Breakage and shear behaviour of intermittent rock joints

The breakage and shear behaviour of intermittent rock joints have been investigated in a series of direct shear tests with a new shear device, specifically designed for this purpose. The tests have been performed on specimens of rock-like material or hard rock, respectively, incorporating idealized non-persistent joints, made up of a number of short cracks in an en-échelon arrangement along the central shear axis.The shear behaviour of such a joint constellation has been found to be composed of different phases. The first phase of shearing is that of the actual rupture, initiated by the formation of wing cracks, starting from the existing cracks and growing into the material bridges, and concluded by the generation of additional new fractures connecting the initial cracks in the zone between the wing cracks. The second phase of shearing is characterized by friction processes and volume increase in the then continuous shear zone. Finally, the third phase of shearing, reached after large shear displacements, is determined by sliding processes inside the strongly fractured shear zone.In a large number of shear tests the geometrical parameters of the discontinuous joints as well as the loading conditions have been found to influence the activated shear resistance in each phase of shearing to a noticeably different extent. The orientation of the initial cracks and the normal stress, however, have been identified as the most influential parameters. Depending on the test conditions, an initially discontinuous rock joint can activate the largest shear resistance not just before rupture but in one of the two subsequent phases of shearing as well.The mechanisms which govern the different shear phases could be identified as (1) tensile rupturing, (2) rolling and sliding friction of dilatant joint zones and (3) sliding within the joint filling composed of brecciated material.     

预制裂纹岩石压剪试验的数值模拟分析

采用岩石破裂过程分析软件RFPA^2D对预制裂隙大理岩试样的压剪试验进行数值模拟。数值分析表明，次生裂纹基本沿裂纹尖端萌生和扩展，所形成翼裂一股逐步趋向与荷载方向一致，这和试验结果是相吻合的。尽管某些数值分析结果和试验结果并不完全符合，但数值分析的模型和参数基本上有效地模拟了真实岩体介质的不均质性等力学特性。同时，采用该软件分析了加工小孔对裂尖应力状况的影响，加工小孔的存在对试验结果有一定影响，但并没有实质性影响。其影响主要在两方面：一是使翼裂起裂角变大，扩展路径变曲折；二是使材料更易于在裂尖引发二次(剪切)裂纹。最后，对锯齿形裂纹模型进行了初步的数值模拟。     

Effect of roughness on the shear behavior of rock joints subjected to impact loading

The shear behavior is regarded as the dominant property of rock joints and is dramatically affected by the joint surface roughness. To date, the effect of surface roughness on the shear behavior of rock joints under static or cyclic loading conditions has been extensively studied, but such effect under impact loading conditions keeps unclear. To address this issue, a series of impact shear tests was performed using a novel-designed dynamic experimental system combined with the digital image correlation (DIC) technique. The dynamic shear strength, deformability and failure mode of the jointed specimens with various joint roughness coefficients (JRC) are comprehensively analyzed. Results show that the shear strength and shear displacement characteristics of the rock joint under the impact loading keep consistent with those under static loading conditions. However, the temporal variations of shear stress, slip displacement and normal displacement under the impact loading conditions show obviously different behaviors. An elastic rebound of the slip displacement occurs during the impact shearing and its value increases with increasing joint roughness. Two identifiable stages (i.e. compression and dilation) are observed in the normal displacement curves for the rougher rock joints, whereas the joints with small roughness only manifest normal compression displacement. Besides, as the roughness increases, the maximum compression tends to decrease, while the maximum dilation gradually increases. Moreover, the microstructural analysis based on scanning electron microscope (SEM) suggests that the roughness significantly affects the characteristics of the shear fractured zone enclosing the joint surface.     

