Phenomenological behavior of rock salt:On the influence of laboratory conditions on the dilatancy onset

Recently,stress-based dilatancy criteria have become essential tools to design underground facilities in salt formations such as gas storage caverns.However,these criteria can depend critically on the volumetric strain measurements used to deduce the dilatancy onset.Results from conventional triaxial compression tests can show different volumetric behavior depending on the loading conditions,as well as on the measurement techniques.In order to obtain a quantitative understanding of this problem,an experimental program was carried out and the testing procedure was investigated numerically under homogeneous and heterogeneous stress states.The experimental results showed that the deviatoric stress corresponding to the dilatancy onset was significantly dependent on the measurement techniques.With a heterogeneous stress state,the simulation results revealed that the strain measurements at different scales(referred to as local,hybrid or global)can provide different volumetric results with moderate to significant deviations from the idealized behavior,and hence different onsets of dilatancy.They also proved that,under low confinement,tensile stresses can take place within the compressed specimen,leading to a great deviation of the dilatancy onset from the idealized behavior.From both experimental and numerical investigations,the difference in sensitivity to the measurement techniques between the deviatoric and the volumetric behaviors is explained by the relatively small values of the volumetric strain.The non-ideal laboratory conditions have more impact on this strain than on the deviatoric one.These findings can have implications for the interpretation of the dilatancy behavior of rock salt,and hence on the geomechanical design aspects in salt formations.     

Rock salt dilatancy boundary from combined acoustic emission and triaxial compression tests

This paper presents an experimental investigation of rock salt dilatancy boundary based on combined acoustic emission and triaxial compression tests carried out on the rock salt samples from the Asse Salt Mine, Germany. The experimental results were evaluated to determine the dilatancy boundary under specified stress, stress loading rate and pore pressure. Pore volume changes caused by deviatoric stresses were measured during triaxial compression tests. The dilatancy boundary was then determined from the maximum compression on a stress–strain curve which separates the compression and dilatancy regions. Variations in acoustic emissions that occur during microcrack development under triaxial compression were recorded with ten sensors mounted on the outer surface of the cylindrical samples. A spontaneous increase observed in the cumulative number of acoustic events is consistent with the dilatancy boundary determined from the minimum pore volume. It is confirmed that the dilatancy boundary depends on both stress loading rate and pore pressure. The dilatancy boundary slightly decreases with increasing the loading rate, but increases with increasing the minor normal stress. High pore pressures accelerate the dilatancy. A Biot coefficient of 0.25 for Asse rock salt was determined from the dilatancy boundary. An analysis of two-dimensional (2D) fractal dimensions determined for several samples shows that the samples with smaller dimensions have slightly higher dilatancy boundaries. The dilatancy boundary values of Asse rock salt are lower than those reported for crystalline rocks, although both rock types show similar dilatancy behaviour.     

剪胀性砂土地震后流滑的机理和模拟

不均匀地层中超静孔压的扩散、重分布过程会导致孔隙水集中渗流累积至局部区域,导致该区域土体强制吸水,从而引发剪胀性砂土边坡地震后侧向流动变形甚至失稳。通过分析边坡流动变形过程中砂土的应力路径特征及强制吸水条件下的体变平衡条件,指出预测剪胀性砂土边坡流动变形的关键是描述砂土保持常剪应力和当前峰值应力比时的剪胀特性和震后再固结体变特性。通过常偏应力下的三轴剪切吸水试验,观察了砂土在流动变形过程中的吸水量与剪应变发展的关系,给出了基于强制吸水体变预测流动剪应变的状态剪胀模型。基于剪切后再固结试验,得出了再固结体变的变化规律和数学描述。基于所提出的机制和数学描述,给出了基于有限差分法的边坡流动变形发展过程直至失稳破坏的模拟方法。     

低渗透岩石三轴压缩过程中的渗透性研究

 采用岩石全自动三轴伺服仪,对低渗透花岗岩进行考虑渗透水压作用的三轴渗流–应力耦合试验。基于试验结果,研究花岗岩在不同围压和渗压下的渗透特性,分析岩石应力、应变变化过程中渗透率随围压、渗压和体积应变的变化规律。试验结果表明：岩石的应力–应变关系具有典型的脆性特征,渗压相同围压不同时,岩石强度随围压增大而增加;围压相同渗压不同时,较低的渗压对低渗透岩石强度影响不明显。岩样体积应变经过压密和扩展2个阶段,最大体积压缩应变随着围压的增加而增加,而岩样渗透率最小值并未出现在最大压密处,而是出现在体积应变拐点前,约在最大压密体积应变的95%处,并给出渗透率与体积应变的关系式。     

