A Laboratory Shear Cell Used for Simulation of Shear Strength and Asperity Degradation of Rough Rock Fractures

Different failure modes during fracture shearing have been introduced including dilation, sliding, asperity cut-off and degradation. Several laboratory studies have reported the complexity of these failure modes during shear tests performed under either constant normal load (CNL) or constant normal stiffness (CNS) conditions. This paper is concerned with the mechanical behaviour of synthetic fractures during direct shear tests using a modified shear cell and related numerical simulation studies. The modifications made to an existing true triaxial stress cell (TTSC) in order to use it for performing shear tests under CNL conditions are presented. The large loading capacity and the use of accurate hydraulic pumps capable of applying a constant shear velocity are the main elements of this cell. Synthetic mortar specimens with different fracture surface geometries are tested to study the failure modes, including fracture sliding, asperity degradation, and to understand failure during shearing. A bonded particle model of the direct shear test with the PFC2D particle flow code is used to mimic the tests performed. The results of a number of tests are presented and compared with PFC2D simulations. The satisfactory results obtained both qualitatively and quantitatively are discussed.     

Effects of Scale and Normal Stress on Shear Behavior of a Soft Fracture Replica

The effects of the fracture scale (size) and normal stress on the shear behavior of a fracture were investigated in direct shear rig using mortar fracture replicas of different sizes (100–200 mm), which were reproduced from a tensile fracture created in granite. Monotonic shear loading was applied at normal stresses of 0.3 and 1 MPa for shear displacements of 2 and 20 mm. The change in surface topography was measured by using a profile measurement system with a laser profilometer to determine the changes in surface damage and the aperture during shear. Additionally, the effect of gouge material on the non-linearity of the closure curve during shear was investigated. The results show that the standard deviation of the initial aperture during shear increases not only with scale but also with the normal stress, since the matedness decreases due to the fact that asperities are more damaged with normal stress, and accordingly, the non-linearity in the closure curve during shear increases with normal stress. The results also show that the non-linearity of the closure curve during shear increases with the presence of gouge material/debris that is produced in the shearing process. This is mainly because gouge is soft and easily deforms under the application of normal stress. Moreover, gouge material is distributed non-uniformly around the contact areas. Thus, the normal stiffness during shear decreases not only with a decrease in the matedness between the surfaces, but also with the presence of gouge material.     

压剪作用下裂隙扩展过程渗流与应力耦合模型研究

裂隙的剪胀特性及扩展演化规律对岩体的渗流特性具有重要影响。为揭示裂隙剪胀及扩展演化对岩体渗流的影响,基于残余强度提出了一种能较好描述岩石硬化-软化特性的全剪切本构关系;结合剪切变形与裂隙开度的关系,利用最小势能原理和立方定律,建立了压剪作用下考虑裂隙剪胀特性的渗流应力耦合模型;假定压剪作用下裂隙发生Ⅰ型扩展,提出了伴有翼型裂纹的渗流模型,该模型不仅考虑了岩石的剪胀特性,更反映了裂隙扩展过程渗流的演化规律。对不同裂隙粗糙度的剪切应力-位移曲线进行分析,全剪切本构模型表现出较高的拟合精度。在剪切应力-位移关系基础上,通过剪切渗流试验数据对压剪作用下渗流模型进行验证,结果表明,该模型能较好地描述岩体剪胀阶段渗透性演化规律。利用等效裂隙简化裂隙网络,并通过试验数据进行验证,证实了裂隙扩展过程渗流-应力耦合模型的准确性与适用性。     

Hydraulic Characteristics of Rough Fractures in Linear Flow under Normal and Shear Load

