Experimental Study of Brittle Behavior of Clay Shale in Rapid Triaxial Compression

The brittle failure behavior of an over-consolidated clay shale (Opalinus Clay) in undrained rapid triaxial compression was studied. The confining stress levels were chosen to simulate the range of confining stresses relevant for underground excavations at the Mont Terri Underground Research Laboratory, and to investigate the transition from axial splitting failure to macroscopic shear failure. Micro-crack initiation was observed throughout the confining stress range utilized in this study at a differential stress of 2.1 MPa on average, which indicates that friction was not mobilized at this stage of brittle failure. The rupture stress was dependent on confinement indicating friction mobilization during the brittle failure process. With increasing confinement net volumetric strain decreased suggesting that dilation was suppressed, which is possibly related to a change in the failure mode. At confining stress levels ≤0.5 MPa specimen rupture was associated with axial splitting. With increasing confinement, transition to a macroscopic shearing mode was observed. Multi-stage triaxial tests consistently showed lower strengths than single-stage tests, demonstrating cumulative damage in the specimens. Both the Mohr–Coulomb and Hoek–Brown failure criteria could not satisfactorily fit the data over the entire confining stress range. A bi-linear or S-shaped failure criterion was found to satisfactorily fit the test data over the entire confinement range studied.     

Laboratory and In Situ Simulation Tests of the Excavation Damaged Zone Around Galleries in Opalinus Clay

In the context of nuclear waste disposal in clay formations, laboratory and in situ simulation experiments were performed to study at reduced scale the excavation damaged zone (EDZ) around tunnels in the indurated Opalinus Clay at Mont Terri, Switzerland. In the laboratory, thick-walled hollow cylindrical specimens were subjected to a mechanical unloading mimicking a gallery excavation. In samples cored parallel to bedding, cracks sub-parallel to the bedding planes open and lead to a buckling failure in two regions that extend from the borehole in the direction normal to bedding. The behaviour is clearly anisotropic. On the other hand, in experiments performed on specimens cored perpendicular to bedding, there is no indication of failure around the hole and the response of the hollow cylinder sample is mainly isotropic. The in situ experiment at Mont Terri which consisted in the overcoring of a resin-injected borehole that follows the bedding strike of the Opalinus Clay showed a striking similarity between the induced damaged zone and the fracture pattern observed in the hollow cylinder tests on samples cored parallel to bedding and such a bedding controlled “Excavation” Damaged Zone is as well consistent with the distinct fracture patterns observed at Mont Terri depending on the orientation of holes/galleries with respect to the bedding planes. Interestingly, the damaged zone observed in the hollow cylinder tests on samples cored parallel to bedding and in situ around URL galleries is found to develop in reverse directions in Boom Clay (Mol) and in Opalinus Clay (Mont Terri). This most probably results from different failure mechanisms, i.e. shear failure along conjugated planes in the plastic Boom Clay, but bedding plane splitting and buckling in the indurated Opalinus Clay.     

三维裂纹布置角度对砂浆材料强度的影响

利用伺服控制试验系统及声发射设备对含有单椭圆裂纹的非均质砂浆材料进行物理试验,获得了单轴加载作用下不同布置角度的三维内部裂纹试样的全应力-应变曲线和声发射数据。找到了声发射数据与应力-应变曲线的对应关系。根据对应关系,判断出试样的起裂应力及破坏应力。分析了裂纹布置角度对材料强度的影响,分析表明：裂纹与加载方向为45º夹角时最难破坏,与加载方向成60º夹角时最容易破坏。此外,裂纹尖端起裂应力为试样极限强度的86.07 %～87.42 %。     

Influence of Sample Height-to-Width Ratios on Failure Mode for Rectangular Prism Samples of Hard Rock Loaded In Uniaxial Compression

