Strength and damage evolution mechanism of rock mass with holes under cyclic loading

The damage and failure law of rock mass with holes is of great significance to the stability control of roadways. This study investigates the mechanical properties and failure modes of porous rock masses under cyclic loading, elucidates the acoustic emission (AE) characteristics and their spatial evolution, and establishes the interrelation among AE, stress, strain, time, and cumulative damage. The results reveal that the rock mass with holes and the intact rock mass show softening and hardening characteristics after cyclic loading. The plastic strain of the rock mass with holes is smaller than that of the intact rock mass, and the stress −strain curve shows hysteresis characteristics. Under uniaxial compression, the pore-bearing rock mass shows the characteristics of higher ringing count, AE energy, b-value peak, and more cumulative ringing count in the failure stage, while it shows lower characteristics under cyclic action. At the initial stage of loading, compared with the intact rock mass, the pore-containing rock mass shows the characteristics of a low b-value. The AE positioning and cumulative damage percentage are larger, and the AE positioning is denser around the hole. The specimen with holes is mainly shear failure, and the complete specimen is mainly tensile shear failure.

Failure behavior of a rock-coal-rock combined body with a weak coal interlayer

Using an MTS 815 testing machine,the deformation and failure behavior of a rock-coal-rock combined body containing a weak coal interlayer has been investigated and described in this paper.Uniaxial loading leads to the appearance of mixed cracks in the coal body which induce instability and lead to bursts in coal.If the mixed crack propagates at a sufficiently high speed to carry enough energy to damage the roof rock,then coal and rock bursts may occur-this is the main mechanism whereby coal bumps or coal and rock bursts occur after excavation unloading.With increasing confining pressure,the failure strength of a rock-coal-rock combined body gradually increases,and the failure mechanism of the coal interlayer also changes,from mixed crack damage under low confining pressures,to parallel crack damage under medium confining pressures,and finally to single shear crack damage or integral mixed section damage under high confining pressures.In general,it is shown that a weak coal interlayer changes the form of overall coal damage in a rock-coal-rock combined body and reduces the overall stability of a coal body.Therefore,the whole failure behavior of a rock-coal-rock combined body in large cutting height working faces is controlled by these mechanisms.

Energy dissipation of coal and rock during damage and failure process based on EMR

The physical and mechanical change processes of coal and rock are closely related to energy transformation,and the destruction and failure of coal and rock is an instability phenomena driven by energy change.However,the energy change of large-scale coal rock in the mine site is hardly calculated accurately,making it difficult to monitor coal-rock systematic failure and collapse from the perspective of energy.By the energy dissipation EMR monitoring system,we studied the damage and failure of coal and rock with bursting liability from the energy dissipation point using the geophysical method-EMR,and explored the energy dissipation characteristics during uniaxial compression and their main influencing factors.The results show that under displacement-control loading mode,there are 2 types of energy dissipation trends for both coal and rock with bursting liability.The type Ⅰ trend is a steady increase one during the whole process,therein,the energy dissipation of rock samples is accelerated at the peak load.The type Ⅱ trend energy is a W-shaped fluctuating one containing 6 stages.Under load-control loading mode,there is one energy dissipation trend of shock downward-steady rise.Besides that,rock samples also present a trend of 4 stages.The energy dissipation characteristics of coal and rockduring loading failure process can be used as effective criteria to assess whether they are in a stable or destructive stage.The factors influencing energy dissipation in the loading failure process of coal and rock mainly include strength,homogeneity,and energy input efficiency.

Effect of confining pressure on deformation and failure of rock at higher strain rate