Summary A hydro-mechanical testing system, which is capable of measuring both the flow rates and the normal and shear displacement of a rock fracture, was built to investigate the hydraulic behaviour of rough tension fractures. Laboratory hydraulic tests in linear flow were conducted on rough rock fractures, artificially created using a splitter under various normal and shear loading. Prior to the tests, aperture distributions were determined by measuring the topography of upper and lower fracture surfaces using a laser profilometer. Experimental variograms of the initial aperture distributions were classified into four groups of geostatistical model, though the overall experimental variograms could be well fitted to the exponential model. The permeability of the rough rock fractures decayed exponentially with respect to the normal stress increase up to 5 MPa. Hydraulic behaviours during monotonic shear loading were significantly affected by the dilation occurring until the shear stress reached the peak strength. With the further dilation, the permeability of the rough fracture specimens increased more. However, beyond shear displacement of about 7 to 8 mm, permeability gradually reached a maximum threshold value. The combined effects of both asperity degradation and gouge production, which prohibited the subsequent enlargement of mean fracture aperture, mainly caused this phenomenon. Permeability changes during cyclic shear loading showed somewhat irregular variations, especially after the first shear loading cycle, due to the complex interaction from asperity degradations and production of gouge materials. The relation between hydraulic and mechanical apertures was analyzed to investigate the valid range of mechanical apertures to be applied to the cubic law. Received June 12, 2001; accepted February 26, 2002 Published online September 2, 2002     

峰前循环剪切作用下岩石节理剪切力学特性

微震、工程爆破等低应力循环剪切荷载作用对节理岩体工程失稳破坏具有重要影响。为研究峰前循环剪切加卸载作用下岩石节理剪切力学特性,采用RDS-200型岩石节理剪切试验系统对人工劈裂黄砂岩节理进行了峰前循环剪切下的直剪试验。通过与未进行峰前循环剪切加卸载时岩石节理力学参数预测值对比,得到峰前循环剪切加卸载作用对峰前剪切刚度、峰值剪切强度、峰值剪切位移与残余剪切强度的影响。结果表明：（1）峰前循环剪切加卸载后,当法向应力为2 MPa时,岩石节理峰前剪切刚度增大,当法向应力为4～10 MPa时,岩石节理峰前剪切刚度在循环剪切应力幅值范围内增大,在超出循环剪切应力幅值时减小;（2）峰前循环剪切加卸载后,峰值剪切强度降低了10%～20%,降低百分比随法向应力增大整体呈对数函数增大;峰值剪切位移增加了2%～40%,增加百分比随法向应力增大整体呈对数函数减小;（3）峰前循环剪切加卸载后,岩石节理残余剪切强度无明显变化,峰值剪切强度与残余剪切强度差值减小,峰后剪切应力做功损失百分比降低。     

A Constitutive Model for Shear Behavior of Rock Joints Based on Three-Dimensional Quantification of Joint Roughness

A new constitutive model to describe the shear behavior of rock joints under constant normal stiffness (CNS) and constant normal load (CNL) conditions is proposed. The model was developed using an empirical approach based on the results of a total of 362 direct shear tests on tensile fractured rock joints and replicas of tensile joints and on a new quantitative roughness parameter. This parameter, the active roughness coefficient C _r, is derived from the features of the effective roughness mobilized at the contact areas during shearing. The model involves a shear strength criterion and the relations between stresses and displacements in the normal and shear directions, where the effects of the boundary conditions and joint properties are considered by the shape indices C _d and C _f. The model can be used to predict the shear behavior under CNS as well as CNL conditions. The shear behavior obtained from the experimental results is generally in good agreement with that estimated by the proposed model, and the effects of joint roughness, initial normal stress, and normal stiffness are reasonably reflected in the model.     

