Experimental and numerical analyses of an opening in a jointed rock mass under biaxial compression

Tunnel construction in a rock mass produces damage around the tunnel by concentration of in situ stress and by construction activity such as blasting. The generated damage changes the mechanical and hydraulic properties of the rock mass. In this study, the rock fracture and joint sliding behaviors of jointed rock masses with an opening under biaxial compression are investigated through experimental and numerical analyses. The tested rock models have a persistent joint set with dip angles of 30°, 45°, and 60° to the horizontal. Under the applied biaxial compression, tensile crack initiation and propagation are the dominant fracture behaviors around the hole in a low joint dip angle rock model (i.e., 30° to the horizontal). The propagation direction of the tensile cracks is roughly normal to the joint surface, and with propagation of tensile cracks, removable rock blocks are generated. The experimental results are simulated using a discrete element code. The numerical analysis simulates several aspects of rock mass cracking and the joint sliding processes around an opening: progressive fracture behaviors in a low joint angle rock model, abrupt initiation and propagation of tensile cracks and joint sliding in a high joint angle rock model (i.e., 60° to the horizontal), propagation of tensile cracks normal to the joint surface, generation of removable blocks in rock segments, an increase of lower hoop stress threshold inducing tensile fractures with a decrease in the joint angle, and an increase of the damage zone around the hole with a decrease in the joint angle.     

Estimation of cracking and damage mechanisms in rock under triaxial compression by moment tensor analysis of acoustic emission

In highly stressed conditions, the excavation damaged zone induced by stress redistribution and disturbance must be evaluated after tunnel excavation. Therefore, the deformation and fracture characteristics of rock must be investigated. In this study, the fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by moment tensor analysis, as well as by the moving point regression technique, both of which were applied to acoustic emission (AE) and strain data obtained from triaxial compression tests. Damage thresholds before the peak strength of rock under triaxial compression were determined by the moving point regression technique using acoustic emission data. The results showed that damage thresholds, except the crack closure stress, increased linearly with confining pressure. The results of the moment tensor analysis showed that shear failure was a major microscopic failure mechanism of rock under triaxial compression. In addition, shear failure became more dominant as the confining pressure increased. In this analysis, the expression of the damage magnitudes in each AE source as relative crack volumes leads to accurate prediction of macroscopic failure mechanisms, as well as major failure planes in the rock. In addition, the orientation of the macroscopic failure plane could be estimated by the orientational distribution of microcracks.     

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。     

Acoustic emission characterization of microcracking in laboratory-scale hydraulic fracturing tests

Understanding microcracking near coalesced fracture generation is critically important for hydrocarbon and geothermal reservoir characterization as well as damage evaluation in civil engineering structures. Dense and sometimes random microcracking near coalesced fracture formation alters the mechanical properties of the nearby virgin material. Individual microcrack characterization is also significant in quantifying the material changes near the fracture faces (i.e. damage). Acoustic emission (AE) monitoring and analysis provide unique information regarding the microcracking process temporally, and information concerning the source characterization of individual microcracks can be extracted. In this context, laboratory hydraulic fracture tests were carried out while monitoring the AEs from several piezoelectric transducers. In-depth post-processing of the AE event data was performed for the purpose of understanding the individual source mechanisms. Several source characterization techniques including moment tensor inversion, event parametric analysis, and volumetric deformation analysis were adopted. Post-test fracture characterization through coring, slicing and micro-computed tomographic imaging was performed to determine the coalesced fracture location and structure. Distinct differences in fracture characteristics were found spatially in relation to the openhole injection interval. Individual microcrack AE analysis showed substantial energy reduction emanating spatially from the injection interval. It was quantitatively observed that the recorded AE signals provided sufficient information to generalize the damage radiating spatially away from the injection wellbore.     