Surface-parallel slabbing is a failure mode often observed in highly stressed hard rocks in underground excavations. This paper presents the results of experimental studies on slabbing failure of hard rock with different sample height-to-width ratios. The main purpose of this study was to find out the condition to create slabbing failure under uniaxial compression and to determine the slabbing strength of hard rock in the laboratory. Uniaxial compression tests were carried out using five groups of granite specimens. The mechanical parameters of the sample rock, Iddefjord granite from Norway, were measured on the cylindrical and Brazilian disc specimens. The transition of the failure mode was studied using rectangular prism specimens. The initiation and the propagation of slabbing fractures in specimens were identified by examining the relationship among the applied stress, strain and the acoustic emission. The stress thresholds identified were compared to those reported by other authors for crack initiation and brittle failure. It is observed that the macro failure mode will be transformed from shear to slabbing when the height/width ratio is reduced to 0.5 in the prism specimens under uniaxial compression. Micro σ ₁-parallel fractures initiate when the lateral strain departs from its linearity. Slabbing fractures are approximately parallel to the loading direction. Labotatory tests show that the slabbing strength (σ _sl) of hard rock is about 60% of its uniaxial compression strength. It means that if the maximum tangential stress surrounding an underground excavation reaches about the slabbing threshold, slabbing fractures may take place on the boundary of the excavation. Therefore, the best way to stop or eliminate slabbing failure is to control the excavation boundary to avoid the big stress concentration, so that the maximum tangential stress could be under the slabbing threshold.     

Crack Initiation and Crack Propagation in Heterogeneous Sulfate-Rich Clay Rocks

Brittle fracture processes were hypothesized by several researches to cause a damage zone around an underground excavation in sulfate-rich clay rock when the stress exceeds the crack initiation threshold, and may promote swelling by crystal growth in newly formed fractures. In this study, laboratory experiments such as unconfined and confined compression tests with acoustic emission monitoring, and microstructural and mineralogical analyses are used to explain brittle fracture processes in sulfate-rich clay rock from the Gipskeuper formation in Switzerland. This rock type typically shows a heterogeneous rock fabric consisting of distinct clayey layers and stiff heterogeneities such as anhydrite layers, veins or nodules. The study showed that at low deviatoric stress, the failure behavior is dominated by the strength of the clayey matrix where microcracks are initiated. With increasing deviatoric stress or strain, growing microcracks eventually are arrested at anhydrite veins, and cracks develop either aligned with the interface between clayey layers and anhydrite veins, or penetrate anhydrite veins. These cracks often link micro-fractured regions in the specimen. This study also suggest that fracture localization in sulfate-rich clay rocks, which typically show a heterogeneous rock fabric, does not take place in the pre-peak range and renders unstable crack propagation less likely. Sulfate-rich clay rocks typically contain anhydrite veins at various scales. At the scale of a tunnel, anhydrite layers or veins may arrest growing fractures and prevent the disintegration of the rock mass. The rock mass may be damaged when the threshold stress for microcrack initiation is exceeded, thus promoting swelling by crystal growth in extension fractures, but the self-supporting capacity of the rock mass may be maintained rendering the possibility for rapidly propagating instability less likely.     

Analysis of Fracture Mechanics Tests on Opalinus Clay

Many studies have recently been conducted to evaluate various mechanical characteristics of the Opalinus Clay (OPA) formation in view of its potential use as the hosting rock for the Swiss nuclear waste repositories. Its sedimentary bedding makes OPA a transversely isotropic rock and its directional mechanical properties need to be measured. This paper reports on an experimental and computational approach that was adopted to define the parallel-to-bedding fracture mechanics (FM) parameters of OPA in Mode-I. OPA cores from Mont Terri Underground Research Laboratory (URL) were submitted to laboratory tests on notched semi-circular specimens under three-point bending (SCB). In these tests, crack propagation is forced along the notch direction. However, the 45° bedding inclination of the specimen axis frequently deviated the crack from the expected direction. An analysis of the SCB tests was performed by means of non-linear FM techniques and the pertinent Mode-I parameters along the bedding were estimated.     

干湿交替作用后砂岩破裂过程实时观测与分析

采用岩石破裂全过程的细观力学试验系统进行了天然状态及干湿交替作用后的完整和含预制圆孔砂岩试件的单轴压缩破裂试验。利用显微镜观察了砂岩裂纹萌生、扩展以及试件的失稳破坏过程。经过干湿交替作用后岩样更容易在较低的应力状态下发生裂纹的扩展,并且裂纹扩展的初始阶段一般表现为拉破裂。完整试件的裂纹是随机性的三维扩展,而有预制圆孔试件一般是从预制圆孔周边开始扩展。天然状态下的预制圆孔试件在起裂时从圆孔的压应力集中区产生剪切裂纹;而干湿交替作用后,起裂时可能先从拉应力集中区产生拉裂纹,并且裂纹演化过程更为复杂一些,破裂形式多样化。与天然状态相比,经过干湿循环作用后的砂岩破坏脆性降低,强度等力学参数也随干湿交替次数增加而逐渐降低。     

