三轴卸荷条件下煤体力学特性和能量耗散演化

孤岛工作面煤体和巷道受周边开采扰动影响,煤体受循环荷载作用存在卸荷力学行为而表现出动态破坏特性.为探讨不同路径下煤体力学特性,利用TAW-2000三轴电液伺服刚性试验机分别进行常规三轴(T)、三轴循环荷载(TC)以及相应卸围压试验(TU、TCU),分析不同围压下煤体卸围压强度、变形、声发射事件以及能量耗散演化特征,开展...     

Mechanical properties of deep-buried marble material under loading and unloading tests

The mechanical properties are essentially different when rock material is subjected to loading or unloading conditions. In this study, loading and unloading tests with various confining pressures are conducted to investigate the mechanical properties of marble material samples taken from the deep diversion tunnels of Jinping II Hydropower Station. The stress-strain relationship, failure characteristics and strength criterion are compared and analyzed based on the experiment results. The results show: in the loading and unloading test, peak strength, lateral strain, axial strain and plastic deformation increase significantly as the confining pressure increases. Lateral strain increased significantly and obvious lateral dilatancy can be observed to the change of confining pressure; The fracture mode is mainly the single shear fracture for the triaxial compression test and post-peak test, angle between the failure surface and the ends of the rock material becomes smaller as the confining pressure increases. Hoek-Brown strength criterion reflects the strength characteristics of marble material under two different unloading conditions, and has some supplementary effects to the rock material of mechanical field.     

三轴加卸载下花岗岩脆性破坏及应力跌落规律

基于2种卸荷应力路径和常规三轴压缩试验,研究了加卸载条件下花岗岩的变形破坏及应力脆性跌落特征。卸荷条件下岩石变形主要是向卸荷（主）方向回弹或拉伸变形为主,而非或次卸荷方向的塑性变形很小,峰后应力应变曲线呈现明显的脆性特征。而加载条件下岩石以轴向压缩变形为主,且压缩塑性变形随围压增大而增大;卸荷条件下破坏岩石各种级别的张拉裂缝较多,张裂面一般垂直于卸荷主方向,高初始围压时双向卸荷甚至在次卸荷方向也可产生环形张拉裂缝。破坏围压较高时破裂面剪性特征相对明显,但剪性裂面一般追踪张性破裂面发展而成,并在剪切面两侧发育较多微张裂缝。而相对较高围压下常规三轴压缩岩石一般为剪切破坏,张性裂缝很少;常规三轴压缩岩石的应力脆性跌落系数随围压的增大而增大,而在卸荷条件下却随初始围压的增大而减小。相同初始围压时,卸荷条件下比加载时的应力脆性跌落系数小得多,方案Ⅱ在初始围压达到30 MPa时甚至出现负值,应力脆性跌落系数R依次为：RⅢ〉RⅠ〉RⅡ。     

Mechanical and damage evolution properties of sandstone under triaxial compression

To study the mechanical and damage evolution properties of sandstone under triaxial compression,we analyzed the stress strain curve characteristics,deformation and strength properties,and failure process and characteristics of sandstone samples under different stress states.The experimental results reveal that peak strength,residual strength,elasticity modulus and deformation modulus increase linearly with confining pressure,and failure models transform from fragile failure under low confining pressure to ductility failure under high confining pressure.Macroscopic failure forms of samples under uniaxial compression were split failure parallel to the axis of samples,while macroscopic failure forms under uniaxial compression were shear failure,the shear failure angle of which decreased linearly with confining pressure.There were significant volume dilatation properties in the loading process of sandstone under different confining pressures,and we analyzed the damage evolution properties of samples based on acoustic emission damage and volumetric dilatation damage,and established damage constitutive model,realizing the real-time quantitative evaluation of samples damage state in loading process.     