单轴压缩砂岩细观裂纹动态演化特征试验研究

岩石细观裂纹的动态演化特征作为岩石破坏的重要信息,其研究对于分析岩爆的孕育过程及预测岩石动力灾害发生有着重要意义。采用应力分析和声发射参数方法,研究了砂岩在单轴压缩条件下的细观裂纹的强度动态演化特征。试验结果表明,应力与砂岩细观裂纹扩展诱发声发射（AE）事件的强度特征有较好的阶段性变化规律,将峰前裂纹动态演化划分为三个阶段。进一步分析裂纹的类型特征,提出三阶段四维演化过程分析方法。细观裂纹多为张拉型,随着时间的增加,细观裂纹向剪切型裂纹转变,裂纹数量迅速增加,裂纹强度逐渐增大。提出将第三阶段AE事件出现的高强度、高RA、低AF特征作为砂岩破裂失稳的定性预警条件。通过矩张量反演对比分析了的细观裂纹数量和类型的动态演化。     

Simulation of the rock microfracturing process under uniaxial compression using an elasto-plastic cellular automaton

In this paper, an elasto-plastic cellular automaton (EPCA) with the associated code was developed to simulate the non-linear fracturing process of rocks under uniaxial compression. It is a useful method for simulating the process of self-organization of the complex system using simple rules. It has the advantages of localization, parallelization, and being able to consider the heterogeneity of rocks. By this means, the fracturing processes, stress–strain curves, acoustic emissions, and compressive strength of 2-D heterogeneous rock specimens under uniaxial compression were numerically investigated using the EPCA code for different cases such as: (1) the influence of the cyclic load; (2) the influence of inhomogeneity; (3) the effect of different softening coefficients; (4) the effect of specimen sizes and height/width ratios; (5) the effect of different spatial distributions of material parameters; (6) the influence of different yield criteria; and (7) the influence of the residual strength coefficient. The results indicate that the numerical simulation reproduced some of the well-known phenomena observed by previous researchers in uniaxial compression tests.     

Crack initiation of granite under uniaxial compression tests: A comparison study

In this study, a combination of acoustic emission (AE) method (AEM) and wave transmission method (WTM) is used to investigate the behaviors of AE and ultrasonic properties corresponding to initial fracturing in granitic rocks. The relationships of AE characteristics, frequency spectra, and spatial locations with crack initiation (CI) are studied. The anisotropic ultrasonic characteristics, velocity distributions in different ray paths, wave amplitudes, and spectral characters of transmitted waves are investigated. To identify CI stress, damage initiations characterized by strain-based method (SBM), AEM and WTM are compared. For granite samples, it shows that the ratio of CI stress to peak strength estimated by SBM ranges from 0.4 to 0.55, and 0.49–0.6 by WTM, which are higher than that of AEM (0.38–0.46). The CI stress identified by AEM indicates the onset of microcracking, and the combination of AEM and WTM provides an insight into the detection of rock damage initiation and anisotropy.     

不同围压下断续预制裂隙大理岩扩容特性试验研究

基于在伺服试验机上获得的不同围压下断续预制裂隙大理岩体积应变-轴向应变全程曲线,分析了围压对断续预制裂隙大理岩扩容特性的影响规律。结果表明,完整和断续预制裂隙大理岩裂纹损伤阈值随着围压的增加均近似呈线性增大,而且裂纹损伤阈值对围压的敏感性低于峰值强度;同等围压下,粒径较小的中晶大理岩具有较高的裂纹损伤阈值;完整或断续预制裂隙大理岩的长期内摩擦角均显著低于瞬时内摩擦角值,但长期黏聚力的降低幅度与裂隙分布密切相关;完整或断续预制裂隙大理岩的峰值强度越高,相应的裂纹损伤阈值也越大,且两者之间具有较好的线性关系。研究结论对于岩石工程设计和数值分析时断续节理裂隙岩体力学参数选取具有一定的参考价值。     

定侧压混凝土双轴抗压疲劳变形特性试验研究

本文通过自行改造的MTS疲劳试验机对混凝土在定侧压下等幅和变幅抗压疲劳的变形性能进行了试验研究。根据试验实测的混凝土循环应力-应变数据,分析了混凝土双轴受压疲劳应变、刚度与超声波速的衰减规律,提出了与侧压水平相关的疲劳第二阶段应变率和混凝土疲劳寿命之间的关系式。根据试验得到的试件疲劳破坏形态分析了定侧压下混凝土双轴受压疲劳破坏的机理。试验结果表明:定侧压下混凝土受压疲劳破坏形态与侧压水平相关,而且疲劳应变发展仍符合三阶段规律,第二阶段应变率与疲劳寿命之间也具有很好的相关性。     

