含预制裂纹的脆性岩石单轴压缩下渐进性破坏过程的试验研究

对含预制裂纹的花岗岩进行单轴压缩试验研究预制裂纹倾角α对脆性岩石渐进性破坏过程的影响。首先埘破坏过程的轴向应力-横向应变曲线进行总结和讨论,然后分析预制裂纹与加载方向夹角α埘岩石的应应力门槛值：裂纹起始应力σci、裂纹扩展应力σed、峰值强度矿σf,由应变片记录的应力-应变曲线和试样的表嘶裂纹扩展情况的影响机制。结果表明,含有预制裂纹的岩石试样进行加载试验过程中,顶制裂纹倾角α的变化成了决定脆性岩石破裂办式的主要因素。故在对含节理、裂隙的脆性岩石的工程应用上,通过对岩休的轴向应力-横向应变向应变典线进行分析,可以对地下开挖工程起到指导设计开挖方式及支护形式的作用。     

Study of the brittle fracture process under uniaxial compression

Static friction along inclined cracks in photoelastic models increases with both the loading and frictional displacement before the larger crack-wall asperities are broken or completely over-ridden. Elastic shocks resulting from successive stick-slip during this stage can be repeated more or less indefinitely with repeated loading and unloading. Locked-in residual stresses, especially around crack tips, result when the model is unloaded, because of frictional coupling between crack walls. Favorable crack arrays for initial growth in the model include particular sets of en-echelon cracks inclined 45° to the stress axis, but the critical orientation may be smaller in brittle rock if crack-wall friction in rock is greater than in the model. Axial growth of en-echelon cleavage cracks, inclined at angles smaller than 45°, was observed in feldspar during deformation of a pegmatite having a mineralogy and texture similar to granite. Their growth follows predictions derived from photoelastic model studies.Crack growth in the pegmatite begins between half and two-thirds of the ultimate strength. The first flaws to grow, however, also include pre-existing axially oriented cleavage cracks in the feldspar. Crack growth occurs randomly throughout the specimen as stress is increased, without much evidence that grain boundaries are activated for crack growth. But when the applied stress approaches the ultimate strength, two new features are observed. There is an abrupt development of finite frictional slip along favorably inclined flaws and grain boundaries, beginning with displacements of the order of the dimensions of grain-boundary asperities. Crack growth still occurs at various locations throughout the specimen at this stage, but there is also a detectable concentration of growth along potential shear zones. Flaw-wall friction appears to be one of the critical factors that determine the pegmatite's ultimate strength and the instability of through-going fracture.     

基于统计岩体力学的岩体强度特征分析

结构面的存在改变了岩体力学性质,影响了岩体强度特征。基于统计岩体力学强度判据,结合摩尔-库仑准则,得到了含单组结构面岩体破坏的4种不同方式和相应的结构面倾角范围,推导了岩体强度由结构面控制转化为应力控制的临界围压表达式。在此基础上,根据岩石和结构面参数之间的关系,将含单组结构面岩体分为4类,并探讨了可能破坏方式和发生破坏的条件。最后,举例分析了含单组结构面闪长岩的强度特征,结果表明,该岩体属于第I类岩体,在垂向受压时先沿结构面后沿岩块破坏,临界围压为9.12 MPa;从全空间分析,该岩体强度各向异性显著,围压增大可使岩体在一些方向上受力时强度由结构控制转化为应力控制。     

Application of Manifold Method (MM) to the stability problems for cut slopes along the national roads

Recently one of the most challenging problems for civil engineers is how to construct new structures/infrastructures adjacent pre-existing ones and how to evaluate the effect of the new construction on the pre-existing structures (so-called neighbouring construction). The Manifold Method (MM) is one of the discontinuum based numerical approaches to simulate the mechanical behaviour of fractured rock masses including large deformation/displacement along fractures as well as stress/strain conditions of the rock blocks/masses. In this study, the MM was used to investigate the stability of the cut slope in the highly fractured rock masses along one of the national roads in Japan, focusing on the effects of new construction on the pre-existing structures/infrastructures. The effect of reinforcement during the construction such as rock bolts, anchors, etc. was also investigated and evaluated quantitatively by numerical simulations using MM.     