Crack propagation trajectories for rocks under mixed mode I-II fracture

Propagation of a crack in engineering materials including rocks can cause failure. Knowledge of the stress state under which a crack can propagate, and the trajectory it may follow during its growth are thus very important for the stability of rock masses/materials and for the safe design of structures in/on rocks. In this paper, the crack initiation angle and subsequent crack propagation path are experimentally investigated for limestone rock specimens. This investigation was conducted under various mixed mode I-II loading conditions, including pure mode-I and pure mode-II.This study includes conducting diametrical compression tests on notched Brazilian disk specimens. Moreover, the effect of confining pressure and temperature on crack initiation and propagation were also studied. The experimental results were compared with theoretical predictions of crack initiation angle. The results showed that limestone behaves in brittle fashion, and the effects of confining pressure and temperature on failure trajectories were not significant. Generally, the crack initiation angle can be predicted by the maximum tangential stress criterion. However, for notched Brazilian disk with high value of crack orientation with respect to loading direction, crack does not propagate from the tip of the crack. This important observation indicated that the tensile-strength failure can become more critical than the fracture-toughness failure.     

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。     

层理面特性对砂岩断裂力学行为的影响研究

层理弱面对层状岩石的力学特性影响较显著，为了研究层理面特性对岩石断裂力学特性的影响，开展了具有不同层理方向的砂岩试样三点弯试验，探讨了砂岩断裂韧度及断裂模式的各向异性。之后基于有限元中的黏聚单元建立了数值模型，采用数值模拟方法研究了层理面强度对各层理角度试样断裂力学行为的影响规律。结果表明：层理方向影响下砂岩的断裂韧度及模式存在各向异性；同一层理方向试样的断裂韧度随层理面强度的增大而增大，且试样的层理面与加载方向夹角越小，断裂韧度受层理面强度变化影响越明显；试样的断裂模式不仅与层理面强度有关，还受层理倾角的控制，层理面与加载方向夹角θ = 0o试样断裂模式基本不受层理面强度影响，θ = 30o试样主要沿层理面张拉或剪切破坏，且沿层理面的破裂长度随层理面强度的降低逐渐增大；层理面强度较大时，θ = 45o试样主要沿层理面张拉破坏，θ = 60o～90o试样主要以贯穿层理的张拉破坏为主；层理面强度较小时，θ = 45o～90o试样均以沿层理面的剪切破坏为主，其中θ = 45o试样沿层理剪切长度最大。另外，通过数值模拟结果分析了层理面强度及方向对试样的起裂角及裂纹扩展路径产生的影响。该研究成果可作为层状岩石断裂力学理论的有益补充。     

Discrete element model for quasi‐brittle rupture under tensile and compressive loading

In the absence of initial cracks, the material behavior is limited by its strength, usually defined in homogeneous conditions (of stress and strain). Beyond this limit, in quasi‐brittle case, cracks may propagate and the material behavior tends to be well described by fracture mechanics. Discrete element approaches show consistent results dealing with this transition during rupture. However, the calibration of the parameters of the numerical models (i.e., stiffness, strength, and toughness) may be quite complex and sometimes only approximative. Based on a brittle rupture criterion, we analyze the biaxial response of uncracked samples. Thus, tensile and compressive strengths are analytically identified and become direct parameters of our discrete model. Furthermore, a physically reliable crack initiation (and subsequent propagation) is shown to be induced during rupture and verified by the simulation of three‐point bending and diametral compression tests. Copyright © 2016 John Wiley & Sons, Ltd.     

Mechanical Behaviour of Reservoir Rock Under Brine Saturation

Acoustic emissions (AE) and stress–strain curve analysis are well accepted ways of analysing crack propagation and monitoring the various failure stages (such as crack closure, crack initiation level during rock failure under compression) of rocks and rock-like materials. This paper presents details and results of experimental investigations conducted for characterizing the brittle failure processes induced in a rock due to monocyclic uniaxial compression on loading of two types of sandstone core samples saturated in NaCl brines of varying concentration (0, 2, 5, 10 and 15 % NaCl by weight). The two types of sandstone samples were saturated under vacuum for more than 45 days with the respective pore fluid to allow them to interact with the rocks. It was observed that the uniaxial compressive strength and stress–strain behaviour of the rock specimens changed with increasing NaCl concentration in the saturating fluid. The acoustic emission patterns also varied considerably for increasing ionic strength of the saturating brines. These observations can be attributed to the deposition of NaCl crystals in the rock’s pore spaces as well some minor geo-chemical interactions between the rock minerals and the brine. The AE pattern variations could also be partly related to the higher conductivity of the ionic strength of the high-NaCl concentration brine as it is able to transfer more acoustic energy from the cracks to the AE sensors.     