Influence of confining pressure from 0 to 28 MPa, which acts on the two lateral edges of rock specimen in plane strain compression, on the shear failure processes and patterns as well as on the macroscopically mechanical responses were numerically modeled by use of FLAC. A material imperfection with lower strength in comparison with the intact rock, which is close to the lower-left corner of the specimen, was prescribed. In elastic stage, the adopted constitutive relation of rock was linear elastic; in strain-softening stage, a composite Mohr-Coulomb criterion with tension cut-off and a post-peak linear constitutive relation were adopted. The numerical results show that with an increase of confining pressure the peak strength of axial stress-axial strain curve and the corresponding axial strain linearly increase; the residual strength and the stress drop from the peak strength to the residual strength increase; the failure modes of rock transform form the multiple shear bands close to the loading end of the specimen （confining pressure=0-0.1 MPa）, to the conjugate shear bands （0.5-2.0 MPa）, and then to the single shear band （4-28 MPa）. Once the tip of the band reaches the loading end of the specimen, the direction of the band changes so that the reflection of the band occurs. At higher confining pressure, the new-formed shear band does not intersect the imperfection, bringing extreme difficulties in prediction of the failure of rock structure, such as rock burst. The present results enhance the understanding of the shear failure processes and patterns of rock specimen in higher confining pressure and higher loading strain rate.

Assessment of rockburst hazard by quantifying the consequence with plastic strain work and released energy in numerical models

Quantifying the rockburst consequence is of critical importance to reduce the hazards with preventative measures in underground mines and deep tunnels. Contours of energy components within a pillar model are plotted at different rockmass damage stages, and plastic strain work and released energy are proposed as indicators of rockmass damage consequence. One pillar model under different loading stiffness is simulated to assess indicators of pillar burst and the resulting damages. The results show the rockmass damage under soft loading stiffness has larger magnitude of plastic strain work and released energy than that which is under stiff loading stiffness, indicating the rockburst consequence can be quantified with plastic strain work and released energy in numerical models. With the quantified rockburst consequence,preventative measures can be taken to avoid severe hazards to mine safety.

Analysis of progressive failure of pillar and instabilitycriterion based on gradient-dependent plasticity

A mechanical model for strain softening pillar is proposed considering the characteristics of progressive shear failure and strain localization. The pillar undergoes elastic, strain softening and slabbing stages. In the elastic stage, vertical compressive stress and deformation at upper end of pillar are uniform, while in the strain softening stage there appears nonuniform due to occurrence of shear bands, leading to the decrease of load-carrying capacity. In addition, the size of failure zone increases in the strain softening stage and reaches its maximum value when slabbing begins. In the latter two stages, the size of elastic core always decreases. In the slabbing stage, the size of failure zone remains a constant and the pillar becomes thinner. Total deformation of the pillar is derived by linearly elastic Hookes law and gradient-dependent plasticity where thickness of localization band is determined according to the characteristic length. Post-peak stiffness is proposed according to analytical solution of averaged compressive stress-average deformation curve. Instability criterion of the pillar and roof strata system is proposed analytically (using) instability condition given by Salamon. It is found that the constitutive parameters of material of pillar, the geometrical size of pillar and the number of shear bands influence the stability of the system; stress gradient controls the starting time of slabbing, however it has no influence on the post-peak stiffness of the pillar.

Evolution of the deformation field and earthquake fracture precursors of strike-slip faults

Seismic hazard analysis is gaining increased attention in the present era because of the catastrophic effects of earthquakes.Scientists always have as a goal to develop new techniques that will help forecast earthquakes before their reoccurrence. In this research,we have performed a shear failure experiment on rock samples with prefabricated cracks to simulate the process of plate movement that forms strike-slip faults. We studied the evolution law of the deformation field to simulate the shear failure experiment, and these results gave us a comprehensive understanding of the elaborate strain distribution law and its formation process with which to identify actual fault zones. We performed uniaxial compression tests on marble slabs with prefabricated double shear cracks to study the distribution and evolution of the deformation field during shear failure. Analysis of the strain field at different loading stages showed that with an increase in the load, the shear strain field initially changed to a disordered-style distribution. Further, the strain field was partially concentrated and finally completely concentrated near the crack and then distributed in the shape of a strip along the crack. We also computed coefficients of variation(CVs) for the physical quantities u, v, and exy, which varied with the load. The CV curves were found to correspond to the different loading stages. We found that at the uniform deformation stage, the CV value was small and changed slowly,whereas at the later nonuniform deformation stage, the CV value increased sharply and changed abruptly. Therefore, the precursor to a rock sample breakdown can be predicted by observing the variation characteristics of CV statistics. The correlation we found between our experimental and theoretical results revealed that our crack evolution and sample deformation results showed good coupling with seismic distribution characteristics near the San Andreas Fault.

