裂隙岩体变形性能及强度的影响具有重要价值。

单裂隙岩石动态强度破坏准则的数值模拟

运用ANSYS/LS-DYNA动力有限元软件,对不同弹速冲击下单裂隙岩石的SHPB动态破坏进行了数值仿真模拟,通过对模拟数据的回归分析确定强度准则数学表达式中的待定系数,并探讨了加载平均应变率与待定系数的关系。结果表明:岩石力学中常用的几种强度准则,包括Mohr-Coulumb准则、Hoek-Brown准则、Griffith准则等并不适用于单裂隙岩石的冲击破坏,而Fairhurst准则具有较好的适用性;随着加载平均应变率的增大,Fairhurst准则中的2个待定系数均近似线性增长,据此构建了适用于单裂隙岩石的动态Fairhurst强度准则。基于计算机仿真模拟对单裂隙岩石的动态强度准则进行拟合分析的方法,为下一步更便捷、高效地得到岩石材料的强度破坏准则提供了新思路。

单孔示踪方法测定裂隙岩体渗透性研究

论述了单孔中测定裂隙岩体渗透系数、等效水力隙宽及静水头的方法与有关的井流理论 ,单孔稀释定理推广到测定岩体渗透流速、流向等参数方面 .推导了钻孔揭露多裂隙组稳定流混合井流理论 ,分别对吸水、涌水及单裂隙 (组 )的情况进行了讨论 ,并且介绍了试验仪器与方法 .将单孔示踪方法应用于群孔之中 ,结合裂隙岩体的走向、倾角、裂隙分布等参数 ,确定出研究区域的裂隙岩体中的渗流场分布 .该项成果已应用于刘家峡左坝肩裂隙岩体渗流场的研究之中 .

单裂隙面渗流特性及等效水力隙宽

首先讨论了描述单裂隙面几何特性的两个最基本参数———隙宽和粗糙性的确定方法 ,系统地总结和分析了现有有关单裂隙面渗流特性方面的研究成果 ,并对不同粗糙性描述下的裂隙等效水力隙宽的确定问题进行了探讨 ,提出了自已的看法 ,并指出进一步要解决的问题。

动水注浆沉积压力特征与注浆效果试验研究

注浆是解决矿井水害的一种有效方法。为了更好地研究动水注浆过程中浆液沉积压力特征与堵水效果,基于自主设计的流体力学综合试验平台,开展了煤岩体裂隙动水注浆试验研究,分析了不同水灰比和注浆压力条件下浆液在煤岩体裂隙中的沉积压力特征,提出一种裂隙动水注浆效果的评价方法。研究结果表明:浆液的沉积压力曲线可分为3类,当曲线呈平塬型分布时,注浆堵水效果最佳;呈山尖型分布时,堵水效果次之;呈台阶型分布时,堵水效果较差。

三维应力作用下砂岩单裂隙渗流规律试验研究

使用劈裂方法在粗砂岩岩样上制造裂隙来模拟粗砂岩天然岩体的张性裂隙,在电液伺服岩石三轴试验机上开展单裂隙渗流应力耦合试验。在三维应力下,检测在不同围压、水压情况下,渗流量随时间的变化情况,对单裂隙岩体在三维应力作用下的渗流规律作了探究。

单裂隙水流作用下岩体稳定温度场理论模型与有限元数值模拟

建立了单裂隙岩体的几何模型,在不考虑热对流、热辐射的前提下用岩体导热微分方程描述了裂隙岩体稳定温度场分布及定解条件,并进行了理论方面的分析;给出了岩体导热偏微分方程的有限元描述,应用FEPG软件对裂隙岩体二维稳定温度场分布进行了数值模拟。模拟结果表明:由于和裂隙水流发生了热量交换作用,裂隙岩体温度场也随之发生变化;裂隙岩体和裂隙水流温度相差越大,温度等值线越密集,岩体温度场变化越大,热量传递越快。

软弱结构面和裂隙岩体直剪破坏机理研究

本文揭示了软弱结构面和裂隙岩体直剪破坏阶段的宏观规律与微观机理，提出测定此类岩体ｃ，φ峰值的直剪单点法的理论依据和特征点判据，从理论上和试验技术上基本解决了此类岩体ｃ，φ峰值直剪单点法的可行性问题。文中还通过单点法与多点法成果对比分析验证了单点法成果的可靠性。

单轴压缩条件下类大理岩裂纹起裂特性与扩展规律

在岩体工程中,裂隙构型属性对新生裂纹起裂和扩展规律、破坏模式及力学行为都有着显著影响。以预制含单裂隙类大理岩为研究对象,结合室内单轴压缩试验与PFC数值模拟试验,针对不同裂隙长度和倾角岩样的起裂、扩展、破坏及力学特性进行研究。结果表明:(1)岩样破坏模式主要受裂隙倾角控制,破碎程度主要受裂隙长度控制。(2)岩样加载变形过程中的起裂应力、损伤应力和峰值应力随裂隙倾角的增加而增加,随裂隙长度的增加而减小;裂纹起裂角随裂隙倾角的增大而减小。(3)裂纹的起裂特性为,在低裂隙倾角(即α<45°)时,裂纹从离预制裂隙中心有一定偏移处开始起裂;在高裂隙倾角(即α≥45°)时,裂纹从预制裂隙的尖端开始起裂,最终形成“弥散式”分布。(4)裂纹扩展规律主要以剪切裂纹为主,在低裂隙倾角(即α<45°)时,裂纹的扩展有明显的聚集带形成;在高裂隙倾角(即α≥45°)时,裂纹聚集带逐渐弱化,朝着“弥散式”方向扩展。研究成果对合理评价岩体结构和设施的安全稳定性具有重要的理论意义。

裂隙岩体强度试验单点法及其前景

本文在回顾裂隙岩体强度试验的基础上，用大量原位测试数据对裂隙岩体直剪破坏机理和裂隙岩体直剪破坏的基本规律及其成因进行了深入探讨；总结出一套行之有效的试验技术；给出了裂隙岩体直剪破坏时的充分必要条件并加以量化。

单裂隙非稳定流分析

采用流体力学基本方程 ,以单裂隙稳定流理论为基础 ,分析由边界条件发生变化引起的单裂隙非稳定流 ,得出单裂隙非稳定流的计算公式 。

孔、围压作用下断续单裂隙岩石破坏数值模拟

采用岩石破裂过程分析系统,研究了围压、孔压对含不同单裂隙分布脆性岩石的破坏影响规律,分析了岩样的裂纹扩展特征。结果表明:断续单裂隙的岩石力学性能显著低于完整岩样,但降低程度与裂隙参数密切相关,随着裂隙长度增加,岩石力学参数均呈降低趋势,而随着裂隙倾角的增加,峰值强度以及峰值应变均呈先减小后增加的趋势,且这种规律不受围压与孔压的影响;围压对岩石峰值强度有强化作用,但同等围压时,孔压对岩石峰值强度有弱化作用;裂隙长度越小或裂隙倾角越大的岩样,其裂纹扩展特征越不明显,且扩展过程受围压与孔压影响显著。研究结论对于认识具有断续节理裂隙岩体的变形破坏机理具有参考价值。