Precursors for rock fracturing and failure—Part II: IRR T-Curve abnormalities

There are many precursors including the abnormality of infrared radiation (IRR) for rock fracturing. The temporal evolution of surface IRR from loaded rock is the combined effect of rock thermoelasticity, pore gas desorbing–escaping, fracture propagation and extension, rock friction, heat transfer and environmental radiation. Being the integral indicator of the surface IRR energy, the mean IRR temperature (AIRT) is selected as the quantitative index to study the temporal evolution of IRR from loaded rock and to identify the precursors for rock fracturing and failure. This paper introduces some typical AIRT curves of uniaxially loaded, compressively sheared and biaxially loaded rock samples. The temporal evolution features of the AIRT curves and three precursor messages, which are short dropping, fast rising and dropping-to-rising, for rock fracturing are analyzed. For theoretical interpretation, the mechanism of AIRT abnormality is studied by taking the actuator, the rock sample and the environment as an independent system in an energy balance state. The heat transfer, as well as the thermoelastic effect based on the sum of principal stresses, has a positive or negative effect on AIRT; the desorbing and escaping of pore gas, as well as the production of new fractures and the expansion of initial fissures, joints and new produced fractures, have a negative effect on AIRT, while the friction action between fractures and between grains has a positive effect on AIRT. It is concluded that the occurrence of precursors is at 77–94% rock strength, i.e., stress peak, and the compressively loaded rock and biaxially loaded rock have clear AIRT precursors for rock fracturing and failure.     

Remote sensing rock mechanics (RSRM) and associated experimental studies

The development of remote sensing rock mechanics (RSRM) was stimulated by meteorological satellite observations using infrared (IR) thermal images during earthquakes. The experimental methods, instruments, and some typical results concerning the relations between the IR radiation and rock stress are introduced. This paper explains that: (1) the IR radiation energy including IR radiation temperature and IR spectral radiation intensity increases with loading; (2) the IR radiation temperature increases with depth inside the rock specimen surface; (3) the IR radiation temperature increases with rock strength and; (4) the IR thermal image can anticipate the stress transfer process and rock fracturing location. Thus, the study and application of RSRM includes detection of material behaviour resulting from heterogeneity and/or stress application as well as mechanical failure and collapse as a time–space forecast. The experiment results have been supported by successful earthquake forecasts. The key issues and future research directions for RSRM are suggested.     

岩石撞击的热红外成像探测研究进展与方向

岩石撞击的热红外成像探测是遥感技术的一项新颖而颇具潜力的应用，亦是遥感-岩石力学交叉学科的重要研究内容。分析岩石撞击热红外成像探测的试验技术原理与特点，介绍前期试验探索成果及新近研究进展。主要包括：热红外技术用于固体撞击瞬态过程监测的试验探索、Hopkinson压杆撞击（包括不同角度、含水岩石靶元的撞击）、岩石与岩石之间的撞击、不同类型岩石的落球撞击、基于红外辐射特征的撞击参数反演研究以及矿山诱灾分析的应用基础研究。讨论岩石撞击辐射升温的物理机制，包括岩石破裂作用机制、岩石热弹效应以及岩石物理学机制。指出岩石撞击热红外成像探测的未来发展方向为辐射规律与物理机制、定量分析与遥感模型、谱段优选与实用技术。     

遥感-岩石力学(IV)--岩石压剪破裂的热红外辐射规律及其地震前兆意义

完整岩体压剪破裂失稳是构造地震的另一种可能孕震机制。以此为模拟对象,采集中国首都圈地区的5种典型岩石,进行了岩石压剪破裂过程的红外辐射成像实验。研究发现:(1) 岩石压剪破裂过程存在热弹和摩擦热两种热-力耦合效应,且压剪作用越强,压剪破裂过程中红外辐射升温现象就越明显。(2) 压应变占优势时,岩石压剪破裂存在红外辐射异常破裂前兆。(3) 压剪破裂前兆有明显的“时空”特征,即:时间上,当载荷增加到约80%破坏应力时,沿剪切区域的平均红外辐射温度开始出现加速升温前兆,且压剪比越高,出现前兆的时间越早;空间上,破裂前沿剪切区域出现的高温异常条带具有前沿推进即“由一端向另一端迁移、扩展”的特点。本研究结果对地震、岩爆、矿爆、边坡滑移等自然与岩石工程灾害的遥感与预报具有重要的基础意义。     