砂岩卸围压变形过程中渗透特性与声发射试验研究

 利用岩石伺服试验系统,对江西红砂岩岩样进行气体渗透三轴试验及声发射监测,研究在常规加载、峰前卸围压和峰后卸围压3种应力路径下,岩样变形破坏过程中的渗透规律和声发射特征。试验结果表明：(1) 随着有效围压的增大,岩石岩样的应力峰值逐渐增大,岩样的应力峰值对有效围压很敏感。(2) 常规加载时,渗透率在岩石屈服前呈现略微下降的趋势,屈服后迅速增长,峰后应变软化阶段有小幅回落;峰前和峰后卸围压时,在卸载之前渗透规律与常规加载时相同,卸载后渗透率均呈急剧增长的趋势,增幅也较大,其中峰前卸围压后渗透率增幅最大。(3) 在相同加载方式下,围压的增大不影响渗透率曲线的发展趋势,只影响渗透率在各阶段量值的大小。(4) 常规加载时,岩石声发射活动在屈服前比较平静,屈服后声发射活动非常活跃,峰后应变软化阶段声发射活动再次趋于平静;峰前卸围压不久后,声发射活动异常活跃、密集,能量数相对值较大并有明显峰值;峰后卸围压过程与常规加载过程中声发射规律相似。(5) 岩样的破坏过程中,随围压增大,脆性减弱、延性增强,在同一围压水平下,峰前卸围压破碎程度最高,脆性最强。(6) 岩石扩容点与渗透率最小值所对应的轴向应变值十分接近,体应变和渗透率随轴向应变的变化趋势对应较好,声发射活动的密集阶段均发生在体积膨胀之后,渗透率、声发射、应力及(体)应变之间存在一定对应关系。     

三轴低频循环荷载下盐岩体积应变特性研究

地下储气库围岩长期处于复杂疲劳应力状态下,盐岩三轴循环荷载下的变形规律对储气库稳定性分析有参考价值。对8个盐岩试块进行了不同围压、不同应力水平和不同频率条件下的循环荷载试验。对每一试块施加恒定围压和轴向低频循环荷载。对试验参数进行了无量纲处理,分析了应力比强度(广义剪应力强度与球应力的比值)、应力比振幅、上限应力水平、荷载频率、循环次数(N)等对体积应变(?v)的影响。利用函数?v=?lg N+?v0,对每一试块的体积应变–循环次数曲线进行了拟合分析,获得了参数?和?v0随应力比振幅、上限应力水平和频率变化的数学表达式。标准化回归系数分析表明,上限应力水平是影响盐岩体积变形的主要因素,其次是应力比振幅。在高上限应力水平三轴状态下,盐岩主要表现出体积扩大现象。     

基于离散元法的真三轴应力状态下砂土破碎行为研究

基于可破碎三维离散颗粒模型模拟了一系列常规三轴试验与真三轴试验,研究了砂土在真三轴应力状态下的破碎行为。数值调查主要关注试样的应力应变特性、级配及相对破碎率的演化。随着围压增大,颗粒破碎率增大,试样应变软化特性和剪胀性逐渐减弱,而超过临界高围压后,由于固结中颗粒大量破碎,试样剪胀性反而增强。真三轴试验中,试样偏应力比峰值均随中主应力参数b值增大而减小。由于破碎随b值增加而明显增大,试样剪胀性随b值增大而逐渐减弱。试样内摩擦角φ随围压增大而减小,其演化关系基本满足对数关系;内摩擦角随b值增大先增大后减小,Lade-Duncan准则较为适合描述其变化规律。此外,试样相对破碎率增大的速率随围压和轴向应变增大而逐渐降低,暗示试样最优终极级配的存在,且相对破碎率与试验输入能量之间存在唯一的双曲线关系。     

基于广义塑性理论的堆石料动力本构模型研究

分析了堆石料在等幅与不等幅应力循环荷载作用下的变形特性,以此为基础,确定了不同加载过程中堆石料的剪胀方程,加载方向,切线模量及塑性模量,建立了一个可以考虑堆石料循环加载特性的广义塑性本构模型.模型将所有的加卸载阶段都视为弹塑性过程,并在剪胀方程中引入老化函数来考虑体积应变积累对剪胀(缩)性的影响.模型共有12个参数,均可通过常规室内单调及循环加载试验确定.为验证模型的有效性,依据试验资料确定了两种不同堆石料的本构模型参数,并对等幅循环三轴压缩与不等幅循环三轴压缩试验进行了模拟.两种材料在不同围压下的模型预测结果与试验数据均吻合良好,表明模型可以有效地反映循环荷载作用下堆石料应力应变曲线的滞回特性与永久变形的积累.     