基于连续-非连续方法的裂隙破坏与相互作用研究

岩体内裂隙等非连续结构面对岩体的强度及变形等力学特性有着显著的影响,研究岩体裂隙起裂、扩展、相互作用和贯通机制,对工程岩体力学行为的表征和工程性能的评价十分重要。本文基于连续介质力学模型的离散元方法,通过考虑裂隙分布、模型加载条件及其与裂隙产状的关系,建立了一系列裂隙力学计算模型,研究了不同模型裂隙扩展演化特征和岩体破裂机制,分析了岩体裂隙扩展规律及其对岩体破坏路径和强度的影响,研究结果表明:（1）裂隙岩体模型加载条件下的破坏起裂点、最终贯通破坏特征及损伤分布受控于裂隙的产状及其与最大主压应力取向角度大小及围压大小。（2）裂隙弱面走向与最大主压应力取向斜交时,裂隙弱面在加载条件下其端部裂隙扩展、贯通破坏表现比较明显,反之,当裂隙弱面走向与最大主压应力取向一致时,裂隙弱面被动影响裂隙模型内新生裂隙的萌生、扩展和贯通模式,自身未出现新的扩展破坏。（3）裂隙数目的增多和围压的增大会显著增加模型内部剪切裂缝的数量和模型破坏后的破碎程度,模型内部的损伤区域主要围绕破裂面呈滑移线型交叉分布,非破裂面区域损伤呈条带状X型分布。（4）裂隙弱面走向与最大主压应力取向斜交时,裂隙对岩体模型强度的弱化程度高于裂隙弱面走向与最大主压应力取向一致的情况,而裂隙模型破坏后的残余强度则正好相反。     

Effects of pre-existing discontinuities on the residual strength of rock mass – Insight from a discrete element method simulation

When rock failure is unavoidable, the designer of engineering structures must know and account for the residual strength of the rock mass. This is particularly relevant in underground coal mine openings. Pre-existing discontinuities play an important role in the mechanical behavior of rock masses and thus it is important to understand the effects of such pre-existing discontinuities on the residual strength. For this purpose, the present study demonstrates a numerical analysis using a discrete element method simulation. The numerical results indicate that fracture intensity has no significant influence on the residual strength of jointed rock masses, independent of confining conditions. As confining pressures increase, both peak and residual strengths increase, with residual strength increasing at a faster rate. The finding was further demonstrated by analyzing documented laboratory compressive test data from a variety of rocks along with field data from coal pillars. A comprehensive interpretation of the finding was conducted using a cohesion-weakening-friction-strengthening (CWFS) model. The effect of rock bolts on rock mass strength was also evaluated by using a discrete element method model which suggested that rock bolts can significantly increases residual strength but have limited effect on increasing the peak strength of rock masses.     

The evolution of rock failure with discontinuities due to shear creep

A two-dimensional brittle creep model for rock provides insight into the initiation of shear fracture along weak discontinuities in rock. The model accounts for material heterogeneity and introduces the concept of a mesoscopic renormalization to capture the cooperative interaction between cracks in the transition from distributed to localized damage. A series of shear creep tests on rock with discontinuities were performed to simulate the initiation and propagation of crack along a pre-existing weakness under sustained shear stress and normal stress. The investigation showed that shear stress level and the normal stress level might have significant effect on the long-term behavior of rock with weak discontinuities. Moreover, a case study of rock slope instability was also investigated, where the numerically simulated instability failure of rock slope with discontinuities showed that both tensile and shear damage at the weakest elements are the trigger for the failure surface initiation in the rock slope. Once damage occurs, redistributed stress concentrations would then intensify fracture propagation and coalescence within these damage zones, leading to the progressive development of a failure surface. Moreover, failure surface extending is not only dominated by the properties and the position of discontinuities but also influenced remarkably by the complex interaction between existing discontinuities and fracture propagation. The results are of general interest because they can be applied to the investigation of time-dependent instability in rock masses, to the mitigation of associated rock hazards in rock engineering, and even to a better understanding of the physical phenomena governing the stability of rock slope.     