Simulation of progressive fracturing processes around underground excavations under biaxial compression

Fractures that develop progressively around underground excavations can be simulated using a numerical code called RFPA (rock failure process analysis). This code incorporates the mesoscopic heterogeneity in Young’s modulus and rock strength characteristic of rock masses. In the numerical models of a rock mass, values of Young’s modulus and rock strength are realized according to a Weibull distribution in which the distribution parameters represent the level of heterogeneity of the medium. Another notable feature of this code is that no a priori assumptions need to be made about where and how fracture and failure will occur – cracking can occur spontaneously and can exhibit a variety of mechanisms when certain local stress conditions are met. These unique features have made RFPA capable of simulating the whole fracturing process of initiation, propagation and coalescence of fractures around excavations under a variety of loading conditions. RFPA is used herein to simulate progressive fracturing processes around three common shapes of underground excavations – circular, elliptical and inverted U-shaped. The results of the simulations show that the code can be used not only to produce fracturing patterns similar to those reported in previous studies, but also to predict fracturing patterns under a variety of loading conditions. Based on these fracturing patterns, failure mechanisms are identified for various loading conditions.     

遥感岩石力学及其新近进展与未来发展

 遥感岩石力学(RSRM) 是一门新兴的交叉科学。重点介绍了RSRM 近年的主要研究进展, 包括岩石单轴压缩红外成像、穿孔岩石单轴压缩红外成像、岩石剪切红外成像、固体撞击瞬间红外成像、岩石钻进过程红外成像、岩石刻划过程红外成像和光弹材料红外成像与等差线图对比研究等。讨论了岩石类材料受力过程中发生电磁辐射变化现象的物理机制; 并提出了RSRM 未来发展的两个主要方向。     

Fracturing process and effect of fracturing degree on wave velocity of a crystalline rock

The present paper investigates the effect of fracturing degree on P-and S-wave velocities in rock.The deformation of intact brittle rocks under loading conditions is characterized by a microcracking procedure,which occurs due to flaws in their microscopic structure and propagates through the intact rock,leading to shear fracture.This fracturing process is of fundamental significance as it affects the mechanical properties of the rock and hence the wave velocities.In order to determine the fracture mechanism and the effect of fracturing degree,samples were loaded at certain percentages of peak strength and ultrasonic wave velocity was recorded after every test.The fracturing degree was recorded on the outer surface of the sample and quantified by the use of the indices P_(10)(traces of joints/m),P_(20)(traces of joints/m~2) and P_(21)(length of fractures/m~2).It was concluded that the wave velocity decreases exponentially with increasing fracturing degree.Additionally,the fracturing degree is described adequately with the proposed indices.Finally,other parameters concerning the fracture characteristics,rock type and scale influence were found to contribute to the velocity decay and need to be investigated further.     

A novel true triaxial test system for microwave-induced fracturing of hard rocks

This study introduces a test system for microwave-induced fracturing of hard rocks under true triaxial stress. The test system comprises a true triaxial stress loading system, an open-ended microwaveinduced fracturing system, a data acquisition system, an acoustic emission(AE) monitoring system, and an auxiliary specimen loading system. Microwave-induced surface and borehole fracturing tests under true triaxial stress were fulfilled for the first time, which overcomes the problem of microwave leakage in the coupling loading of true triaxial stress and microwave. By developing the dynamic monitoring system, the thermal response and fracture evolution were obtained during microwave irradiation. The monitoring system includes the infrared thermometry technique for monitoring rock surface temperature, the distributed optic fiber sensing technique for monitoring temperature in borehole in rock, the AE technique and two-dimensional digital speckle correlation technique for monitoring the evolution of thermal damage and the rock fracturing process. To validate the advantages of the test system and investigate the characteristics of microwave-induced fracturing of hard rocks, the study demonstrates the experimental methods and results for microwave-induced surface and borehole fracturing under true triaxial stress. The results show that thermal cracking presented intermittent characteristics(calm eactiveecalm) during microwave-induced surface and borehole fracturing of basalt. In addition, true triaxial stress can inhibit the development and distribution of thermal cracks during microwave-induced surface fracturing. When microwave-induced borehole fracturing occurs, it promotes the distribution of thermal cracks in rock, but inhibits the width of cracks. The results also prove the reliability of the test system.     