Field observations and interpretation of extensional fracture in hard rock surrounding deep underground openings

Extensional fracturing often occurs in hard rock masses during excavation at depths,for example,>1000 m below the ground surface.Surface-parallel fractures are created in the surrounding rock mass,which is typically subjected to stresses parallel to the free rock surfaces after excavation.These are called extensional fractures because the strains perpendicular to the fracture planes are extensional and the opposite surfaces of each fracture tend to separate from each other as soon as the fracture is created.These fractures predominantly propagate parallel to the maximum principal stressσ1 in the surrounding rock mass.This study analyses extensional fractures observed during excavations in cut-and-fill mining stopes in a deep metal mine.This analysis explores the process of extensional fracturing during excavation in an undisturbed rock mass.In general,intensive spalling occurred on the roof surfaces immediately after the excavation of the undisturbed rock mass.This spalling terminated after a certain depth of rock failure,while burst sounds intermediately emitted from the surrounding rock mass,indicating that rock fracturing was ongoing at depth.In the subsequent cutting slices,the spacing between the extensional fractures decreased with increasing mine-out space in the stope.An extensional fracturing criterion was proposed based on microscopic observations of microcrack development in the rock in response to applied stress.The crack initiation and extensional fracturing processes are associated with two critical extensional strains which are related to the secondary stress state in the position.In areas close to the free rock surface whereσ3=0,the stress for crack initiation is(σ1+σ2)=0.4σc,whereas the stress for extensional fracturing is(σ1+σ2)=0.8σc.

急倾斜深埋巨厚煤层掘巷冲击地压前兆特征及其灾害防治

随着冲击地压矿井逐渐向深开采,其巷道掘进伴随的冲击显现愈发强烈。针对巷道掘进过程中的冲击地压有效防治问题,以新疆乌东煤矿急倾斜煤层矿井为例,运用微震监测对巷道掘进的冲击地压时空前兆特征加以分析。结合巷道掘进的应力与能量变化数值模拟分析,揭示巷道掘进的冲击地压发生机理,提出急倾斜巨厚煤层巷道冲击地压防治策略,并完成现场工程实践验证。研究结果表明:急倾斜巨厚煤层巷道掘进的冲击地压发生前第2~5天出现微震总能量极低值,或存在至少4 d的能量潜伏期;冲击地压发生前5 d普遍存在3 d以上的最大能量占比高频波动期。冲击地压发生前存在明显缺震现象,发生位置集中分布在距离掘进工作面较近的微震能量极小值区间范围内,或位于微震能量极值区间附近的微震频次极小值区间范围内,且冲击地压事件位于冲击变形能指数较高区域。急倾斜巨厚煤层水平分段综放开采的坚硬覆岩结构不易破断,使得巷道掘进存在上水平采空区两侧“双翼型”应力集中,掘进工作面前方与巷道底部受顶底板岩层相互挤压的应力集中分布且能量积聚显著,随着巷道掘进深度增加其应力集中与能量积聚进一步增强,容易诱发冲击地压等动力灾害。综合分析形成急倾斜巨厚煤层巷道掘进的工作面爆破卸压、巷道钻孔卸压与补强支护、复杂区域架蓬的冲击地压防治策略。结合冲击地压时空前兆异常为及时加强卸压力度提供时机。通过工作面与巷道卸压使得掘进期间未发生单日累计1×105 J以上微震能量,在对支护优化调整与复杂区域重点防护后,巷道掘进日均微震能量降至2.2 kJ,其1 kJ以上微震事件占比下降且巷道断面整体平整。研究结果为急倾斜巨厚煤层巷道安全掘进提供了科学依据。