An Experimental Study of the Fracture Coalescence Behaviour of Brittle Sandstone Specimens Containing Three Fissures

To analyse the fracture coalescence behaviour of rock, rectangular prismatic sandstone specimens (80?×?160?×?30?mm in size) containing three fissures were tested under uniaxial compression. The strength and deformation behaviours of the specimens are first analysed by investigating the effects of the ligament angle β₂ on the peak strength, peak strain and crack initiation stress of the specimens. To confirm the sequence of crack coalescence, a photographic monitoring technique is used throughout the entire period of deformation. Based on the results, the relationship between the real-time crack coalescence process and the axial stress–strain curve of brittle sandstone specimens is also developed, and this relationship can be used to evaluate the macroscopic deformation characteristics of pre-cracked rock. The equivalent strain evolution fields of the specimen, with α?=?β₁?=?45° and β₂?=?90°, are obtained using the digital image correlation technique and show good agreement with the experimental results of pre-cracked brittle sandstone. These experimental results are expected to improve the understanding of fracture mechanisms and be used in rock engineering with intermittent structures, such as deep underground excavated tunnels.     

不同类型储层岩石三轴压缩变形破裂与声发射特征研究

随着页岩气开采、废水回注和CO₂地质封存工程活动的进行,储层应力环境改变诱发地震的问题得到广泛关注,研究储层岩石变形破裂和声发射规律对于理解诱发地震活动具有重要意义。本文选取了页岩、致密砂岩和白云岩开展三轴压缩声发射试验,获取了空间裂缝形态,揭示了不同类型储层岩石的变形破裂过程和声发射特征。结果表明：(1)页岩的扩容应力与峰值应力比最高,其次为白云岩和致密砂岩,表明页岩主要发生脆性破坏。(2)岩石结构对裂缝扩展和强度具有明显影响,页岩层理发育,抗压强度和裂缝形态各向异性显著。当层理角度为0°时,试样发生剪切-拉张复合型破坏。当层理角度为30°和60°时,试样主要发生剪切破坏。当层理角度增大至90°时,试样主要发生拉张破坏。致密砂岩破裂形成剪切主裂缝,白云岩形成两条剪切主裂缝和微裂缝。(3)不同类型储层岩石破裂过程声发射特征差异显著。页岩在扩容应力点附近有少量声发射活动,达到峰值应力时,声发射活动迅速增强。致密砂岩仅在破裂瞬间有少量声发射活动。相比之下,白云岩在裂缝非稳定扩展阶段和峰后阶段声发射活动显著。因此,在工程实践中需要根据微震监测调整施工措施,避免页岩作为储...     

一种测量岩石压缩裂纹扩展过程声波波速的连续测量方法

中低围压或单轴压缩荷载条件下,可以将脆性岩石材料的裂纹扩展过程划分为原生裂纹压密、新生裂纹起裂并稳定扩展、不稳定裂纹扩展和交互贯通以及峰后5个主要发展阶段。含预制裂纹试样的直接观测法、应变监测、声发射（AE）监测、声波波速测试、CT（computerized tomography）扫描、微电镜观察等间接监测方法被用来研究裂纹起裂和扩展过程。本文基于在中、低围压以及单轴压缩荷载条件下,脆性岩石材料首先产生张拉裂纹,并优先沿加载方向发育的规律,提出采用声波波速连续测量方法进行岩石加载过程波速的连续观测,有助于分析裂纹扩展过程中声波波速的变化,为研究裂纹扩展过程提供一种间接测量手段。     