Growth of geologic fractures into large-strain populations: review of nomenclature, subcritical crack growth, and some implications for rock engineering

Several aspects of fracture arrays are reviewed briefly and discussed. The terminology applied to progressive or multi-stage brittle deformation in rock masses is improved by noting fundamental mechanical differences in fracture type and the kinematic coupling between dilatant mixed-mode crack displacements and wing cracks developed at the fracture tips. An array of initially mixed-mode (I–II) cracks will evolve under remote tensile least principal stress and with increasing strain to a dilatant, mode-I crack array oriented approximately perpendicular to the remote tensile stress. This progressive fracture growth thus defeats predictions of fracture-set orientation and displacement based only on a Mohr circle estimate of initial elastic stress (valid in the rock mass only at the earliest stages of fracture nucleation). Slow, subcritical crack growth in rock is associated with distinctive changes in fracture population geometry, as shown by published numerical simulations of fracture–network evolution. An increase in the stress corrosion index promotes joint clustering and significant changes in joint length–frequency that may lead to characteristic differences in the statistics of large-strain fracture populations. These geometric clues can be used to refine estimates of strength and deformability of rock masses and to infer classes of physico-chemical processes acting at the fracture tips during the development of the fracture population.     

遥感-岩石力学(I)——非连续组合断层破裂的热红外辐射规律及其构造地震前兆意义

以构造地震孕震机制之一即非连续组合断层破裂发震为模拟对象，利用双轴加载实验系统和红外热像仪，对雁列和同线非连通这两种非连续组合断层在双轴加载破裂失稳过程中的红外辐射特征进行了模拟实验研究。研究表明：双轴加载过程中岩石表面红外辐射温度的变化对应岩石应力变化，具有阶段性规律，包括初始、弹性、应力闭锁、解锁和屈服破裂5个阶段。自弹性阶段开始至解锁阶段，岩石表面的平均红外辐射温度(AIRT)的动态曲线表现为“上升-下降-再上升”的演变特征：而断层雁列区和非连通区在解锁阶段其热像出现高温异常条带，非连通区的高温异常条带还具有“增强-减弱(平静)-再增强”的演变特征。其地震前兆的时空意义为：(1)构造地震活动区卫星遥感AIRT的上升趋势可作为地震中期预警指标；(2)热像局部正异常条带预示未来震源位置；(3)AIRT下降与热像局部热异常条带的减弱(平静)一起，可作为构造地震的短临前兆。     

Influence of intermediate principal stress on rock fracturing and strength near excavation boundaries—Insight from numerical modeling

The influence of the intermediate principal stress on rock fracturing and strength near excavation boundaries is studied using a FEM/DEM combined numerical tool. A loading condition of σ₃=0 and σ₁≠0, and σ₂≠0 exists at the tunnel boundary, where σ₁, σ₂, and σ₃, are the maximum, intermediate, and minimum principal stress components, respectively. The numerical study is based on sample loading testing that follows this type of boundary stress condition. It is seen from the simulation results that the generation of tunnel surface parallel fractures and microcracks is attributed to material heterogeneity and the existence of relatively high intermediate principal stress (σ₂), as well as zero to low minimum principal stress (σ₃) confinement. A high intermediate principal stress confines the rock in such a way that microcracks and fractures can only be developed in the direction parallel to σ₁ and σ₂. Stress-induced fracturing and microcracking in this fashion can lead to onion-skin fractures, spalling, and slabbing in shallow ground near the opening and surface parallel microcracks further away from the opening, leading to anisotropic behavior of the rock. Hence, consideration of the effect of the intermediate principal stress on rock behavior should focus on the stress-induced anisotropic strength and deformation behavior of the rocks. It is also found that the intermediate principal stress has limited influence on the peak strength of the rock near the excavation boundary.     

Measurements of induced stress and strength in the near-field around a tunnel and associated estimation of the Mohr–Coulomb parameters for rock mass strength

The state of induced stress measured by the compact conical-ended borehole overcoring technique in the immediate roof of an approach tunnel excavated under high rock stress is described. During the measurements, core disking was observed. An X-ray Computed Tomography (CT) scanner was used to select strain data uninfluenced by the core disking; then the induced rock stress was estimated from selected strain data. From these results, it is shown that the non-destructive investigation using X-ray CT is effective for visualization of the fracturing within cores and the selection of strains measured during overcoring. Furthermore, the Mohr–Coulomb failure criterion parameters for the rock mass were estimated by comparing the measured stresses with the shear strength of in situ rock and the uniaxial compressive strength determined in laboratory tests.     