考虑T应力的岩石压剪裂纹起裂机理

传统断裂理论在研究岩石压剪裂纹起裂机理时,往往仅考虑裂纹尖端应力场Williams展开式中的r~(1/2)奇异应力项,而忽略了非奇异应力项(T应力)的影响,造成理论预测值与试验结果不符。在对压剪应力下裂纹受力特征进行分析的基础上,将T应力引入传统断裂力学的最大周向应力准则,提出了考虑T应力的修正最大周向应力准则。同时考虑压剪应力下的裂纹应力传递特点,在上述准则中又引入裂纹面法向刚度及切向刚度等变形参数。最终建立了能够同时考虑岩石性质和裂纹几何参数(如裂纹倾角、长度等)、强度参数(裂纹面摩擦系数)及变形参数(裂纹面法向及切向刚度)的最大周向应力准则,更好地反映了岩石压剪裂纹起裂机理。算例表明由该方法计算得到的翼裂纹起裂角与试验结果吻合较好,同时通过参数敏感性分析发现裂纹尖端相对临界尺寸对翼裂纹起裂角的影响最大。     

Over-consolidation effect on shear behavior of rock joints

Although many researchers have studied the normal and shear behavior of fractures under stresses, the over-consolidation effect on the slip/shear behavior of discontinuities has not been considered. The over-consolidation behavior of non-planar rock fractures should be considered when deposition–consolidation–erosion (or excavation) sequences occur. Plaster replicas of representative natural rock joint surfaces were prepared for this study. In this case, the surface roughness and other geometrical properties remain constant during the laboratory direct shear tests. It was observed that the shear strength within a large range of roughness, joint wall strength and normal stress values significantly increases with increasing over-consolidation ratio. According to the test results, a new model is developed as an extended form of Barton's shear failure criterion for rock joints. This model considers the effect of various paths of normal loading/unloading before shearing and over-consolidation ratio in a fracture. A new joint over-closure (JOC) parameter is also introduced as the ratio of closure in over-closed to normally closed conditions.     

岩石节理剪切力学行为的颗粒流数值模拟

 利用颗粒流程序生成岩石节理直剪试验数值模型,进行不同法向应力作用下节理直剪试验的颗粒流数值模拟,研究节理的宏细观剪切力学行为,以及不同法向应力作用下微裂纹的发育及演化规律。结果表明：模型试件在直接剪切过程中表现出类似于真实节理的宏观力学行为,试件的抗剪强度和剪切峰值剪胀角对法向应力的依存关系体现出与JRC-JCS模型预测结果良好的一致性;模型试件主要在节理面两侧的微凸体附近产生接触压力集中现象。法向应力越大,接触压力集中区越多,集中程度越高;随法向应力不断增大,试件内微裂纹的发育速度逐步提高,剪切裂纹发育数目在微裂纹总数中所占的比例逐步增大,但仍远少于张性裂纹;节理面上接触压力的分布特征与模型试件内微裂纹的发育规律一致;节理在外荷载作用下的剪切破坏是节理面上微裂纹汇集贯通的结果,“压致拉”效应在此过程中起主导作用。     

单轴压缩条件下锚杆影响脆性岩体破裂的细观机制

锚杆支护系统是控制深部脆性围岩动力灾害的重要措施,但锚固理论研究仍滞后,锚杆支护下的脆性岩体破坏问题困扰着深部岩体工程实践。根据实际工程中锚杆支护下脆性围岩的浅表局部破坏特点,通过室内相似模型试验研究单轴压缩条件下锚杆杆径影响完整脆性岩体的破坏特性,试验表明,锚杆杆径对脆性岩体弹性模量和强度的提升存在最优匹配的特点,一味强调增大锚杆直径并不能达到理想的围岩控制效果;锚杆改变了脆性岩体单轴压缩破坏模式,宏观上由劈裂破坏转为剪切破坏,杆径对试样剪切破坏的程度有所影响。从细观角度,建立了含两条固有主裂纹的裂纹扩展分析模型,加锚试样单轴压缩破裂模式的改变,可以归结为锚杆锚固止裂效应对试样内部裂纹扩展的抑制作用,使翼裂纹与主裂纹长度比η变小。根据最易开裂角度ζ的计算结果,翼裂纹较长时,翼裂纹朝外载作用方向扩展,产生劈裂破坏,翼裂纹较短时,翼裂纹偏离外载作用方向扩展,产生剪切破坏。从细观上很好地解释了锚杆改变脆性岩体破裂模式的作用机制。     