轴向循环加载条件下人工制备结构性土力学特性

为探讨结构性土的力学特性和变形机理,对人工制备的弱胶结结构性土样进行了轴向加载—卸载—再加载的三轴压缩试验,分别在围压为25和100 kPa时进行了固结排水和固结不排水压缩试验.结果表明:随着荷载的逐渐增加,结构性土样的结构逐渐弱化,其特性逐渐趋向重塑土的特性;在固结排水条件下,在整个轴向加载—卸载—再加载过程中,结构性土样总体上体积收缩.但在轴向应变较小,颗粒间的胶结保持较完好时,在卸载时会出现局部剪胀或体积收缩不明显,区别与重塑黏土的特性;在固结不排水条件下,产生正的孔压,轴向卸载时,在应变较小的情况下,孔隙压力减小,在应变较大的情况下,孔隙压力增大,且围压大时孔隙压力增加的量值大.     

卸围压条件下花岗岩强度特性及三维裂隙演化规律

为研究卸围压条件下花岗岩强度特性和三维裂隙演化规律,对花岗岩开展了常规三轴压缩、卸围压-加轴压和分级卸围压-加轴压循环加卸载3种不同应力路径力学试验,获得对应的轴、径向应力-应变曲线;采用CT扫描三维重构技术获得岩石卸围压过程中和破坏后内部裂隙分布三维图像.结果表明:相对于常规三轴压缩试验,试件在卸荷条件下脆性破坏特征更加显著,分级卸围压-加轴压循环加卸载会增大花岗岩的峰后延性,降低破坏轴压和破坏剧烈程度;两种卸围压方案都会使花岗岩的承载能力降低30%左右;卸荷作用下花岗岩宏观破裂为拉剪组合状,拉剪过渡不明显,表观裂隙是内部裂隙向外扩展的结果;花岗岩在卸荷作用下峰前产生的裂隙量较少,大量裂隙在峰后产生,破裂具有突发性和瞬时性,围压较低时宏观裂隙首先在试件边缘产生,围压较高时宏观裂隙首先在试件中部产生.     

Experimental study on the deformation behaviour,energy evolution law and failure mechanism of tectonic coal subjected to cyclic loads

Compared to intact coal, tectonic coal exhibits unique characteristics. The deformation behaviours under cyclic loading with different confining pressures and loading rates are monitored by MTS815 test system, and the mechanical and energy properties are analysed using experimental data. The results show that the stress–strain curve could be divided into four stages in a single cycle. The elastic strain and elastic energy density increase linearly with deviatoric stress and are proportional to the confining pressure and loading rate; irreversible strain and dissipated energy density increase exponentially with deviatoric stress, inversely proportional to the confining pressure and loading rate. The internal structure of tectonic coal is divided into three types, all of which are damaged under different deviatoric stress levels, thereby explaining the segmentation phenomenon of stress–strain curve of tectonic coal in the cyclic loading process. Tectonic coal exhibits nonlinear energy storage characteristics, which verifies why the tectonic coal is prone to coal and gas outburst from the principle of energy dissipation. In addition, the damage mechanism of tectonic coal is described from the point of energy distribution by introducing the concepts of crushing energy and friction energy.     

吸附气体对受载含瓦斯煤渗流特性影响的试验研究

利用自主研发的含瓦斯煤岩三轴压缩试验系统,进行了大量受载瓦斯煤的渗透特性室内试验,对比分析了CO2,CH4和N2的渗透率之间的异同.研究结果表明,在恒定瓦斯压力条件下,煤样渗透率随围压的增大而减小,均服从负指数函数变化规律;在恒定围压条件下,煤样渗透率随瓦斯压力的增加而减小,并且表现出幂函数变化规律;吸附性强弱不同的气体所表现出来的渗透性也不一样,气体吸附性越强,渗透性越弱;在轴向加载情况下,不同气体的渗透率都表现出先减小后增大的现象,并且具有一般的"V"字型变化规律.研究结果对深入认识煤层瓦斯运移规律具有一定的理论价值.     

