Excavation-induced deep hard rock fracturing: Methodology and applications

To analyze and predict the mechanical behaviors of deep hard rocks,some key issues concerning rock fracturing mechanics for deep hard rock excavations are discussed.First,a series of apparatuses and methods have been developed to test the mechanical properties and fracturing behaviors of hard rocks under high true triaxial stress paths.Evolution mechanisms of stress-induced disasters in deep hard rock excavations,such as spalling,deep cracking,massive roof collapse,large deformation and rockbursts,have been recognized.The analytical theory for the fracturing process of hard rock masses,including the three-dimensional failure criterion,stress-induced mechanical model,fracturing degree index,energy release index and numerical method,has been established.The cracking-restraint method is developed for mitigating or controlling rock spalling,deep cracking and massive collapse of deep hard rocks.An energy-controlled method is also proposed for the prevention of rockbursts.Finally,two typical cases are used to illustrate the application of the proposed methodology in the Baihetan caverns and Bayu tunnels of China.     

Influence of microwave treatment on mechanical behaviour of compact basalts under different confining pressures

The realisation of microwave-induced fracturing of hard rocks has potential signiflcance for microwave-assisted mechanical rock fracturing and stress release in deep rock masses.In this context,compact basalts were treated by microwave heating in a multi-mode cavity at a frequency of 2450 MHz,and then,we investigated the mechanical behaviour of basalt samples after microwave treatment under uniaxial compression and conventional triaxial compression(CTC)tests.After microwave exposure,cracks appeared on the surface and inside of the rock sample,and the temperature of the sample's surface was unevenly distributed.The results show that the conventional triaxial compressive strength(CTCS)of basalt samples decreased linearly with microwave exposure time,and the higher the confining pressure,the smaller the reduction in the strength of basalt samples after microwave treatment.Under uniaxial compression,microwave exposure greatly affected the axial deformation,suggesting that deformation resistance of the samples gradually decreases with increasing microwave exposure time.Under triaxial compression,some microcracks induced by microwave exposure closed due to the effect of confining pressure,resulting in the confining pressure inhibiting any rightward shift of the axial deformation curve.Furthermore,under uniaxial compression,the elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear manner with elapsed microwave exposure time.Under triaxial compression,microwave exposure has slight influence on elastic modulus and Poisson's ratio.After microwave treatment,the changes in rock strength and deformation mainly result from changes in between the mineral structures.Confining pressure results in the closure of microcracks produced by microwave exposure,so that effects of microwave treatment on strength and deformation decrease,thus reducing the influence on elastic constants.The cohesion decreases with increasing microwave exposure time and shows an approximately linear decrease over time.In the basalt samples,new microcracks in various directions generated by microwave exposure can increase the discreteness of test results,while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.     

定向水力压裂控制煤矿坚硬难垮顶板试验

 针对煤矿坚硬难垮顶板控制的研究现状及存在的问题,进行定向水力压裂控制煤矿坚硬难垮顶板井下试验。通过在压裂孔两侧布置监测孔和在压裂过程中实时监测泵压变化,深入分析煤矿坚硬难垮顶板水力压裂特点。试验结果表明：(1) KZ54型切槽钻头能够在坚硬岩层中预制横向切槽,可有效降低裂缝破裂所需压力;(2) 采用跨式膨胀型封隔器可对岩层坚硬段分段逐次压裂,压裂过程中可在顶板中产生多条裂缝,从而有效弱化顶板;(3) 随着压裂处与孔口距离的增大,裂缝破裂和扩展所需的压力也相应增大,裂缝的扩展半径最大可达20 m;(4) 在压裂过程中,由于岩层均匀性、渗透性、地应力场、岩层结构面等影响因素的变化,压力–时间曲线呈现出多种形态;(5) 岩石抗拉强度与地应力值较为接近时,岩石强度对水力压裂有较大影响。     

硬岩应力–应变曲线真三轴仪研制关键技术研究

基于茂木式岩石真三轴试验原理,介绍真三轴试验机设计中的岩石体积变形测量、端部摩擦效应、应力空白角效应以及真三轴条件下的峰后行为捕捉关键试验技术。此试验系统具有高刚度、大吨位及高响应等特点,能获得真三轴条件下多种硬岩的全应力–应变曲线。同时,利用该试验系统,对不同减摩方案下的摩擦系数进行测试,并评估端部摩擦效应对岩石变形的影响。结果表明:硬脂酸和凡士林混合物具有较好的减摩效果,真三轴试验中,建议采用此减摩方案。端部摩擦往往造成岩石变形的不均匀分布,且对岩石的变形模量和强度有所提高。     