粗糙节理剪切性质的颗粒流数值模拟

 在二维颗粒流程序PFC2D中生成粗糙节理剖面并模拟其剪切性质。通过数值直剪试验,从细观角度观察节理的宏观破坏过程。作为模型试验的一种补充,可以观测到粗糙表面微凸体的剪切破坏及微裂隙发育的情况。宏观剪切区域的产生主要是由细观剪裂纹的累积形成的,剪切破坏区域集中在爬坡效应显著的位置,即剪切应力集中的区域,剪切过程中形成的剪裂纹的数量在出现剪切峰值应力后显著增加,此时粗糙节理面的破坏最为显著。颗粒流数值试验能较好地再现模型试验的结果,采用PFC2D数值试验能部分替代真实岩石试样的模型试验,将其作为一种预测真实岩石节理抗剪强度的手段,从而解决天然粗糙节理形貌难以重复的问题。     

Application of the expanded distinct element method for the study of crack growth in rock-like materials under uniaxial compression

The expanded distinct element method (EDEM) was used to investigate the crack growth in rock-like materials under uniaxial compression. The tensile-shear failure criterion and the Griffith failure criterion were implanted into the EDEM to determine the initiation and propagation of pre-existing cracks, respectively. Uniaxial compression experiments were also performed with the artificial rock-like samples to verify the validity of the EDEM. Simulation results indicated that the EDEM model with the tensile-shear failure criterion has strong capabilities for modeling the growth of pre-existing cracks, and model results have strong agreement with the failure and mechanical properties of experimental samples. The EDEM model with the Griffith failure criterion can only simulate the splitting failure of samples due to tensile stresses and is incapable of providing a comprehensive interpretation for the overall failure of rock masses. Research results demonstrated that sample failure primarily resulted from the growth of single cracks (in the form of tensile wing cracks and shear secondary cracks) and the coalescence of two cracks due to the growth of wing cracks in the rock bridge zone. Additionally, the inclination angle of the pre-existing crack clearly influences the final failure pattern of the samples.     

钢筋套筒挤压连接装配整体式梁柱中节点抗震性能试验研究

为研究钢筋套筒挤压连接的装配整体式叠合梁-预制柱中节点的抗震性能,进行了2个装配整体式中节点和1个现浇中节点试件的拟静力试验,观察其破坏现象,研究其滞回性能、承载力、变形能力及钢筋应变发展过程。结果表明：所设计的核心区剪切破坏和梁端弯曲破坏节点均实现了预期的破坏形态;核心区剪切破坏的装配整体式梁柱节点与现浇对比节点的破坏过程、裂缝分布基本一致,滞回曲线、承载力、刚度、变形、耗能等性能指标基本相同;核心区剪切破坏、梁端弯曲破坏的装配整体式梁柱中节点的承载力试验值与相应的规范公式计算值的比值分别为1.62、1.31,等效极限层间位移角分别为1/26和1/27;试验结束后,凿开混凝土发现套筒没有出现裂纹,钢筋在套筒内未见滑移,套筒挤压接头可有效传递钢筋拉、压力。     