遥感-岩石力学(II)——断层双剪粘滑的热红外辐射规律及其构造地震前兆意义

以构造地震孕震机制之二即断层粘滑发震为模拟对象,利用双轴加载实验系统和红外热像仪,对4类断层组合条件下双剪粘滑过程中的红外辐射温度场的时空演变特征进行了模拟实验研究.研究表明断层表面温度场的时空演变不仅与应力水平有关,还与组成摩擦面的两侧岩性及其粗糙度有关.具体表现在(1) 应力集中和摩擦作用强的区域,其红外辐射较强;而应力松弛和摩擦作用弱的区域其红外辐射较弱.(2) 当两断层岩性及摩擦面条件对称时,红外热像呈双蝶翼形;当摩擦面粗糙,红外热像呈非对称、非均衡时空演变特征,如串珠状、针状、倒针状、条带状、单蝶翼形或其依次变化.卫星热红外遥感与差分干涉雷达(D-INSAR)技术相结合,并以活动断层预滑及其后续粘滑为监测重点,可望成为构造地震遥感监测和地震短临时-空-强预报的新途径.     

The elasto-plastic response of underground excavations in rock masses that satisfy the Hoek–Brown failure criterion

This paper is intended to illustrate the relationship between the Hoek–Brown parameters describing the strength of rock masses and the mechanical response of underground openings.A formulation of the elasto-plastic behavior of rock in terms of the Hoek–Brown criterion is presented. The analysis assumes that the joint system present in the rock mass has no preferred orientation so that the medium can be considered to behave as an isotropic continuum. It is shown that appropriate scaling of the Hoek–Brown parameters leads to considerable simplification in defining the elasto-plastic response of the rock mass.The classical case in which the excavation process is treated as a uniform reduction of internal pressure in symmetrically loaded cylindrical and spherical cavities is considered. Closed-form expressions are given for the extent of plastic behavior and the related stress and displacement fields. A dimensionless graphical representation of these solutions is provided that allows accurate estimates of the response of excavations in Hoek–Brown materials to be made quickly and easily. Examples are given to illustrate the use of the graphs.Illustrative applications of the derived closed-form solutions are also described. The construction of ground reaction curves for the design of cylindrical tunnels according to the convergence–confinement method and a case study of stability analysis of spherical cavities produced by underground nuclear explosions in French Polynesian atolls are discussed.     

多暗色矿物类岩石单轴加载过程中红外辐射定量研究

对首都圈地区的多暗色矿物类岩石(花岗闪长岩、辉长岩及片麻岩)进行了单轴加载至破裂过程的热红外遥感监测实验,发现:(1) 在加载至应力峰值过程中,岩石平均红外辐射温度与应力呈线性正相关关系;(2) 在加载至应力峰值过程中,岩石平均红外辐射温度与所受机械功呈三次曲线关系;(3) 应力峰后-破裂过程中,岩石沿剪切破裂带辐射温度升高,而在应力松弛或张性区,辐射温度有所降低;(4) 损伤岩石在单轴加载过程中红外辐射温度变化与完整岩石相比总体规律相似,但波动性较大。基于实验分析,建立了岩石应力与红外辐射温度以及机械能与红外辐射温度的定量关系;并对地震遥感及其热红外前兆的物理-力学机制进行了讨论。     

混凝土破裂与渗水过程的红外辐射特征

 对内含水体的混凝土试样进行单轴压缩加载,利用红外热像仪并辅以声发射仪,观测加载过程中试样的红外辐射与声发射变化特征,研究混凝土破裂与渗水的红外异常前兆。实验结果显示,随着应力的阶段性发展,试样表面的红外辐射呈现早期整体性均匀上升、中期局部高温异常、后期低温异常的主要特征,声发射相应出现早期微量增加、中期均匀增加、后期快速增加的特征。加载中、后期红外热像出现的局部辐射温度“先升后降”现象是混凝土破裂–渗水的重要红外异常前兆,其出现的时间要早于应力和声发射前兆。整个破裂与渗水过程中,高温区域包围低温区域的温度场分布是热像的主要特征。基于实验结果,分析试样红外辐射阶段性变化以及破裂、渗水异常前兆的机制,讨论应力、红外及声发射灾变前兆时间差异性的原因。实验结果对水库大坝以及其他储水混凝土工程破裂与渗水的遥感监测与灾变预警具有重要的意义。     