岩爆机理声发射试验研究

采用单向和三向应力状态下的岩石声发射测试技术,对不同岩性岩石的岩爆机理进行了系统地试验研究。研究表明,强度不同、结构差异、不同岩性的岩石,其在单向和三向应力状态下的声发射特性明显不同,而同种岩性岩石在单轴压缩和围压条件下的声发射特性则极为相似,其声发射特征应力值之间也存在一定的规律性关系,对于强度高的岩石和强度低的岩石这一规律都存在,由此总结出不同岩性岩石在不同应力状态下的岩爆发生特征。试验研究表明,采用岩石声发射测试技术追踪岩爆的产生和发展过程,是进行岩爆机理研究的一种科学、有效的方法。     

坚硬顶板水压致裂控制理论与成套技术

煤矿坚硬顶板具有硬度大、整体性好、分层厚度大等特点,导致诸多围岩控制与安全难题。水压致裂可改造顶板岩体结构,控制工作面顶板的冒落;提出坚硬顶板水压致裂控制的理论与成套技术框架。采用真三轴实验系统等研究揭示水压裂缝的扁椭球体典型形态和空间转向扩展形态,给出围压主应力差、排量、层面与原生裂隙等对水压裂缝扩展的影响规律,考虑围岩的应力应变状态与控顶效果,阐明定向压裂临空坚硬悬顶的断顶线位置适当内错煤柱的原理。给出采动岩体水压致裂的时空关系及确定方法。针对裂缝形态的控制要求,提出定向水力割缝致裂等系统的控制致裂方法,在此基础上研制煤矿井下高压(60 MPa)水压致裂的成套装备。针对不同类型工程特点,控制水压主裂缝扩展、翼型裂纹扩展和吸水湿润作用,使顶板及时充分冒落,实现围岩弱化、应力转移、诱导矿压破煤等功能。研发了工作面端头悬顶、切眼及中部坚硬顶板、坚硬顶煤弱化、综放面初采瓦斯、临空巷道冲击地压和大变形等控制成套工艺技术。成套技术与装备已在大同矿区、神东矿区等推广应用。与传统爆破弱化顶板相比,水压致裂弱化顶板管理简单、扰动小、安全性高、工程量少、作用范围大、控制距离远,且经济成本不到其1/10,在深部开采中更具有优势。     

2种岩石直接拉压作用下的力学性能试验研究

 利用Instron1342液压伺服机对2种典型硬岩和软岩试样进行单轴试验,包括单轴压缩试验和直接拉伸试验,研究这两种岩石在直接拉压作用下的力学性能,对比2种岩石的单轴抗压强度和单轴拉伸强度。试验过程中监测岩石试样的轴向应变和水平应变,并记录岩石试样的声发射特征,得到2种岩石在单轴拉压下的应力–应变曲线和声发射计数率曲线,对比2种岩石在单轴拉压下的声发射变化规律。试验发现,直接拉伸下2种岩石在加载初期较大范围内基本无声发射事件发生,直到破坏前声发射事件数才突然增大。讨论2种岩石在不同加载模式下的弹性模量和泊松比变化关系,发现单轴压缩下,硬岩的弹性模量随载荷变化先增大而后趋于稳定,当载荷超过单轴抗压强度的80%时又变小;而单轴拉伸下,硬岩的弹性模量初始较大,随后随着载荷增大而逐渐减小。单轴压缩下2种岩石的泊松比为0.2～0.3,而单轴拉伸下岩石的泊松比很小,几乎可以忽略。比较2种岩石的破坏角、内摩擦角以及黏聚力,讨论2种岩石在直接拉压作用下的不同破坏模式。利用三维表面形貌扫描仪对直接拉伸试样的破坏断面进行三维扫描,得到破坏面细观结构图和裂纹面表面粗糙度曲线。     

高位硬厚岩层破断规律及其动力响应分析

 以杨柳煤矿10416工作面高位硬厚岩浆岩条件为背景,建立高位硬厚岩层三边固支一边简支弹性薄板力学模型,利用瑞利–里兹法,推导出硬厚岩层挠曲函数与应力近似解析式,得到破断跨度的计算式,并根据覆岩破裂形态提出破断步距的计算方法。采用微震、支架压力及地表下沉等监测分析,揭示高位硬厚岩层破断失稳规律及其动力响应。研究表明：高位硬厚岩层破断前的挠度最大点为(x =a /2,y =13b/(10π)) (a为硬厚岩层走向悬露长度,b为硬厚岩层沿倾向的悬露长度);当a＜1.049b时,硬厚岩层首先沿倾向固支边发生破断,否则首先沿走向固支边发生破断;硬厚岩层初次破断形式为沿走向对称而沿倾向非对称的“O-X”型,且破断后侧向跨度固支侧大于简支侧。高位硬厚岩层破断及运移过程中微震活动加剧,产生强微震活动,破断失稳期间支架压力显著升高,并引起地表下沉明显变化。高位硬厚岩层破断失稳引起强烈的动力响应,采用理论计算和微震监测可以进行分析预测。     