岭澳核电三期强风化角岩边坡岩体直剪试验研究

采用堆载法进行天然状态与饱和状态下强风化角岩边坡岩体的现场直接剪切试验,获得不同正应力水平下剪切应力-剪切变形关系曲线和剪切强度参数,对其剪切应力-剪切变形关系曲线特征和不同正应力作用下剪应力随正应力的变化规律以及水-力耦合作用对剪切强度与变形特性影响进行了分析。试验结果表明：岩体峰值剪切强度和屈服剪切强度均随正应力的增大而增大;天然状态和饱水状态下剪应力随不同正应力的变化趋势基本相同;岩体剪切强度随含水率的增大而减小,在低法向应力下尤其敏感;水对岩体强度参数中黏聚力c的弱化作用更加明显,同时加大了岩体变形量,延长了岩体变形过程;通过现场直剪试验测得的法向变形可以估算岩体的压缩模量,为边坡稳定性分析提供参数。     

基于统一强度理论深埋圆形隧道围岩的剪胀分析

基于连续介质理论中岩体的剪胀角与围压和塑性剪切应变密切相关,隧洞周边岩体的应力状态因开挖卸荷而发生应力重分布,迫使其围压由原地应力逐渐衰减,塑性剪切应变不断增加,引起剪胀效应呈非线性变化。首先,基于统一强度理论和非关联流动法则,将潜在塑性区围岩按等围压释放划分为若干同心圆,提出了考虑中间主应力和非线性剪胀性的有限差分法,计算应变软化围岩的力学问题,并以实例验证其正确性。其次,通过参数分析,研究塑性区内岩体的剪胀角受中间主应力、临界软化系数和支护力的影响规律。研究结果表明,中间主应力主要影响剪胀角的峰值,随着中间主应力效应增加,剪胀峰值增加;临界软化系数主要影响剪胀角的变化率,随着临界软化系数的增加,剪胀角变化缓慢;中间主应力和临界软化系数共同影响塑性区剪胀角的变化;随着支护力的增加,洞壁处的剪胀角增加;双剪强度理论计算的位移值较小,应谨慎采用,同时采用Mohr-Coulomb强度准则时可以适当考虑围岩的承载潜力。     

含双裂隙岩石裂纹演化机理的离散元数值分析

采用离散单元法探讨了预制双裂隙岩石的裂纹演化机理。用近期从试验资料提取的无胶结厚度含抗转动能力的岩石微观力学模型和相应的离散单元法商业软件,模拟了含不同预制倾角的双裂隙岩石试样在单轴压缩作用下裂纹的扩展与贯通规律,揭示了裂纹演化的宏微观机理。同时,将离散元法DEM岩石试样的裂纹的扩展和贯通规律以及强度特性与室内试验结果进行了比较分析。结果表明,预制裂隙之间以及端点处的拉应力集中是导致裂隙岩石破坏的主要原因,且DEM数值试验得到裂纹的演化规律与室内试验结果较为一致。含30°的预制裂隙的岩石试样最容易起裂,含75°的预制裂隙的岩石试样最困难起裂,造成此种现象的原因可能是裂纹在垂直于主应力方向上的长度不同导致试样受拉区域大小不同。     

Modeling of a jointed rock mass under triaxial conditions

One of the most important aspects of designing a structure on or in a rock mass is based on the strength response of a jointed rock mass. Understanding this important aspect, the present study was undertaken to understand the strength response of a jointed rock mass with the help of a finite difference package FLAC^3D. In the present work, an attempt has been made to understand the effect of discontinuity angle on the failure mode and strength of the rock mass. For this purpose, stress and displacement in the model were studied and various stress–strain histories were recorded at constant strain loading rate. Rock discontinuity plays a critical and vital role to understand physico-mechanical characteristics of a rock mass. It has wider application in the rock excavation engineering, e.g., caverns, tunnels, slope stability, dams, etc. Simulated rock results are compared with the analytically calculated results of the jointed rock mass and found in good agreement.     