DDA子块体开裂模拟算法的优化与验证

非连续变形分析(DDA)是计算离散可变形块体系统力学响应的数值计算方法,其在连续岩体开裂破坏模拟中的应用也已得到研究。在以往的预离散子块体DDA开裂模拟方法中,通过采用虚拟节理对子块体进行粘结以模拟连续体的变形,进而根据虚拟节理面上的块间接触力判断沿预设节理面的拉伸或剪切破坏。该算法能够较好的模拟岩体的开裂路径和破坏形态,但受预设虚拟节理面方向对块间接触力大小的影响,由此得到的岩石开裂强度与实际之间可能产生较大差异。本研究不再根据块间接触力进行开裂判断,而改进为根据邻近子块体的应力状态进行开裂判断,并仍假定裂纹沿虚拟节理面产生。用新的DDA程序对压缩载荷作用下圆盘试件的拉伸开裂破坏和方形试件的剪切开裂破坏进行了模拟。算例表明,改进后的开裂算法具有高的开裂计算准确性,并大大减小了预设虚拟节理分布对开裂破坏强度及破坏路径模拟结果的影响。     

含预制裂隙大理岩破坏过程声发射特征研究

采用含预制裂隙大理岩块试件 ,对压剪应力场中试件破坏过程声发射特征进行研究 ,试验表明预制裂纹对试件的影响是显著的。试验观测到不同试件的不同声发射特征 ,这表明它们的损伤断裂过程的不同机制 ,对于一些以张拉型翼裂萌生、扩展为破坏机理的岩样 ,还表现出明显的二次峰值现象。锯齿型裂纹的情况较为复杂 ,其破坏具有不同于其他试件的特征 ,破坏之前有相当长一段时间 ,声发射也较为活跃 ,且增幅越来越大 ,破坏时出现最高峰值 ,破坏一瞬间的振铃累积数几乎达到全过程的一半 ,这表明含锯齿形裂纹的材料的破坏更具有突然性和不可预测性 ,对岩石材料强度以及岩体稳定性有关键的影响     

直剪作用下不共面断续节理岩桥破断试验与数值研究

在伺服控制剪切加载系统下对不共面类岩石断续节理试件进行正向、反向直剪试验,研究直剪下不共面断续节理的岩桥破断机理和剪切规律,试验研究发现,直剪作用下不共面断续节理岩桥破坏过程具有明显的阶段性,经历线弹性阶段、裂纹起裂扩展阶段、岩桥断裂贯通阶段、剪切面爬坡咬合阶段和残余摩擦阶段5个阶段,正向剪切下岩桥呈齿形破断面,反向剪切作用下岩桥产生沿直剪方向贯通的带形破断面,与正向剪切相比,反向剪切下节理的初裂抗剪强度和峰值抗剪强度较大,裂纹倾角、法向应力和相邻节理搭接比例是影响试件初裂抗剪强度和峰值抗剪强度的主要因素。采用FLAC3D对正向、反向直剪作用下不共面断续节理的岩桥破断、剪切破断面的形成过程进行数值试验,数值试验结果和类岩石直剪试件的试验结果基本吻合,数值试验揭示了直剪作用下不共面断续节理岩桥的拉裂破坏和破断面的剪切屈服机理。     

节理起伏角对非贯通节理岩体力学特性的影响

对非贯通节理岩体进行直剪试验,在相同法向应力作用下研究具有不同节理起伏角的非贯通节理岩体的强度特性、变形特征。并采用颗粒流数值模拟软件(PFC 2D)进一步研究非贯通节理岩体的细观扩展机理。两种试验研究表明:(1)非贯通节理岩体的破坏形态受节理起伏角的影响显著,随着节理起伏角增大,岩体的破坏程度逐渐加重,岩体破坏时的张拉裂纹越大,裂纹数目也越多,张拉节理与两节理面之间的夹角将越大,表面破损也越明显;(2)非贯通节理岩体的变形特征受节理起伏角的影响显著,当节理起伏角不同时,非贯通节理岩体的法向变形将不同,随着节理起伏角增大,非贯通节理岩体的峰值切向位移逐渐减小;(3)非贯通节理岩体的强度特性受节理起伏角的影响显著,随节理起伏角的增大,非贯通节理岩体的峰值剪切强度增大,岩体抗剪强度增大。     