Fracture propagation in sandstone and slateeLaboratory experiments,acoustic emissions and fracture mechanics

Fracturing of highly anisotropic rocks is a problem often encountered in the stimulation of unconventional hydrocarbon or geothermal reservoirs by hydraulic fracturing. Fracture propagation in isotropic material is well understood but strictly isotropic rocks are rarely found in nature. This study aims at the examination of fracture initiation and propagation processes in a highly anisotropic rock, specifically slate. We performed a series of tensile fracturing laboratory experiments under uniaxial as well as triaxial loading. Cubic specimens with edge lengths of 150 mm and a central borehole with a diameter of13 mm were prepared from Fredeburg slate. An experiment using the rather isotropic Bebertal sandstone as a rather isotropic rock was also performed for comparison. Tensile fractures were generated using the sleeve fracturing technique, in which a polymer tube placed inside the borehole is pressurized to generate tensile fractures emanating from the borehole. In the uniaxial test series, the loading was varied in order to observe the transition from strength-dominated fracture propagation at low loading magnitudes to stress-dominated fracture propagation at high loading magnitudes.     

岩石破坏声发射平静期及其分形特征研究

 通过岩石加载的室内试验方法,测试不同岩石破裂全过程的力学特征及其声发射特性,得到岩石破坏全过程力学特性——岩石的全应力–应变曲线、声发射事件累积数、声发射事件率等相关曲线及参数,给出声发射事件数、事件率与应力水平、时间之间的关系。着重讨论一次性加载过程中塑性变形阶段明显的岩石在加载接近峰值强度时单位时间内的应力增长速度减小,也即这一阶段出现明显的“耗时”现象;并且在此阶段监测到的声发射事件率出现明显下降,出现声发射相对平静阶段;而对于塑性变形阶段不明显的岩石来说,这一阶段则基本不存在明显的“耗时”现象,声发射的监测中也没有发现声发射相对平静期现象。另外,还运用分形理论,研究分析处于不同加载应力比的岩样在各个阶段的声发射分形维数,特别是研究声发射平静期维数变化情况。指出在加载初期分形维数处于较低值,且分形维数随加载应力增加而逐步增加;在加载到峰值应力的40%左右时,分形维数开始下降;在加载接近到峰值应力时,即处于声发射平静期阶段时分形维数逐步降到最低,且此时预示着岩石的破坏。此外,结合室内试验,还对现场岩体失稳破坏声发射监测中的一些实际问题进行总结和分析,为更好地应用声发射手段进行岩体稳定性现场监测预报提供理论依据、方法和手段。     

急倾斜坚硬岩柱动态破裂“声–热”演化特征试验

急倾斜坚硬岩柱稳定性预测对科学采矿具有重要意义。以乌鲁木齐矿区急倾斜特厚煤层安全开采为背景,基于平面组合加载试验平台,构建急倾斜坚硬岩柱动态破裂"声–热"演化特征模型试验,采用岩石破裂声发射和红外热像综合监测方法,揭示开采扰动作用下岩柱破裂过程中的声发射与温度演化规律。研究表明:坚硬岩柱破裂过程中红外热像呈现温度辐射区(可分为低温边界区、中温过渡区和高温中心区);岩柱破裂经历弹性变形、微破裂至破裂失稳过程,该过程中温度辐射区温度逐渐降低,AE能率呈持续性增长;动态破裂"声–热"演化实质为热弹效应和摩擦热效应。这对急倾斜特厚煤层安全开采具有科学意义。     

CO2–水两相条件下砂岩致裂特征与有效应力模型的试验研究

 CO2–水两相条件下的岩石力学行为是CO2地质封存中的关键科学问题之一。利用自主研发的厚壁圆筒式两相致裂仪进行CO2–水两相条件下砂岩压裂试验,并研究CO2–水两相有效应力模型。通过向试样加载内压、围压和轴压形成与井壁围岩类似的最小主应力为负的真三轴应力状态,采用半透隔板法进行CO2驱水试验以精确控制试样孔隙中CO2–水两相流体的组分压力和饱和度,最后通过内压致裂试样获得致裂压力。还进行了不同有效围压下含单相水孔流试样的致裂试验,得到其致裂压与有效围压的拟合函数。为研究CO2–水两相有效应力模型,选用4种可应用于岩石的非饱和介质有效应力公式与拟合函数联立推导得到破裂压力预测模型,与试验结果对比以间接验证这4种有效应力公式对于表征CO2–水两相流体条件下有效应力的适用性。结果表明,考虑Biot系数的Bishop非饱和有效应力模型能够较好地描述CO2–水两相流体下所测试砂岩的张性开裂行为,可直接作为CO2–水两相有效应力模型使用。     