岩石临界抗渗强度的测定及其在工程中的应用

通过岩石伺服渗透试验获得渗透率-应变关系和应力-应变关系曲线,对比分析岩石破坏前的渗透率、应力与应变之间的关系特点,提出了岩石临界抗渗强度的概念,阐述了将其作为岩石变形过程渗透性特征参数的工程意义,并实际应用于杨村煤矿煤层底板带(水)压开采隔水层阻水能力的评价。      

崖腔型悬崖的稳定性估算

崖腔型悬崖破坏,是一种崩塌地质灾害,对线路工程与居住环境的安全性必须进行评估。假设张应力呈线性三角形分布,提出了一种评估崖腔型悬崖体稳定性的估算方法：首先计算张力矩,应用张力矩与压力矩相等的力矩平衡原理,计算出张力面上的总张力,然后按张力线性分布原理求出最大张应力。崖腔型悬崖的破坏是顶部最大张应力拉裂岩石而发生的。所以稳定系数定义为抗拉强度与最大张应力之比,用以评价崖腔型悬崖的稳定性。用算例讨论了影响崖腔型悬崖张应力大小的因素：悬崖体厚度H、岩石容重γ与崖腔深度L,分析了它们对不同厚度岩体与不同崖腔深度条件下最大张应力变化与破坏的一般变化规律,以及节理裂隙发育程度对岩体抗拉强度的影响,并提出了相关建议。算例表明本方法与悬臂梁的弹性力学解方法结果相差很大,证明弹性力学解对“深梁”型悬崖是不适用的。     

高应力硬岩卸荷岩爆模式及损伤演化分析

针对岩爆形成机制研究的不足,结合岩爆实例和真三轴加、卸荷试验,分析了卸荷岩爆模式,建立了围岩卸荷岩爆发生劈裂破坏和剪切破坏的损伤变量表达式,即损伤演化方程。结果表明,岩爆破裂演化表现为劈裂和剪切的复合破坏形式,不同之处只是劈裂和剪切哪一个所占的成分更大而已;随着剪切成分的增大,爆坑由呈现平底到弧形甚至三角形或V形;随着应力的增加,岩爆倾向性围岩有一段时间的平静期（无损伤）,具体平静期存在时间的长短与围岩卸荷前后的应力水平有关,损伤一旦开始则加速度明显,具有较强的临界敏感性;劈裂总是优先发生的,围岩卸荷后的应力水平越高含剪切成分越大,岩爆也越强烈。     

预应力锚索锚固段荷载传递解析算法

锚固是岩体工程增稳的主要措施,对充分发挥岩体的自承潜力,调节和提高岩体自身强度和自稳能力有着十分重要的作用。由于锚固工程本身的复杂性和多样性,导致目前锚固机制、设计理论以及计算方法都不够完善。现有的锚固段荷载传递解析解存在两大难题,一是没有反映锚固段应力变化过程,随着荷载不断加大,接触面是逐渐被破坏,剪应力的峰值将由端口逐渐向内转移;二是在端口处是一个应力奇异点,难以很好地解决。通过对当前多种锚固段荷载传递解析解的适用性及局限性进行分析归纳,指出应将锚固段应力分布划分为弹性、塑性和破坏3个阶段,在不同的阶段应力分布形式不一样,不能一概而论。在此基础上,基于传递系数,针对沿锚固段剪应力呈非均匀性分布形式,提出了一种能反映这3个阶段变化规律的荷载传递解析公式,获得了锚固段剪应力和轴力的分布规律和影响因素。对各解析解方法求出的临界锚固长度进行了算例对比,证实所提算法是合理和有效的。该算法适用于预应力锚索锚固段工程设计。     

预制节理岩体试件强度及破坏模式的试验研究

采用相似材料模型试验对不同节理倾角、节理贯通度、节理组数、载荷应变率、试件长径比、节理充填物厚度及类型等7种工况下的预制节理岩体在单轴压缩下的峰值强度及破坏模式进行了研究。结果表明：节理岩体的破坏模式及峰值强度与节理构造形态密切相关。贯通节理岩体将产生沿节理面的剪切破坏或穿切节理面破坏,且与第1种破坏模式对应的岩体峰值强度更低。非贯通节理岩体的强度介于完整岩体和贯通节理岩体之间。随着平行节理组数的增加,岩体峰值强度逐渐下降。随着载荷应变率的增加,岩体峰值强度逐渐增大,相应地试件的破坏模式也变得更加复杂。试件长径比基本没有改变其破坏模式,完整试件仍主要是以张拉破坏为主,而节理试件仍以剪切破坏为主。随着长径比增加,试件峰值强度逐渐增大。随着节理充填物厚度增加,试件峰值强度降低。不同节理填充物对试件峰值强度也有一定影响。     