循环剪切荷载作用下含二阶起伏体模拟岩石节理力学特性研究

利用人工材料浇注含二阶起伏体的模拟岩石节理试样,进行常法向荷载循环剪切试验,研究节理剪切力学特性在循环剪切过程中的劣化规律。试验结果表明：二阶起伏体对节理循环剪切力学特性有重要影响,剪切强度、剪切刚度、剪胀角随剪切循环次数增大而衰减,衰减趋势随着二阶起伏度的增大而加快;法向应力、二阶起伏度较大时,二阶起伏体对剪切力学特性的影响主要体现在第1轮剪切循环中,在随后的剪切循环中影响不明显;法向应力、二阶起伏度较小时,二阶起伏体的影响在前几轮循环剪切过程中均有较清晰的体现。基于Hertz接触力学理论,提出了节理面微凸体球面接触细观模型,揭示了节理循环剪切宏观试验现象的力学机制     

岩石节理在固定法向压力刚度条件下的剪切理论研究

节理在固定法向压力刚度（CNS）条件下的剪切过程比较复杂,剪切中由于剪胀的存在使得法向压力增大,而法向压力的增加又限制了剪胀的发生。根据CNS剪切过程中某一瞬时状态下法向压应力大小,以节理在该法向压应力下法向剪胀位移与剪切位移的关系为基础,建立一个楔形物理模型。通过循环迭代求得CNS剪切过程中每一步的法向压应力值,进而得到整个过程的剪切应力值。通过该模型,讨论了节理各参数对CNS剪切过程的影响,结果表明,其剪切应力值受法向压应力刚度与节理法向变形参数共同协调控制（包括单轴压缩变形及剪胀角的磨损）,节理在剪切过程中越不易发生压缩变形,其剪切应力值越大。     

An Elasto-Plastic Constitutive Model for Rock Joints Under Cyclic Loading and Constant Normal Stiffness Conditions

An elasto-plastic constitutive model is introduced for rock joints under cyclic loading, considering the additional shear resistance generated by the asperity damage in the first forward shear cycle and sliding mechanism for further shearing. A series of cyclic loading direct shear tests was conducted on artificial joints with triangular asperities and replicas of a real rock asperity surface under constant normal stiffness (CNS) conditions. The model was calibrated and then validated using selected data sets from the experimental results. Model simulations were found to be in good agreement with the rock joints behaviour under cyclic loading and CNS conditions both in stress prediction and dilation behaviour. In addition, dynamic stability analysis of an underground structure was carried out, using Universal Distinct Element Code and the proposed constitutive model.     

A Fractal-Based Model for Fracture Deformation Under Shearing and Compression

This study presents an improved model for calculating mechanical deformation of individual fractures subject to shearing and compression processes. Considering that the fracture-surface roughness can be characterized by fractals, a relationship between asperity angle and size of a fracture with fractal surfaces is derived. In addition, plastic work is employed to account for the effects of fracture-asperity degradation during the shearing process. Based on these developments, a scale-dependent relationship between shear stress and displacement is proposed, and also a formulation to calculate fracture aperture during shearing and compression processes is developed, by relating normal displacement to shear displacement for fractal surfaces. The usefulness of this model is demonstrated by agreements between calculated results and observations from laboratory experiments under different test conditions.     

Laboratory modelling of shear behaviour of soft joints under constant normal stiffness conditions

Shear behaviour of regular sawtooth rock joints produced from casting plaster are investigated under constant normal stiffness (CNS) conditions. Test results obtained in this investigation are also compared with the constant normal load (CNL) tests. It is observed that the peak shear stress obtained under CNL conditions always underestimates the peak shear stress corresponding to the CNS condition. Plots of shear stress against normal stress show that a nonlinear (curved) strength envelope is acceptable for soft rock joints subjected to a CNS condition, in comparison with the linear or bilinear envelopes often proposed for a CNL condition. Models proposed by Patton (1966) and Barton (1973) have also been considered for the predictions of peak shear stress of soft joints under CNS conditions. Although Patton's model is appropriate for low asperity angles, it overestimates the shear strength in the low to medium normal stress range at higher asperity angles. In contrast, while Barton's model is realistic for the CNL condition, it seems to be inappropriate for modelling the shear behaviour of soft joints under CNS conditions. The effect of infill material on the shear behaviour of the model joints is also investigated, and it is found that a small thickness of bentonite infill reduces the peak stress significantly. The peak shear stress almost approached that of the shear strength of infill when the infill thickness to asperity height ratio (t/a) reached 1.40. This paper also introduces an original, empirical shear strength envelope to account for the change in normal stress and surface degradation during CNS shearing. © Rapid Science Ltd. 1998     