三轴压缩大理岩局部化变形的试验研究及其分岔行为

常规三轴压缩过程 ,岩石试件破坏常常是以狭窄剪切带形式存在的局部化变形为先导。结合常规三轴压缩试验 ,对大理岩破坏过程中局部化变形的发展过程进行研究 ,其结果表明 :岩石局部化变形分为三个阶段 ,即稳定阶段、稳定增长阶段和快速发展阶段 ;局部化变形在体积膨胀点之前开始稳定增长 ;达到一定程度后 ,岩石表现出短暂的应变硬化 ,岩石开始剪胀。最后应用对称性群论分岔理论对此过程中的滑移线等分岔现象进行了初步研究     

Microcrack fracturing of coal specimens under quasi-static combined compression-shear loading

Coal pillars are usually loaded under combined compression-shear stresses at underground coal mines. Their long-term stability is critical to the utilization of underground structures, such as underground reservoirs at coal mines. In this study, a modified rock property testing system was used to explore the mechanical properties of coal specimens under quasi-static combined compression-shear loading conditions. The acoustic emission technique was applied to investigating the microcrack fracturing of coal specimens at various inclination angles. The experimental results show that specimen inclination has remarkable effects on the microcrack initiation, microcrack damage and ultimate failure of the coal specimen. The failure mode of the coal specimen tends to transit from axial splitting to shear failure with increasing specimen inclination, and its peak strength is closely associated with the microcrack damage threshold. In practice, it is recommended to consider coal strength under combined compression-shear loading when using empirical pillar strength formulae so that the effect of pillar inclination can be included.     

Experimental investigations on effects of gas pressure on mechanical behaviors and failure characteristic of coals

The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction. In this study, triaxial compression and seepage tests were carried out on coal at different gas pressures. The mechanical properties and failure process of coal were studied, as well as the acoustic emission (AE) and strain energy. The influence of gas pressure on the mechanical parameters of this coal was analyzed. Based on the conventional energy calculation formula, the pore pressure was introduced through the effective stress formula, and each energy component of coal containing gas was refined innovatively. The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described. Finally, the influence of gas pressure on coal strength was theoretically analyzed from the perspectives of Mohr–Coulomb criterion and fracture mechanics. The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction (positive) and in the confining pressure direction (negative), as well as that induced by the pore pressure (initially negative and then positive). The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal. The quiet period of AE in the initial stage shortens, and AE activity increases during the pre-peak stage under high gas pressure. The fractal characteristics of AE in three stages are studied using the correlation dimension. The AE process has different forms of self-similarity in various deformation stages.     

不同围压下岩石声发射不可逆性及其主破裂前特征信息试验研究

 通过对花岗岩在不同围压下循环加卸载声发射(AE)试验,得到岩石加卸载损伤破坏过程中高、低频通道中AE累计振铃计数、岩石应力与时间的关系。基于此,研究岩石AE的不可逆性特征。同时运用快速傅里叶变换FFT逆变换对Kaiser点AE信号进行消噪,并通过FFT分析消噪后信号的频谱特征,探求岩石主破裂前特征信息。研究结果表明：(1) 两通道中接收到的AE振铃计数整体变化趋势基本相同,所揭示的Kaiser效应和Felicity效应规律基本一致;两通道中AE振铃计数特征主要区别在于数量不同;(2) Kaiser点主频分布在46.39～70.80与151.37～166.99 kHz范围内。岩石主破裂前,随轴向应力水平增加,低频通道中Kaiser点主频整体变化趋势由较低频向较高频转移,高频通道中由较高频向较低频转移;(3) 花岗岩Kaiser效应应力上限值为极限强度的65%左右。Kaiser点的主频特征及变化规律,可为岩石的损伤破坏评价提供依据。     