中部锁固岩桥三轴加卸荷力学特性及裂纹扩展研究

 锁固型高陡岩质边坡内部岩桥破坏机制复杂,研究边坡中部锁固段的破坏规律及其对边坡整体变形破坏机制具有重要意义。为表征滑坡后缘拉裂缝和前缘蠕滑破坏,在完整岩样端部预制裂纹形成中部岩桥,开展3种不同长度岩桥试样的三轴加载和三轴加卸荷试验,分析2种应力路径下的应力–应变特征、强度特征和裂纹扩展模式,从断裂力学角度揭示了裂纹扩展机制。结果表明：随围压和岩桥长度的增加,试样峰值强度和对应的应变增大,且三轴加卸荷峰值和应变均大于三轴加载;应力–应变曲线呈现出“突发式破坏”和“峰后回升”现象,部分试样还表现出“双峰值”特征;岩桥试样呈现贯通岩桥、贯通试样上端面、向外环向破坏、向内环向破坏及贯通试样下端面等5类裂纹扩展模式;岩桥试样在下部节理尖端应力集中处产生张拉裂纹和剪切裂纹,大部分裂纹起裂角集中在40°～50°范围。中部岩桥三轴加卸载力学试验表明,边坡锁固段并非一次剪断破坏,可能呈现逐次多级破坏模式,本研究获得的岩桥裂纹扩展及破坏机制,可为锁固型岩质边坡开挖卸荷的破坏机制和变形特征提供理论支撑。     

类岩石材料有序多裂纹体单轴压缩破断试验与翼形断裂数值模拟

为研究断续岩石裂纹产状特性对岩体强度的影响和岩桥破断规律,在水泥砂浆中预制有序多裂纹体,开展单轴压缩下类岩石材料有序多裂纹体破断试验。研究发现：有序多裂纹体破断模式主要为排间翼形拉裂纹贯通、排间拉伸—剪切裂纹贯通和排内倾斜剪切裂纹贯通。当裂纹倾角较小（如倾角为25°和45°）时,随裂纹密度的增加,试件表征峰值强度总体上呈衰减趋势,而残余强度总体上呈增加走势;裂纹倾角较大（如倾角为75°和90°）时,裂纹密度对表征峰值强度无显著影响,其残余强度特性表现不明显;相同裂纹密度下倾角从25°变化到90°,试件表征峰值强度总体上呈增加趋势。提出主控岩桥贯通模式的概念,倾角25°试件的主控岩桥贯通模式大都是斜对角线上排间拉伸-剪切裂纹贯通;倾角45°试件的主控岩桥贯通模式为：翼形裂纹贯通和斜对角线方向上共面次生剪切裂纹贯通两种模式。裂纹尖端应力-应变集中特性揭示了压剪裂纹尖端的拉应变集中是岩石翼形裂纹萌生的本质原因,而裂纹端部的双向压应力-应变集中导致次生剪切裂纹萌生。从岩石断裂力学基本理论出发,引入点剪切安全系数,构建基于ANSYS的岩石多裂纹体翼形断裂扩展的数值分析模型,阐明了单轴压缩下有序多裂纹体翼形断裂贯通的力学机制,其数值结论与物理试验基本相吻合。     

Fracture coalescence in rock-type materials under uniaxial and biaxial compression