Effects of inelastic volume increase on fractured rock behaviour

This study investigated the correlation between inelastic volumetric deformation and the bearing capacity of fractured rock. Triaxial compression tests on marble specimens have been performed under constant and controlled deformation rates using a servocontrolled loading machine supported by an electro-hydraulic volume-measuring unit connected to a data acquisition system. The triaxial compression tests were carried out on two different marble samples under constant confining pressures ranging from 1–12.5 MPa. The results indicate that stress–inelastic volumetric strain curves are generally linear through the initial part of the post-failure curve where fractured rock behaviour is dominant. It was found that brittle rock loses its strength in the post-failure region depending on volumetric deformation; the slopes of post-failure curves change with an increase in confining pressure. As a result it is concluded that there is a relationship between strength and volumetric strain of fissured rock. Electronic Publication     

Influence of acidic environment and temperature on mechanical behavior of cement-treated Masado and numerical modeling with a thermoelasto-viscoplastic model

Cement-treated Masado (CTM), as a common cement-mixed geomaterial, is usually only used in ground improvement for temporal structures in Japan. However, in recent years, it has been used for new permanent structures, such as the supporting ground of pile foundations and direct foundations. Masado, a completely decomposed granite, is distributed widely in Japan. However, the long-term stability of CMT has not been thoroughly investigated, despite that it has the risk of exposure to environmental changes in acid conditions and temperature, which are very common in volcanic areas in Japan. In this study, to investigate the influence of acidic environments and temperatures on the mechanical behavior of CMT, a systematic test program using uniaxial and triaxial tests was conducted on CMT specimens under different acidic environments, temperatures, and confining pressures. To avoid the extra influence of water on the mechanical behavior of CMT, a special double-cell was designed within the pressure chamber of the triaxial loading device so that the volumetric strain of the specimens completely sealed with rubber sleeve can be accurately measured in triaxial compression and creep tests. In addition, the chemical components of all the tested specimens are investigated by X-ray fluorescence spectrometry analysis to identify the influence of calcium leaching and hydration reactions in the curing period. Combined with the influence of the initial confining pressure, the influence of the acidic environment and temperature on the strength and dilatancy of CMT is carefully investigated. Based on the test results, a relation between the stress ratio at the critical state and the influential factors, including the initial confining pressure, acidic environment and temperature, is proposed by regression analyses. Meanwhile, an existing thermoelasto-viscoplastic model is modified to properly describe the influence of the abovementioned influential factors on the mechanical behavior CMT. The applicability of the modified model is then verified by triaxial compression and creep tests.     

Stress–dilatancy relationships of sand in the simulation of volumetric behavior during cyclic torsional shear loadings

In order to describe the volumetric behavior of soil subjected to shearing, a relationship that deals with the ratio of plastic strain increments to stress ratio (i.e. a stress–dilatancy relationship) is required in addition to the stress–shear strain relationship. In view of the above, stress–dilatancy relationships during cyclic torsional shear loadings were experimentally investigated in the current study. Based on the experimental results, a bilinear non-unique stress–dilatancy model was proposed for stress controlled drained cyclic torsional shear loading. The stress–dilatancy relationships during virgin loading and subsequent cyclic loading were modeled separately by considering the effects of stress history (over-consolidation or normal consolidation). Then the volume change of Toyoura sand specimens subjected to cyclic torsional shear loading was simulated by combining the simulation of stress–shear strain relationship with the proposed stress–dilatancy relationships. It was observed from the comparison of the experiment results with the simulation of volumetric strain that, after combining with accurate modeling of stress–shear strain relationship, the proposed stress–dilatancy relationship can reasonably simulate the volumetric behavior of sand subjected to various drained cyclic torsional shear loadings.     

超固结土三轴排水不排水试验数值分析

在临界状态土力学的框架内,采用Hashiguchi提出的下负荷面本构模型,建议了一个简化的塑性硬化规律,使之能较合理地描述超固结粘土在正常屈服面和下负荷面之间的变化规律.数值模拟结果表明,修正后的硬化规律能够较好地描述超固结粘土的许多力学特性如应力应变关系、应变软化以及应力剪胀性等.数值预测结果与室内三轴排水试验结果相...     