基于EMD的粉砂岩单轴压缩破裂损伤分析

通过RMT-150C岩石力学测试系统与SAEU2S型声发射仪器,对粉砂岩进行了单轴压缩声发射试验。运用荷载—时间—振铃累计计数关系对岩样损伤阶段进行划分,利用经验模态分解法(EMD)对不同损伤阶段的典型信号源进行了分解,并结合EMD能量熵分析声发射信号能量分布,进而对岩体损伤阶段进行描述。结果表明,粉砂岩单轴压缩破裂可分为起始损伤期、损伤发展期、损伤加剧期、试件屈服期及试件断裂层剥离期五个阶段,其EMD能量熵主要能量所占优势频率分布情况呈现先往高频发展、再往低频发展的趋势。利用EMD能量熵能有效分辨岩样损伤过程中的声发射源信号特征,为评估岩体损伤变化奠定了基础。     

混凝土梁破坏机制的声发射特性实验研究

记录了钢纤维混凝土,素混凝土和砂浆试件断裂过程中的声发射（AE）信号,并对其进行了分析和处理,通过对声发射信号持续时间设置滤值,以声发射信号的振幅分布特性为判据,并依据事件峰值对应的振幅大小,将它们的声发射信号持续时间分别划分为9个,7个和5个区段,并通过细胞细观层次上破坏机制的分析,分析得到了各破坏机制所对应的声发射参数特征。     

不同截面形状砂岩试样的力学特性试验研究

人们通常采用实心圆柱试样进行静态及动态加载试验分析岩石的力学特性,然而实际工程岩体中方形和空心圆柱体试样也很常见。为了研究不同横截面形状对岩石力学特征的影响,本文对相同高度和横截面面积的完整圆柱试样、方形试样和厚壁圆筒试样进行单轴压缩试验和声发射检测。试验结果表明,方形截面菱角、厚壁圆筒试样孔道自由面有利于岩石的局部变形,方形试样强度比实心圆柱体试样偏低6.2%而比厚壁圆筒试样偏高10.0%,方形试样弹性模量分别比实心圆柱体试样和厚壁圆筒试样偏低4.7%、2.8%。实心圆柱试样和厚壁圆筒试样在单轴压缩过程中峰值强度之前可释放弹性能占吸收总能量比值比方形试样大,耗散能所占比值比其小。实心圆柱试样和厚壁圆筒试样破坏以张拉复合破坏为主,方形试样出现了“沙漏”状的破坏形式,试样菱角处都出现明显的剪切面。研究结果为理解岩石的失稳破坏提供了参考。     

岩石声发射的Kaiser效应研究进展

简要介绍了声发射技术的原理、发展史,从Kaiser效应应用方面,还介绍了声发射技术的研究成果.还介绍了岩石的Kaiser效应及声发射技术在岩土工程中的应用.最后讨论了声发射技术未来的研究方向.     

Time-sensitivity mechanism of rock stress memory properties under tensile stress

In deep underground engineering,understanding of time-related stress memory properties is critical to evaluate the in situ stress conditions of a rock mass.In this study,the time-sensitivity mechanism of the rock stress memory properties under tensile stress was investigated.It was found that the material property(Poisson's ratio) and crack angle were the controlling factors of the Kaiser effect(KE) under tensile stress.In particular,the time-sensitivity of the stress memory properties was closely related to the crack growth path.When the failure of the rock specimen was dominated by tensile microcracks and the crack development direction was deflected by up to 30° in the successive loading process,the stress memory capacity was likely to be time-independent for a sandstone specimen.The distribution of the Felicity ratio in a Brazilian test was more discrete than that in a three-point bending test It also showed that the changes in the crack path,rather than the time interval between successive loading cycles,led to inaccuracy of the detected KE.This study provides insights into stress memory-related issues under uniaxial or more complex stress conditions and thus facilitates development of methods for testing in situ mechanical behaviors of rocks with acoustic emission(AE) technology.     

混凝土梁声发射特性试验研究

观测了不同切口相对深度的三点弯曲梁的声发射（ＡＥ）特性，发现切口相对深度对声发射特性有明显的影响．随切口相对深度增加，声发射事件突发性明显减小．采用Ｗｅｉｂｕｌ函数方法定量分析了切口相对深度对声发射特性和混凝土试件断裂特性及脆性的影响．结果表明，参数θ，ｍ具有明显的物理意义，θ表征由试件几何尺寸引起的试件脆性，ｍ值则表征材料的脆性．