不同应力路径下岩桥试验的声发射特征研究

 岩质边坡中岩桥贯通是导致边坡失稳的重要因素,通过开展不同岩桥长度岩样的常规三轴加荷、三轴卸荷以及三轴加卸荷试验,研究在不同应力路径下岩桥贯通破坏过程中的声发射特征,以及围压和岩桥长度对声发射特征的影响。结果表明：岩石压密阶段与线弹性变形阶段声发射事件较少,塑性阶段声发射事件明显增多,破坏阶段声发射计数率、累计能量以及幅值均达到峰值。声发射特征的明显变化可为紧接的岩样破坏提供预警作用,幅值变化较其他指标更为敏感,因此幅值的监测对于各阶段演化以及破坏预警更有效。本试验中,岩桥试样达到峰值强度后不会立即跌落至残余强度,而是出现2次应力跌落,应力跌落均对应声发射特征达到峰值,2次应力跌落中会出现“平静期”或“峰后回升”现象,其声发射特征与塑性阶段相似,但幅值、计数率与累计能量均大于塑性变形阶段,表明在这一阶段岩样裂纹仍以较快速率扩展,最终导致岩桥贯通破坏。不同应力路径下累计能量由大到小依次为：三轴加卸荷、三轴卸荷和常规三轴。随着岩桥长度与围压的增加,声发射计数率峰值和累计能量逐步增长,破坏程度更加剧烈。     

Acoustic emission monitoring of hydraulic fracturing in laboratory and field

In the hydraulic fracturing tests, water was injected into four kinds of granitic rock specimens with different grain sizes. The fault plane solutions of acoustic emission (AE) indicated that the shear fracturing is dominant in the specimens with larger grains whereas the tensile fracturing is dominant in those of smaller grains. In addition, the experiments were conducted in granitic rock specimens of the same kind, under three different conditions: injection of water, injection of viscous oil and pressurization via a urethane sleeve. The shear fracturing was dominant due to water injection and the pressurization whereas the tensile fracturing was dominant due to viscous oil injection. Further in a field, AE was monitored by a borehole sonde in the hydraulic fracturing to measure rock stress. The AE count rates increased with closing and reopening hydraulically induced cracks. The result suggested that AE monitoring could improve the reliability and the accuracy of hydraulic fracturing stress measurements.     

基于细观结构表征的岩石破裂热–力耦合模型及应用

 岩石热破裂的研究只有考虑各种矿物组分造成的岩石的非均匀性,才能更客观地反映岩石热破裂的本质。利用数字图像处理技术数字化表征岩石内部矿物颗粒的几何形态,充分考虑岩石真实的细观结构,结合细观损伤力学和热弹性理论,建立能更客观的分析岩石热–力耦合作用下破裂过程的数值模型。以花岗岩为例,运用数值模型研究花岗岩在温度和压缩荷载共同作用下的力学行为和破裂过程。研究结果表明,温度对岩石的力学性质和破裂演化过程影响显著,热破裂裂纹多发生在矿物颗粒边界处,并沿颗粒边界扩展,局部会形成闭合多边形,其热破裂演化过程与试验结果基本相符,从而验证了数值模型的合理性和有效性,该数值模型为细观尺度定量研究岩石热破裂提供一种新的方法。     

不同应力路径下大理岩破坏过程的声发射特性

 对大理岩试样进行常规三轴和卸围压破坏过程的声发射参数测试,研究加荷和卸荷两种应力路径下大理岩破坏过程的声发射特性差异。结果表明,常规三轴试验中,声发射幅值随着围压的增加逐渐增大,岩样破坏前的声发射累计释放能量呈线性增加,最大振铃计数率和能量计数率不是出现在峰值,而是出现在峰后应力跌落阶段,峰值应力前的屈服阶段和残余强度前各存在一个平静期,振铃计数率的每个突增都与应力降相对应。卸围压试验中,岩样破坏后声发射幅值明显增大,卸荷开始后振铃计数率和能量计数率出现突增,声发射累计释放能量呈非线性迅速增加,根据声发射累计释放能量增速可以将岩样破坏过程分为3个阶段：弹性阶段、弹塑性阶段和塑性阶段,在大规模声发射出现前期会出现平静期,两者会交替发生。与常规三轴试验相比,卸荷声发射振铃计数率更大,累计释放能量更高,说明大理岩卸荷破坏更加剧烈。     