Fracture coalescence, which plays an important role in the behavior of brittle materials, is investigated by loading pre-fractured specimens of gypsum, used as a rock model material, in uniaxial and biaxial compression. Several new phenomena and their dependence on geometry and other conditions are observed. The specimens have two pre-existing fractures or flaws that are arranged in different geometries, and that can be either open or closed. Two different test series are performed with these flaw geometries, one under uniaxial loading and one with biaxial loading in which confining stresses of 2.5, 5.0, 7.5 and 10 MPa are applied. As the vertical (axial) load is increased, new cracks emanate from the flaws and eventually coalesce. Flaw slippage, wing crack initiation, secondary crack initiation, crack coalescence, and failure are observed. Two types of cracks occur: wing cracks, which are tensile cracks, and secondary cracks which initiate as shear cracks in a plane roughly co-planar with the flaw. The secondary cracks usually propagate as shear cracks in the same plane but, depending on the geometry, they also propagate out of plane as either tensile or shear cracks. The wing cracks initiate at the flaw tips for uniaxial or low confinement biaxial conditions but move to the middle of the flaw and disappear completely for higher confining stresses. Three types of coalescence, which depend on the geometry of the flaws and to some extent on stress conditions, occur; they can be distinguished by different combinations of wing cracks and secondary cracks. For closed flaw specimens, at least partial debonding and slippage of the flaws is required prior to initiation of a crack. In uniaxial compression coalescence and failure occur simultaneously, while failure in biaxial compression occurs after coalescence.     

非贯通节理Jennings强度准则的岩桥弱化和节理面起伏角修正

 通过对具有不同粗糙程度(以节理面起伏角表示)的共面非贯通人工节理进行不同法向应力水平下的直剪试验,研究节理面起伏角对非贯通节理剪切强度的影响,分析在剪切过程中岩桥力学参数的弱化机制。现有Jennings准则将岩桥视为完整岩块,计算所得的剪切强度与直剪试验结果存在较大偏差。在试验结果的基础上,考虑剪切过程中岩桥力学参数的弱化和节理面起伏角的影响,对Jennings准则进行修正。将修正Jennings准则计算的非贯通节理剪切强度结果与直剪试验结果对比,结果表明：修正Jennings准则的计算结果与试验结果吻合较好,能较好地预测具有规则起伏角的非贯通节理剪切强度。     

类岩石裂纹压剪流变断裂与亚临界扩展实验及破坏机制

进行双轴压缩条件下类岩石裂纹的压剪流变断裂实验,采用双扭试件的常位移松弛法对类岩石材料进行亚临界裂纹扩展与断裂韧度试验。在实验室尺度上证实了类岩石裂纹流变断裂现象的存在,并且得到了翼形裂纹–翼形裂纹贯通、翼形裂纹–原生裂纹贯通和翼形裂纹–翼形裂纹–剪切裂纹贯通的 3 种流变断裂贯通模式。类岩石材料的流变断裂是一种稳定的裂纹扩展,其本质原因是类岩石裂纹的亚临界扩展。以黏弹性断裂力学、流变力学和能量准则为理论依据,推导以应力强度因子、翼形裂纹长度和时间为内变量的相应势函数,建立多种破坏机制的 压剪岩石裂纹的流变断裂 判据和计算模型。 利用 流变断裂 实验对计算模型进行验证,得出裂纹流变 贯通的 理论时间与实验时间较为吻合,当翼形裂纹的扩展方向与最大压应力方向偏离较大时实验结果与理论模型误差较大。提出的计算方法和理论判据为研究岩石裂纹的流变断裂的细观机理及岩体工程流变破坏的宏观机制提供了一个新而实用的研究手段。     

含预制裂纹大理岩的压剪试验分析

采用含预制裂纹大理岩块试件对单轴压缩荷载作用下的裂纹扩展及裂纹搭接进行试验研究。试验表明含预制裂纹试件的临界失稳荷载、破坏时的应变及弹性模量都明显降低,原生裂纹的方位对产生何种裂隙有显著的影响,岩桥区尺寸对翼裂的萌生和扩展有显著的影响,使试件表现出完全不同的破坏方式。试验中观察到预制裂纹尖端萌生的裂纹有张拉型的翼形裂纹、压剪型的二次裂纹和翼裂反向裂纹等多种形态。试验观测还表明大理岩翼形裂纹的起裂角为52°～68°之间,而模型材料试验的结果为77°左右,这表明模型材料并不能完全模拟真实岩体的特性。