Rock support design based on the concept of pressure arch

A metal mine stope, located at a depth of about 1000 m below the surface, experienced drastic changes in roof displacement and wall fracturing within a short period of time. Thus, the stope needed additional reinforcement in order to remove the remaining ore. It was revealed from rock mechanics assessment that the stability problem was owing to the relatively low strength of the rock as well as the high in situ stresses. It was believed that both the roof and the hanging wall were heavily fractured. It was then proposed that the unstable section of the stope be reinforced with bolt–shotcrete ribs. The concept of the design was to form a pressure arch in the failed rock with the help of six bolt–shotcrete ribs. Displacement measurements showed that the roof displacement reduced from about 2 mm/day to a level of about 0.25 mm/day immediately after the reinforcement operation. Two hanging wall collapses occurred a few months later in the areas outside the bolt–shotcrete reinforced section in the stope. The collapses indirectly proved the effectiveness of the bolt–shotcrete ribs in reinforcing the failed rock.     

爆破方法改变分区破裂巷道围岩应力传递

深部开采中的高地应力对巷道围岩的稳定造成很大的影响。地应力的大小虽然不可以改变,但是其分布却可以通过爆破方式,人为干预的方法,造成的局部的改变。笔者提出了一种新的解决高地应力下解决巷道围岩稳定的方法:通过巷道外部围岩内的装药结构,引爆后通过装药的内部作用在巷道顶部的装药区域贯通,造成分区破裂;分区破裂的强度降低,在周围高应力作用下,起到“孔洞”的作用,改变局部地应力的传递路径,一定程度上起到应力“间隔”的作用。计算结果表明,分区破裂的“孔洞”效应,改变了地应力在局部的传递路径,改变了地应力对巷道围岩的作用,使得巷道围岩及其支护趋于安全。本文提出的方法对维持巷道的安全、稳定具有很大的实用价值。     

Simulating stress corrosion with a bonded-particle model for rock

A numerical model for rock that extends the formulation of the bonded-particle model (BPM) to include time-dependent behavior by adding a damage-rate law to the parallel-bond formulation is described. The BPM represents rock by a dense packing of non-uniform-sized circular or spherical particles that are bonded together at their contact points and whose mechanical behavior is simulated by the distinct-element method using the two- and three-dimensional discontinuum programs PFC2D and PFC3D (Particle Flow Code in 2/3 Dimensions). The extended model is called the parallel-bonded stress corrosion (PSC) model, because it mimics the stress-dependent corrosion reaction that occurs in silicate rocks in the presence of water. Force transmission through rock, and through a BPM, produces many sites of microtension, and it is postulated that stress-corrosion reactions may be occurring at these sites. The stress–corrosion process is implemented by removing bonding material at a specified rate at each parallel bond that is loaded above its micro–activation stress, and the form of the damage-rate law arises from considerations of chemical reaction-rate theory. Global force redistribution occurs throughout the process, and parallel bonds are removed from the system either by breakage (when their strength is exceeded) or by complete bond dissolution. The PSC model parameters can be chosen to match both the static–fatigue curve (time-to-failure versus applied load) and the damage mechanisms and deformation behavior (a creep curve showing primary, secondary and tertiary creep) of Lac du Bonnet granite.     

利用岩石的Kaiser效应测定储层地应力

 用单轴压缩试验测试岩石Kaiser效应特征,进而确定岩体地应力状态的方法,在岩体工程实践中得到广泛的应用。长庆靖安油田长6储层属于低孔低渗储层,需要进行水力压裂后才能得到高效地开发,为此必须研究该储层的地应力。用岩石的Kaiser效应,结合岩芯古地磁定向来研究储层19口井的地应力。研究结果表明,岩石声发射得到的最小主应力与水压致裂瞬时停泵法进行比较,大小吻合较好,而用岩芯古地磁定向结合岩石的声发射测试,能准确测定出地层中主应力分布的方向,与地层倾角测井解释成果比较接近。