Rockburst characteristics of several hard brittle rocks:A true triaxial experimental study

Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation.Rock lithology is a control factor of rockburst.In this paper,rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks,i.e.granodiorite,granite,marble,basalt,sandstone and limestone,under one-free-face true triaxial loading conditions.With the use of high-speed cameras,an acoustic emission(AE)system and a scanning electron microscope(SEM),rockburst of different rocks was investigated.The results show that the strainbursts of granodiorite,granite and marble were accompanied by tensile splitting near the free face,and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release.For basalt,sandstone and limestone,failure was primarily dominated by shear rupture.The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release;while no burst failure occurred on limestone due to its relatively low peak strength.Rock strength,fracturing and fragmentation characteristics,and failure modes of different rocks can significantly affect rockburst proneness and magnitude.The AE evolution coupled with SEM analysis reveals that the differences in the inhe rent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.     

花岗岩应变岩爆声发射特征及微观断裂机制

 对真三轴应力状态下的突然卸载应变岩爆试验监测到的声发射原始波形数据进行频谱分析和时频分析。根据三亚花岗岩岩爆试验前后样品SEM微观结构照片,岩爆过程的声发射频谱特性及声发射参数RA值(声发射撞击上升时间/幅度)的不同,分析其破坏过程的微观机制。在试件相对稳定阶段,产生以低幅、低能量释放为特征的波,对应微裂纹的滑移或局部的微裂纹开裂;在岩爆发生时声发射波除低频部分的幅值明显增大之外,高频幅值也有增大趋势,表明有高能量释放,试件内产生宏观破裂;岩爆前RA值增加,岩爆时降低。高RA值是张裂纹形成产生的波,低值是剪切裂纹形成的波。试验结果揭示岩爆过程中同时产生大量的高频低幅特征的波和低频高幅特征的波,分别对应形成穿晶或解理微裂纹,以张裂纹为主及沿晶或穿晶宏观裂纹,以剪切裂纹为主,显示高能量释放及低RA值特征。     

循环加载高应力对大理岩Kaiser效应影响的试验研究

声发射法(AE)广泛用于地应力测量。当单次加载Kaiser效应不明显时,可采用循环加载方式。通常各循环峰值加载应力大于岩石在地壳中所受最大应力?m,将大于?m的加载应力称为高应力。为研究高应力对岩石Kaiser效应的影响,对取自铜绿山矿的垂直钻孔岩芯进行循环加载试验,根据声发射特征曲线变化趋势及变形率分析法(DRA)地应力测量结果,对比水压致裂法所测地应力量值,确定不同加载阶段Kaiser效应点位置。研究结果表明:当循环加载峰值应力大于?m时,高应力将削弱与?m对应声发射事件的能量,使后续加载循环Kaiser效应不明显,同时会改变岩石之前记忆的应力值,使AE法地应力测量结果产生偏差;多次循环加载可导致岩石内部微结构面的破坏,其产生的大量声发射事件会混淆Kaiser效应点的辨识;因岩石差异性,高应力可能会取代岩石之前所受最大应力,成为岩石记忆的新应力。鉴于此,采用循环加载方式时,首次加载峰值应力A?应不超过?m,同时应使岩石跨过压密阶段进入线弹性阶段。铜绿山矿所取岩样循环加载试验结果表明,A?较合适的取值范围为岩石单轴抗压强度的6%~28%。此外,还给出声发射法和DRA法联合用于地应力测量的合理加载方式。     

页岩水力压裂物理模拟与裂缝表征方法研究

 采用真三轴岩土工程模型试验机、压裂泵伺服控制系统、Disp声发射三维空间定位技术、试验前后工业CT扫描水力压裂缝扩展形态的方法,建立一套页岩水力压裂物理模拟与压裂缝表征方法。由页岩水力压裂物理模拟试验可得：(1) 采用Disp声发射八探头三维空间定位监测方法,能实时有效地监测水力压裂缝的起裂位置;(2) 采用水力压裂后追踪红色示踪剂痕迹的方式,可实现水力压裂缝的空间形态描述;(3) 当水力压裂未形成沿天然层理面的贯通压裂缝时,易形成与天然层理面相交的压裂缝,并与层理面开裂后交叉形成网络裂缝。建立的页岩水力压裂物理模拟试验与表征方法,可进行页岩压裂施工参数的优化设计,为页岩气储层水力压裂开采提供技术支持。