Cyclic deformation behaviour of natural K_0-consolidated soft clay under different stress paths

Characteristic of cyclic loading due to passing wheels is associated with one-way loading without stress reversal,which includes a simultaneous cyclic variation of vertical normal stress and horizontal normal stress lasting for a long period of time and generally takes place in partially-drained conditions.Therefore,it is of great practical relevance to study the deformation behaviour according to the characteristic of traffic loading.In this work,a series of one-way stress-controlled cyclic triaxial tests with a simultaneous variation of the vertical and horizontal stress components during cyclic loading were conducted to investigate the deformation behaviour of natural K_0-consolidated soft clay in partially-drained conditions.Test results demonstrate that not only the deviator part of the stress rules accumulation but also the volumetric part significantly contributes.While the deviator part of the stress amplitude is held constant,the increase amplitude of cyclic confining pressure will promote the development of both permanent volumetric strain and axial strain significantly.Furthermore,the effects of cyclic confining pressure on the deformation of natural K_0-consolidated soft clay was quantified.Finally,an empirical formula for permanent axial strain considering the effects of cyclic confining pressure was proposed which can be used for feasibility studies or for the preliminary design of foundations on K_0-consolidated soft clay subjected to traffic loading.     

Space–time evolution rules of acoustic emission location of unloaded coal sample at different loading rates

By using MTS815 rock mechanics test system, a series of acoustic emission (AE) location experiments were performed under unloading confining pressure, increasing the axial stress. The AE space–time evolution regularities and energy releasing characteristics during deformation and failure process of coal of different loading rates are compared, the influence mechanism of loading rates on the microscopic crack evolution were studied, combining the AE characteristics and the macroscopic failure modes of the specimens, and the precursory characteristics of coal failure were also analyzed quantitatively. The results indicate that as the loading rate is higher, the AE activity and the main fracture will begin earlier. The destruction of coal body is mainly the function of shear strain at lower loading rate and tension strain at higher rate, and will transform from brittleness to ductility at critical velocities. When the deformation of the coal is mainly plasticity, the amplitude of the AE ringing counting rate increases largely and the AE energy curves appear an obvious “step”, which can be defined as the first failure precursor point. Statics of AE information shows that the strongest AE activity begins when the axial stress level was 92–98%, which can be defined as the other failure precursor point. As the loading rate is smaller, the coal more easily reaches the latter precursor point after the first one, so attention should be aroused to prevent dynamic disaster in coal mining when the AE activity reaches the first precursor point.     

卸围压条件下砂岩变形破坏特性试验研究

从可释放弹性应变能角度对岩石卸围压条件下破坏特性进行研究,利用MTS815电液压伺服可控制刚性试验机进行保持轴向变形不变的卸围压试验,根据卸围压试验数据,分析了该砂岩卸围压过程中变形、强度、弹性模量及能量变化特征。结果表明:随着围压逐渐降低,岩样发生侧向不断扩容;轴向应力逐渐降低,呈现出非线性特征;弹性模量在初始阶段几乎不变化,越过破坏点之后大幅降低;可释放的弹性应变能在初始阶段增大比较缓慢,当围压降低至一定程度时急剧增大;推导出基于可释放弹性应变能的卸荷岩石的整体破坏准则Ue0。     

Behavior of High Performance PVA Fiber Reinforced Cement Composites in Triaxial Compression

Based on an extensive experimental program,the paper studies the behavior of HPFRCC under triaxial compression. The experimental parameters are lateral confining pressure and PVA fiber content by volume. The test results indicate that ultimate strength and peak strain are significantly improved with the increases of confining pressure. The confining effect introduced by the fibers becomes minor in triaxial compression tests,where there is relatively high external confining pressure. The axial stress-strain curves with different confining pressure and different PVA fiber content by volume are obtained. Lateral confining pressure constraints the lateral expansion of HPFRCC,so there is a big plastic deformation with its ultimate strength improved. At lower confining pressure,PVA fiber content by volume has some effect on the decreased section of stress-strain curve. According to test results,the paper establishes formula of confining pressure with ultimate strength and axial peak strain respectively.     