Critical review on shear strength models for soil-infilled joints

An infilled rock joint is likely to be the weakest plane in a rock mass. The presence of infill material within the joint significantly reduces the friction of the discontinuity boundaries (i.e. rock to rock contact of the joint walls). The thicker the infill, the smaller the shear strength of the rock joint. Once the infill reaches a critical thickness, the infill material governs the overall shear strength, and the joint walls (rock) play no significant role. Several models have been proposed to predict the peak shear strength of soil-infilled joints under both constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions, taking into account the ratio of infill thickness (t) to the height of the joint wall asperity (a). CNS models provide a more realistic picture of the soil-infilled joint behaviour in the field. This paper presents a critical review on the existing mathematical models for predicting the shear strength of soil-infilled rock joint and verifies the normalised peak shear stress model with further laboratory investigations carried out on idealised saw-tooth rock joints at the University of Wollongong. Based on the prediction of the experimental data, the normalised peak shear stress model is slightly modified by the authors. A simplified approach for using this model in practice is presented and a new expression for prediction of dilatation at peak shear stress is suggested.     

一种双折减系数的强度折减法研究

传统的强度折减法采用相同的折减系数对黏聚力和内摩擦角进行折减,边坡达到临界状态时的折减系数就是边坡的安全系数,此单一折减系数的折减方法被定义为等比例折减。对于一个边坡几何构型和岩体重度确定的边坡而言,使边坡处于临界状态的黏聚力和摩擦系数组合并不惟一,等比例强度折减法所得到边坡临界状态的黏聚力和摩擦系数只是其中可能的一种潜在组合,因此,其只是众多寻找临界状态方法中的一种折减形式,并不一定就是最优的折减方式。基于以上认识,首先建立不同边坡形状、不同岩体重度条件下临界状态边坡的 - 曲线,假设岩体的黏聚力和内摩擦角在衰减破坏的过程中应沿着距离 - 临界曲线最短的路径折减,进而建立黏聚力和内摩擦角的配套折减原则,提出了一种双参数强度折减法。最后,通过3个算例比较了等比例强度折减法与双参数强度折减法的差异,结果表明,等比例强度折减法和双参数强度折减法得到的边坡临界状态并不相同,后者得到的临界滑动面范围要比前者得到的滑动面范围大。     

Tensile strength and its anisotropy measured by point and line-loading of sandstone

Tensile fracture is an ubiquitous feature of rock failure ranging in size from microfracture around Griffith cracks to the large-scale explosive failure accompanying rock bursts in underground cavities. Hence the resistance of rock to failure in tension, the tensile strength, is one of the fundamental parameters of rock strength. The tensile strength of rock is, however, a most elusive parameter which is difficult to measure and to define.

The tensile strength, and its variation with direction, was determined by the pointloading and the line-loading (Brazilian test) methods for nine rocks from five sandstone formations in New Brunswick, Canada. Altogether 319 double point-load and 357 line-load tests were conducted on discs having a diameter of 22.9 mm and thickness of 7.9 mm.

The tensile strength determined by the point-load test and as calculated by the Frocht formula was found to be consistently lower than that obtained by the line-loading technique. To some extent, the difference is due to the fact that most of the sandstones are anisotropic with respect to tensile strength and that the constraints in the point-load test are such that fracture occurs along the plane of minimum tensile strength; the computed tensile strength therefore represents a minimum value. To a greater extent, the lower tensile strength calculated from the point-load test is due to the difference in the states of stress induced by the two techniques. When correction is made for the anisotropy, the tensile strength from the Brazilian test is still 3.76 times larger than the one obtained from the point-load test.

The Brazilian test seems to yield a more accurate definition of both the tensile strength and its variation with direction. The point-load test is more suitable for the determination of the minimum value of tensile strength and the direction of the preferred fracture plane.