Classification of microseismic events in high stress zone

For the purpose of having a better understanding of failure mechanisms of rock fracturing in mines, the equivalent point source models of tensile, shear and explosive seismic events were established, and the relationship between far-field seismic displacements of the waves and the corresponding equivalent forces were analyzed as well. Based on the results of a microseismic monitoring carried out in the mining progress of 9202 working face under the upper remnant coal pillar in Sanhejian Mine, the waveform features of the seismic events associated with different failure modes were further analyzed. The results show that the signals corresponding to different failure mechanisms have different radiation patterns of the seismic displacements, and different characteristics in waveform features, such as dominant frequency, energy released, the ratio of S- to P-wave energy, and so on. In addition, the rock burst happened in the high stress zone is mainly the result of the strong shear fracturing in the mining process. The results of this study have significantly improved the understanding of the characteristics of the failures associated with underground mining, and will greatly benefit the prevention and control of rock burst hazards in burst-prone mines.

巨厚硬岩下综放工作面超前效应失效机制研究

以河南义马煤田上赋巨厚硬岩综放工作面为工程背景,采用现场实测、理论分析和数值模拟等方法,探索工作面开采中的超前效应失效机制,并提出了相应防控策略。研究表明,低位大厚度软岩的易垮性、高位巨厚硬岩的整体性和超前高强卸压是导致超前效应失效的根本原因,低位大厚度软岩随推采即时垮落,高位巨厚硬岩未达到极限跨距时以整体静载作用于下方煤体,煤体所承受的覆岩自重应力被均化,进而导致超前效应显现不明显;超前高强卸压不但阻断了传力路径,同时降低了应力集中程度。据此提出了设计合理推采长度、留设合理隔离煤柱以减少远场悬露岩层的生成,近场实施超前强卸压降力的防控策略,将大幅减少或消除巨厚硬岩下工作面超前效应的影响,对冲击地压防治具有重要意义。

深部煤矿泡沫充填复合承载结构强度特征及应用研究

深部煤矿开采带来的复杂环境和开采扰动积累的超高能量瞬间释放易造成冲击地压灾害。为了解决煤矿开采过程中上覆岩层移动及高能量瞬间释放冲击现象,采用实验室实验、数值模拟和现场实践相结合,研究了深部煤矿开采过程中顶板断裂的煤体承载区、协同承载区和泡沫充填体承载区承载特性。通过单轴压缩和声发射试验系统研究了泡沫充填体复合承载结构和破坏特征,揭示泡沫充填体的强度变化及裂纹扩展规律;探究了变形与能量耗散特性关系,应力越大,能量耗散越大,应变越大。采用数值模拟研究了泡沫充填法和直接垮落法应变能密度分布。结果表明:泡沫充填开采应变能密度分布不集中,比直接垮落法峰值低,泡沫充填可有效降低上覆顶板断裂能量释放。以三道沟煤矿45205综采工作面为例,提出了泡沫材料充填体压实吸能控制措施,可降低上覆岩层和采动过程中能量的瞬间释放;对45205综采工作面地表沉降及微震能量监测结果表明,当充填体充填率控制为80%时,与直接垮落法相比,工作面微震总数和释放能量显著降低。研究表明,泡沫充填体能够有效地减少采动岩石能量释放,从而实现矿山冲击地压管理和深部煤矿可持续开采。

不同倾角煤岩组合体冲击破坏特征研究

针对煤岩复合结构冲击地压等动力灾害的防治,选用煤与红砂岩2种材料制作节理倾角为0°、30°、60°、90°的体积比1∶1的煤岩组合体试件,利用霍普金森压杆试验系统进行动载冲击破坏特征试验。试验结果表明:煤与红砂岩及其组合体试件满足动载平衡条件,煤的动态抗压强度、破坏程度与冲击速度呈正比。煤岩组合体的抗压强度与倾角条件和受力形式密切相关。煤先受力、岩先受力2种加载方式下,抗压强度与倾角的变化曲线呈近似中心对称关系,冲击破坏形式主要为压剪破坏。为井下复合煤岩结构冲击地压防治提供一定的参考。