CO<Subscript>2</Subscript> permeability of fractured coal subject to confining pressures and elevated temperature: Experiments and modeling

The CO₂ permeability of fractured coal is of great significance to both coalbed gas extraction and CO₂ storage in coal seams, but the effects of high confining pressure, high injection pressure and elevated temperature on the CO₂ permeability of fractured coal with different fracture extents have not been investigated thoroughly. In this paper, the CO₂ permeability of fractured coals sampled from a Pingdingshan coal mine in China and artificially fractured to a certain extent is investigated through undrained triaxial tests. The CO₂ permeability is measured under the confining pressure with a range of 10–25 MPa, injection pressure with a range of 6–12 MPa and elevated temperature with a range of 25–70°C. A mechanistic model is then proposed to characterize the CO₂ permeability of the fractured coals. The effects of thermal expansion, temperature-induced reduction of adsorption capacity, and thermal micro-cracking on the CO₂ permeability are explored. The test results show that the CO₂ permeability of naturally fractured coal saliently increases with increasing injection pressure. The increase of confining pressure reduces the permeability of both naturally fractured coal and secondarily fractured coal. It is also observed that initial fracturing by external loads can enhance the permeability, but further fracturing reduces the permeability. The CO₂ permeability decreases with the elevation of temperature if the temperature is lower than 44°C, but the permeability increases with temperature once the temperature is beyond 44°C. The mechanistic model well describes these compaction mechanisms induced by confining pressure, injection pressure and the complex effects induced by elevated temperature.     

循环加卸载下纤维增强泡沫轻质土变形特性

为了研究聚丙烯纤维增强泡沫轻质土的损伤变形特性,选用UNSAT非饱和土三轴仪开展50 kPa围压作用下的循环加卸载试验. 以泊松比、弹性模量提高率及弹塑性变形趋势为参数,分析不同纤维质量分数条件下聚丙烯纤维增强泡沫轻质土的力学特性和变形规律. 研究结果表明：当纤维质量分数达到0.50%~1.00%时,初次弹性模量提高率为56.88%~69.43%,纤维对泡沫轻质土内部初始缺陷的改善作用最佳;纤维增强泡沫轻质土试样的弹性模量提高率随着循环次数的增加呈先增大后减小的趋势,泊松比随着循环次数的增加而增大,在经历最初的2、3次循环荷载作用后趋于稳定直至试样破坏;随着循环次数的增加,纤维增强泡沫轻质土的轴向弹性应变与总应变之比逐渐降低,试样内部损伤变形逐渐累积.     

超静孔隙水压下软土卸荷蠕变特性试验研究

考虑超静孔隙水压作用的软土卸荷力学特性对富水软土地区地下空间开挖变形和稳定性分析具有重要作用。以深圳地区淤泥质软土为研究对象,开展不同初始超静孔隙水压作用下的K₀固结不排水三轴卸荷强度试验和卸荷蠕变试验。试验结果表明：初始超静孔隙水压越大,固结围压越小,软土卸荷破坏越具有突然性;软土卸荷强度应力-应变曲线大致呈双曲线型,其双曲线函数拟合结果表明,卸荷强度随着初始超静孔隙水压的增大而大致线性减小。卸荷蠕变对初始超静孔隙水压敏感性很大,卸荷蠕变破坏时的偏应力约为卸荷强度试验中偏应力的90%。UU0.5应力路径相对于UU0.0应力路径更容易发生卸荷强度破坏和卸荷蠕变破坏,在实际工程中,应尽可能控制软土侧向卸荷比和超静孔隙水压的大小。     

Bounding surface model for ballast with additional attention on the evolution of particle size distribution

Particle size distribution significantly influences the mechanical response of the ballast under low confining pressure. However, particle breakage usually occurs and unfavorably degrades the particle size distribution of the ballast when sufficient load is applied. To model the triaxial stress-strain behavior and its associated evolution of particle size distribution of the ballast, a specific bounding surface model is proposed. The proposed model is based on the traditional bounding surface plasticity and a modified particle breakage index, which correlates the initial gradation and the ultimate gradation together with the current gradation. Simulation of the experimental results from the triaxial compression tests shows that the proposed model can predict the strain softening and volumetric expansion of the ballast under relatively lower confining pressure. It is also able to simulate the strain hardening and volumetric compression of the ballast under relatively higher confining pressure. Most importantly, the proposed approach was observed to have a great potential in predicting the evolution of the particle size distribution of the ballast.     