Contact mechanism of a rock fracture subjected to normal loading and its impact on fast closure behavior during initial stage of fluid flow experiment

Fast closure of rock fractures has been commonly observed in the initial stage of fluid flow experiments at environmental temperatures under low or moderate normal stresses. To fully understand the mechanisms that drive this fast closure, the evolution of local stresses acting on contacting asperities on the fracture surfaces prior to fluid flow tests needs to be evaluated. In this study, we modeled numerically the asperity deformation and failure processes during initial normal loading, by adopting both elastic and elastic–plastic deformation models for the asperities on a real rock fracture with measured surface topography data, and estimated their impact on initial conditions for fluid flow test performed under laboratory conditions. Compared with the previous models that simulate the normal contact of a fracture as the approach of two rigid surfaces without deformations, our models of deformable asperities yielded smaller contact areas and higher stresses on contacting asperities at a given normal stress or normal displacement. The results show that the calculated local stresses were concentrated on the contacts of a few major asperities, resulting in crushing of asperity tips. With these higher contact stresses, however, the predicted closure rates by pressure solution are still several orders of magnitude lower than that of the experimental measurements at the initial stage of fluid flow test. This indicates that single pressure solution may not likely to be the principal compaction mechanism for this fast closure, and that the damages on contacting asperities that occur during the initial normal loading stage may play an important role. Copyright © 2015 John Wiley & Sons, Ltd.     

法向刚度对土与结构接触面三维特性影响的试验研究

运用80 t大型三维接触面试验机,对不同法向刚度下粗粒土与结构接触面三维静动力学特性进行了试验研究,重点分析了法向刚度对接触面力学特性的影响规律。法向常刚度条件下,接触面在单调剪切时均先剪缩再剪胀、法向应力先减小后增大;循环剪切时接触面不可逆性剪切体变呈单调增长、可逆性剪切体变峰值逐渐减小后趋于稳定,切向应力峰值不断减小,抗剪强度逐渐减小,主应力比峰值则基本保持不变。法向刚度主要影响剪切体变、可逆性剪切体变、切向应力、主剪应力等接触面力学性能参数的大小;法向刚度越大,单调剪切时法向应力变化越大、切向应力峰值越大、剪缩和剪胀量越小;循环剪切时不可逆性剪切体变增长越慢、最终值越小,抗剪强度减小越快、达到0时对应的循环周次越少。法向刚度对剪切体变、可逆性剪切体变、主剪应力、主应力比等性能参数与切向位移的关系形式及接触面摩擦角基本没有影响。     

Physical Modelling of Stress-dependent Permeability in Fractured Rocks

This paper presents the results of laboratory experiments conducted to study the impact of stress on fracture deformation and permeability of fractured rocks. The physical models (laboratory specimens) consisted of steel cubes simulating a rock mass containing three sets of orthogonal fractures. The laboratory specimens were subjected to two or three cycles of hydrostatic loading/unloading followed by the measurement of displacement and permeability. The results show a considerable difference in both deformation and permeability trends between the first loading and the subsequent loading/unloading cycles. However, the micrographs of the contact surfaces taken before and after the tests show that the standard deviation of asperity heights of measured surfaces are affected very little by the loadings. This implies that both deformation and permeability are rather controlled by the highest surface asperities which cannot be picked up by the conventional roughness characterization technique. We found that the dependence of flow rate on mechanical aperture follows a power law with the exponent n smaller or larger than three depending upon the loading stage. Initially, when the maximum height of the asperities is high, the exponent is slightly smaller than 3. The first loading, however, flattens these asperities. After that, the third loading and unloading yielded the exponent of around 4. Due to the roughness of contact surfaces, the flow route is no longer straight but tortuous resulting in flow length increase.     