厚煤层掘进工作面强震动诱发底煤卸压失效机理研究

针对厚煤层掘进工作面底煤爆破断底卸压后仍发生冲击地压的现象,以新疆硫磺沟煤矿厚煤层掘进工作面为工程背景,采用现场调研、理论分析、数值模拟等方法,对厚煤层掘进工作面强震动诱发底煤卸压失效机理进行研究。研究结果表明:由于掘进导致围岩卸荷过程中煤体剪切破坏产生强烈震动,震动波能量主要被松散底煤吸收,最终导致底煤发生冲击;提出了强震动诱发底煤卸压失效的能量判据,基于此计算得到硫磺沟煤矿厚煤层掘进工作面底煤抛出高度=1.26 m,验证了理论分析的准确性,同时得到卸压底煤冲击的临界震源能量hc=13.7 k J;最后通过FLAC3D模拟了不同震源能量对卸压底煤的扰动效应,得到当震源总能量为22, 32, 42 k J时,底煤最大垂直速度分别为4.32, 6.91, 16.65 m/s,底煤最大位移量分别为0.11, 0.57, 1.40 m;底煤垂直速度随着震源总能量的增加不断增大,随着动载作用时间的增加呈现先迅速增大后迅速减小最后衰减为零的趋势;底煤位移随着震源总能量的增加而增大,随着动载作用时间的增加其增大趋势逐渐变缓,在动载作用时间为3/5个周期时达到最大值,辅助验证了理论分析的准确性。

中国西部深埋隧洞地应力研究与实践

深埋隧洞高地应力岩爆、大变形灾害已经成为制约地下工程施工的重要影响因素,对地应力的研究了解深部岩体破坏机理及相应的防治对深埋隧洞的设计和施工具有实际意义。依托西安市引蓝济李引水工程高地应力测试成果结合施工期深部岩体的破坏形式进行分析研究,并提出相应的防治处理措施,可为类似工程的勘察设计施工提供经验与指导。

Mechanical properties and charge signal characteristics in coal material failure under different loading paths

Rock burst is a catastrophic dynamic disaster caused by sudden failure and instability of coal, loading paths play an important role in the failure of coal, the coal failure process is associated with charge exception infonnation. Hence, violent coal failure mechanics and time-frequency domain distribution of charge signal such as rock burst under different loading paths should be studied in-depth. In this paper, grade and cyclic loading test were carried out for coal with impact tendency samples produced by blocks cored from 800 depth in Xiaoqing coal mine of the Tiefa coal group in northeast China. Theory discussion was carried out for the result of stress and strain, frequency-spectra analysis was conducted for the wavelet charge data, figures showing the evolution mechanism of mechanical properties and the relationship of timefrequency domain amplitude of charge signals in coal with different loading paths and stage were obtained. The failure process and characteristics of coal under different loading paths were summarized. It found that the loading path changed the manner of energy accelerate-release, there were more plastic strain generation in coal under cyclic loading than that under grade loading, the former was more likely to cause greater damage and failure, then the strength of coal under cyclic loading is generally lower than that under grade loading, an energy conversion mechanical model of stress, damage and deformation was developed and explained the effect of the loading path. Charge signal was primarily distributed in the strengthening and peak stages, where there was a high amplitude pulse at each stress drop. The charge pulse was a type of low frequency signal with a primary frequency distribution range of 1 -100Hz. Discussion on the charge generating mechanism from the perspective of friction slip, it demonstrated that the charge obtained during the coal failure process directly to stress loaded on and damage, the result verified it better. We propose that the research results in this study could be efficiently applied to daily mining activities, to provide an early warning and effectively avoid rock burst disaster.

变直径卸压钻孔卸压参数模拟研究

常规大直径卸压钻孔对巷道浅部围岩扰动较大,增加了支护成本。以变直径卸压钻孔为研究对象,采用理论分析、数值模拟的方法,分析了变直径卸压钻孔卸压机理,讨论了变直径钻孔损伤形状、卸压半径、钻孔布置方式和钻孔长度等因素,同时比较了不同钻孔方案的优劣。结果表明:变直径钻孔在围岩浅部使用小直径钻孔,可以减小浅部围岩变形,对浅部围岩扰动更小,在煤层深部采用大直径,可以为煤体提供更大的能量释放空间,卸压效果好;在垂直应力大于水平应力的情况下,钻孔损伤为“蝴蝶”型,因此采用双排三花布置效果最佳;小直径段长度不超过应力降低区,其长度为3 m,其总长度不超过应力集中区,为25 m;变直径卸压钻孔可以在有效卸压的同时减小对巷道浅部围岩的扰动。