TBM滚刀贯入过程中泥岩破坏特征试验研究

为研究无侧限和有侧限条件下软岩贯入破坏特征,结合声发射检测技术和电镜扫描试验进行泥岩立方体试样常截面滚刀贯入试验,分析试验过程中荷载-贯入度曲线和声发射参数特征,并对岩石渣块断口进行细观电镜扫描,研究了破坏时细观和宏观裂纹形成过程。试验结果表明,（1）无侧限和有侧限贯入试验中荷载-贯入度曲线在跃进荷载处均出现明显峰值,跃进荷载峰值以后无侧限试样完全破坏,有侧限试样尚未完全破坏;（2）当宏观裂纹产生时无侧限试样声发射参数出现明显峰值,主要源于该宏观裂纹在试样内部的扩展。有侧限试样破坏过程中出现多次声发射参数峰值,对应多条宏观裂纹形成和扩展;（3）常截面滚刀贯入过程中试样内部出现微滑动,剪切破坏现象比较明显,可以认为滚刀贯入过程中泥岩破坏模式以剪切破坏为主。     

Influence of inclination angles and confining pressures on mechanical behavior of rock materials containing a preexisting crack

To deeply understand the cracking mechanical behavior of brittle rock materials, numerical simulations of a rock specimen containing a single preexisting crack were carried out by the expanded distinct element method (EDEM). Based on the analysis of crack tips and a comparison between stress- and strain-based methods, the strain strength criterion was adopted in the numerical models to simulate the crack initiation and propagation processes under uniaxial and biaxial compression. The simulation results indicated that the crack inclination angle and confining pressure had a great influence on the tensile and shear properties, peak strength, and failure behaviors, which also showed a good agreement with the experimental results. If the specimen was under uniaxial compression, it was found that the initiation stress and peak strength first decreased and then increased with an increasing inclination angle α. Regardless of the size of α, tensile cracks initiated prior to shear cracks. If α was small (such as α ≤ 30°), the tensile cracks dominated the specimen failure, the wing cracks propagated towards the direction of uniaxial compression, and the propagation of shear cracks was inhibited by the high concentration of tensile stress. In contrast, if α was large (such as α ≥ 45°), mixed cracks dominated the specimen failure, and the external loading favored the further propagation of shear cracks. Analyzing the numerical results of the specimen with a 45° inclination angle under biaxial compression, it was revealed that lateral confinement had a significant influence on the initiation sequence and the mechanical properties of new cracks.     

基于巴西劈裂试验的页岩强度与破坏模式研究

为了研究页岩横观各向同性性质对劈裂强度及破坏模式的影响,对7组不同倾角下的试样进行了巴西劈裂和声发射测试。研究表明：层理的方向对试样的劈裂强度和最终破坏模式有着重要的影响。随着层面倾角 的增加,页岩试样的劈裂强度逐渐减小。在不同倾角下的破坏模式可分为直线形（ 0°、90°）、月牙形（ 15°）和曲弧形（ 30°、45°、60°、75°）。通过分析不同倾角下圆盘中心点沿层理分解得到的正应力和剪应力,当 0°时,试样的破坏方式为基质的拉伸破坏;当 90°时,试样的破坏方式为层理的拉伸破坏,当15° 75°时,试样的破坏为复合型破坏。观察声发射空间累计定位演化分布情况,直线型的声发射点较均匀地分布在试样中轴线的两侧;曲弧型的声发射点在圆盘的一端较密而另一端较稀疏;月牙型则在试样的中部和一端都累积了部分声发射点;分布特征与宏观破坏方式具有相似性。分形维数D可以较好地反映试样的破坏模式,从倒V形的变化趋势可知,直线形试样D值接近于2,而月牙形和曲弧形试样D值介于2.2～2.7。因此,D值越大,说明裂纹的曲率半径越大;相反D值越小,裂纹越趋于一条直线,破裂形态也就越简单。     

A relationship between tensile strength and loading stress governing the onset of mode I crack propagation obtained via numerical investigations using a bonded particle model

Numerical models based on the discrete element method are used to study the fracturing process in brittle rock‐like materials under direct and indirect tension. The results demonstrate the capacity of the model to capture the essential characteristics of fracture including the onset of crack propagation, stable and unstable crack growth, arrest and reinitiation of fracturing, and crack branching. Simulations of Brazilian indirect tension tests serve to calibrate the numerical model, relating macroscopic tensile strength of specimens to their micromechanical breakage parameters. A second suite of simulations reveals a linear relationship between the tensile strength of specimens and the loading stress for which mode I tensile crack propagation ensues. Based on these results, a crack initiation criterion for brittle materials is proposed, prescribing the stressing conditions required to induce tensile failure. Such a criterion, if broadly applicable, provides a practical means to rapidly assess the failure potential of brittle materials under tensile loads.