Directivity effect of unloading bedding coal induced fracture evolution and its application

In the coal mining process, the gas contained in the coalbed is one source of the most serious accident hazards. Stress releasing from the coal deposit is the main controlling factor that leads to such accidents.Based on the bedding of coal samples, the gas permeability well describes the evolution of fracture, so the paper carries out research on the permeability properties of coal under different unloading directions.The research obtains that when the stress unloading direction is perpendicular to bedding, more penetrating fractures and bedding fractures occur, and the permeability significantly increases. Although the axial stress reduced, the confining pressure makes the permeability of the bedding plane fracture exist under constant. The permeability obtained when the unloading direction was perpendicular to the bedding is 52 times larger than that when it is parallel to the bedding. The results show that the efficiency of gas drainage is impacted by the relative direction of gas drainage drilling in relation to the bedding orientations. The maximum amount of gas extraction when drilling is perpendicular to the bedding is 1.3 times than that when drilling is oblique to the bedding, and 1.75 times than that when drilling is parallel to the bedding.     

北山花岗岩短长期常规三轴力学特性试验研究

为研究北山花岗岩的物理力学特性，开展了常规三轴、蠕变和松弛试验，研究过程与结论如下：（1）开展北山花岗岩在不同围压条件下的常规三轴压缩试验，对比分析试样的变形和破坏形式等力学特征，得出北山花岗岩岩石破坏时的不可恢复变形较小，且随着围压增大未出现脆-延性转变现象的结论；（2）开展北山花岗岩单级蠕变试验及不同围压条件下的多级蠕变试验，分析不同偏应力的形变和单级蠕变的蠕变速率，得出北山花岗岩的蠕变形式为非稳定蠕变，其应变曲线出现了典型的蠕变三阶段特征的结论；（3）开展北山花岗岩单级应力松弛试验及循环松弛试验，分析偏应力的变化规律，得出北山花岗岩在应力松弛过程的初始松弛阶段，其偏应力下降速率很快，在较短时间内即能够达到稳定状态的结论。     

煤基质收缩对渗透率影响的实验研究

在具有围限压力的情况下进行了煤岩体氦气和甲烷渗透率的平行实验；采用控制有效应力的方法,消除了因流体压力降低和气体解吸引起的渗透率变低问题；同时,利用克林伯格公式，校正了因气体分子沿壁面滑动而影响的渗透率，并定量地推导了煤基质收缩引起的渗透率变化情况。结果表明：渗透率增量随绝对渗透率的增加而增大，随流体压力的减少而呈对数形式减少；煤岩体氦气的绝对渗透率大于甲烷的克氏渗透率，在有效应力不变的情况下，流体压力愈小，滑脱效应愈明显；滑脱效应引起的渗透率增量越大;氦气滑脱效应大于甲烷。     

软岩S形应力-应变曲线的数值模拟

针对软岩在荷载作用下的变形特征,结合三轴试验提出了软岩四参数本构方程。从数学理论上严格证明了该方程描述的应力-应变曲线的S形特征。该模型的四个参数中,峰值应力及其相应应变可由试验直接获得,另两个参数则需通过优化反演方法获得。在此基础上建立了四个参数与围压的经验关系式,从而可对不同围压作用下的软岩应力-应变曲线进行估算。数值模拟表明该模型能够有效模拟软岩应力-应变曲线的S形特征,能够反映软岩强度与弹性模量随围压增大的特点。