陇西黄土三轴剪切过程微观变化研究

为了解黄土三轴剪切过程中的微观变化,采用PFC3D建立黄土三轴试验模型,模拟围压分别为0、50、150、300 kPa的三轴剪切试验,并与室内试验进行对比分析。分析结果表明：PFC3D能够较好地模拟出不同围压下从开始到破坏到残余变形整个过程中应力-应变的变化规律,且位移场及接触应力场的变化规律与室内三轴试验宏观现象较一致;发现弹性模量、泊松比及峰值强度与数值模型中微观参数有着密切的联系,如法向刚度kn控制试样宏观弹性模量,kn /ks值控制泊松比,摩擦系数控制峰值强度;通过体应变-轴向应变曲线发现,随着围压的增加应变能增大,试样呈现出由体积膨胀到体积减缩的变化规律。其研究结果为进一步探究黄土的应力-应变性状及抗剪强度特性提供参考。     

The Influence of Shearing Velocity on Shear Behavior of Artificial Joints

In this paper, the effects of shear velocity on the shearing behavior of artificial joints have been studied at different normal stress levels. Here, artificial joints with planar and rough surfaces were prepared with the plaster (simulating soft rock joints) and concrete (medium-hard rock joints) materials. The rough joints had triangular shaped asperities with 10° and 20° inclination angles. Direct shear tests were performed on these joints under various shear velocities in the range of 0.3–30 mm/min. The planar plaster–plaster and planer concrete–concrete joints were sheared at three levels of normal stress under constant normal load boundary condition. Also, the rough plaster–plaster and concrete–concrete joints were sheared at one level of normal stress under constant normal stiffness boundary condition. The results of the shear tests show that the shearing parameters of joints, such as shear strength, shear stiffness and friction angle, are related to the shear velocity. Shear strength of planar and rough plaster–plaster joints were decreased when the shear velocity was increased. Shear strength of concrete joints, except for rough joints with 10° inclination, increased with increasing shear velocity. Regardless of the normal stress level, shear stiffness of both planar plaster–plaster and concrete–concrete joints were decreased when the shear velocity was increased.     

充填物性质影响结构面剪切特性试验研究

通过自主研发的煤岩剪切-渗流耦合试验装置,开展了无充填结构面和充填石膏、岩屑、黄泥结构面的剪切试验。利用三维光学扫描技术,研究了结构面的三维形貌特征和裂隙开度演化。结果表明：充填结构面的峰值剪应力和法向位移由充填石膏、岩屑到黄泥依次递减,受充填物强度影响较大;无充填结构面由于直接接触,剪切后表面发生明显磨损,结构面粗糙度系数（JRC）整体减小且变化最为显著,充填石膏和岩屑结构面的JRC变化次之,充填黄泥结构面几乎没有变化;充填物破坏和结构面磨损主要受裂隙开度演化和局部应力集中的影响。随着剪切位移的增大,在裂隙开度较小的区域,应力分布较大,充填物易沿此处破碎,无充填结构面沿此处严重磨损。     

剪胀对复杂裂隙岩体渗透性影响的离散元分析

裂隙岩体流固耦合问题是目前国内外研究热点之一,采用离散元软件UDEC对裂隙岩体发生节理剪胀的渗透性变化规律进行了模拟分析。基于现场调查的裂隙信息统计生成裂隙网络岩体模型。 通过固定垂直应力、不断增加应力比RS(RS=水平应力/垂直应力)使岩体出现剪胀,采用库伦滑移节理模式对岩体在剪胀过程中的渗透性变化情况进行模拟。结果发现:当应力比较小(RS3.1)时,节理水力隙宽、流速、渗透系数等参数都随着应力比的增加表现出明显的降低; 而当岩体出现剪胀现象之后(应力比大于3.1),发生剪切滑移和剪胀现象的节理控制着裂隙岩体的总体渗流行为,与不考虑节理剪胀的计算结果相比,岩体渗透能力出现了显著增长。这一结果表明,剪胀对裂隙岩体渗透性的影响是显著而不可忽视的。