基于地音监测技术的多类型冲击地压前兆特征研究

针对煤矿多类型冲击地压地音监测预警问题,基于冲击地压启动理论和现场地音监测数据,分析了不同类型冲击地压的失稳启动机制,得出冲击地压发生前煤岩体应力变化主要分为持续加载型、动静加载型和循环加卸载型3种;结合实验室煤岩体全应力-应变曲线与声发射监测数据,对煤岩体试件破坏前发生3种类型应力变化的地音前兆特征进行了细化研究,并通过现场实测数据,对3种不同类型冲击地压失稳破坏前地音活动演化规律进行了分析。结果表明:现场冲击地压发生前地音活动演化规律与实验室煤岩体试件破坏前地音前兆特征具有一致性。因此,基于地音监测技术的多类型冲击地压前兆特征研究可以对相应类型冲击地压灾害的防控实现不同程度的指导作用,为煤矿现场防控冲击地压灾害提供一定的理论支撑。

多孔弱胶结粉砂岩疲劳强度确定方法及疲劳破坏先兆

为了研究多孔弱胶结粉砂岩的疲劳力学特性,通过单轴阶梯加卸载试验建立了一种新的特征应力定量计算方法,然后开展上限荷载位于不同应力区间的单轴等幅循环加卸载试验,研究粉砂岩的疲劳强度确定方法和疲劳破坏先兆。新建特征应力定量计算方法将岩石的体应变分离为以岩石骨架为主的固相结构线弹性体应变、以裂纹为主的气相结构非线弹性体应变及不可逆的塑性体应变,并提出气相非线弹性体应变最大点、塑性体应变最大点、塑性体应变由正转负点、总体应变由正转负点分别对应粉砂岩的闭合应力、启裂应力、损伤应力和绝对扩容应力。该方法具备科学合理的物理意义,并解决了特征应力划分受主观因素影响的难题。由新建方法获得的损伤应力和绝对扩容应力可以分别视为单轴循环荷载下发生高周和低周疲劳破坏的临界上限应力。当上限应力低于损伤应力时,循环荷载作用下粉砂岩骨架颗粒间发生错动滑移,使试件细观结构重新排列、体积逐渐压缩、体积柔量逐渐增大,随着循环次数增多,下限应力处残余体应变、体积柔量和声发射累计计数等逐渐趋于稳定。当上限应力高于损伤应力时,声发射累计计数持续增加,粉砂岩内部裂纹快速扩展使残余体应变和体积柔量迅速减小,试件最终发生疲劳破坏,而体积柔量由正变负可视为发生疲劳破坏的早期先兆。循环加卸载过程中声发射事件数与k呈对数线性关系,其中斜率b_(k)可用于描述岩石破裂模式,而b_(k)的陡增可视为发生疲劳破坏的临界征兆。

中国冲击地压灾害防治体系的现状及发展

深地资源开发是我国未来科技发展的重要方向。随着煤矿开采深度的增加,冲击地压灾害频发,严重威胁矿井的安全、高效开采。因此,对冲击地压发生机理、监测预警及卸压减冲技术的研究尤为重要。本文首先以时间为顺序综述了煤矿冲击地压的发生机理,明确了理论之间的联系。基于此,从试验角度对冲击地压机理进行总结,细化了冲击地压发生机理及类型,提出煤岩体材料效应和结构效应是冲击地压发生的关键影响因素。随后,针对几种类型的发生机理,利用综合微震、应力监测、电磁辐射和地音监测等手段和方法,比较分析了现有冲击地压监测预警系统存在的局限性。基于此,提出了将前兆信息在时间和空间上进行融合,并以特征向量作为权重,建立冲击地压前兆识别大数据分析及综合监测预警系统。最后,针对我国在冲击地压防治工作中面临的监测精度不足、复杂环境难以控制、卸压减冲方案优化不足、支护方法不理想等问题,提出了未来防治冲击地压的建议,为防治冲击地压发生提供参考。