岩石应变软化及渗透率演化模型和试验验证

为预测承载岩石的应变软化和渗透率演化,基于Gebdykes白云岩的三轴试验结果,分析了围压对岩石弹性模量、破坏应变、峰值强度、强度退化过程、残余强度和剪胀扩容的影响规律。将岩石变形全过程简化为3阶段,使用强度退化指数、脆性模量系数和扩容指数改进FLAC中的SS模型,建立了考虑围压影响的岩石应变软化模型。基于淮南潘一矿煤、凝灰岩、巴里坤砂岩、山西安家岭泥岩的渗透率与体积应变实验数据,建立了基于体积应变增透率的岩石渗透率演化模型,与改进SS模型结合,建立了考虑围压影响的岩石应变软化和渗透率演化模型。利用本文模型分别模拟了安家岭泥岩和Gebdykes白云岩的三轴压缩、渗透率演化和体积扩容过程,结果表明:1体积应变渗透率演化模型能较好地描述体积应变与渗透率的关系;2本文模型能较好地模拟围压对岩石残余强度、峰后强度退化过程和剪胀扩容的影响,能较准确预测承载岩石的渗透率演化。     

不同应力路径下花岗片麻岩渗透特性的试验研究

 采用全自动三轴伺服仪,对花岗片麻岩开展渗流应力耦合试验,研究常规三轴压缩和轴压循环加卸载2种应力路径下,渗透率与渗压、围压、有效围压、体积应变及应力路径等因素的关系。结果表明：(1) 在2种不同应力路径下,岩石渗透率演化规律有差异性和一致性,同种路径下变形各阶段渗透率随有效围压增大而减小,但渗透率曲线的形态保持不变;(2) 渗压和围压对渗透率的影响,通过对岩石变形过程中内部微裂纹和孔隙变化产生作用,有效应力系数发生改变,有效围压效应随之改变;(3) 循环加卸载试验中,卸载渗透率均明显大于相应加载渗透率,体积应变转折前,加载渗透率减小,卸载后渗透率增加,形成比较完整的渗透率回滞环,体积应变转折后,加载渗透率增大,卸载渗透率降低不能够完全恢复;(4) 体积应变较轴向应变更清楚和灵敏反映渗透率变化规律,可把体积转折应变或其对应应力作为岩石渗透率变化的一项指标。试验研究旨在为岩石工程渗流–应力耦合稳定性分析提供参考。     

热–力耦合作用下黏性土体积变形特性试验研究

采用温控三轴仪,对饱和黏性土在热–力耦合作用下的体积变形特性进行了研究。试验中共考虑了温度循环后的应力加卸载试验、升高或降低不同温度后的应力加卸载试验3种方案。结果表明,黏性土温度体积变形的大小与围压无关。温度循环会造成黏性土塑性体积变形持续发展,但单周塑性应变增量随温度循环周数的增加而减小,体现了温度历史的影响。将土样继续加载后,温度的历史效应将被覆盖,后续温度循环中产生的塑性体积应变量值及发展规律与初始循环下的类似。不同温度下土体的压缩指数和回弹指数基本不变。同一塑性体积应变下,屈服应力随温度的增加而减小。温度变化引起的塑性体积应变也会造成屈服应力提高,可近似采用与力学塑性体积应变硬化一致的规律进行描述。     

堆石体修正Rowe剪胀方程与南水模型

在分析堆石体三轴试验成果的基础上,探讨了堆石体的体积变形特性,提出了形式简洁的堆石体修正Rowe剪胀方程。将所建立的修正Rowe剪胀方程引入到沈珠江院士提出的"南水模型",克服了该模型由于采用体变–轴向应变抛物线假设所带来的一些缺陷。应用堆石体常规三轴试验和典型复杂应力路径三轴试验成果对修正后模型的合理性进行了验证。     

高围压下岩石卸荷的扩容性质及其本构模型研究

通过岩石试样高围压下的卸荷试验,研究卸荷条件下岩石的扩容性质。研究结果表明:初始扩容点即最大压缩体应变对应的变形模量可作为弹性阶段的弹性模量。根据试验研究,将岩石卸荷试验应力–应变全过程曲线分为弹性、应力屈服、峰后脆性及残余理想塑性4个阶段,并根据各阶段特征得到相应段的本构方程,最后得到卸荷岩体全过程的本构模型。用数据拟合的方法,得到不同卸荷试验下模型曲线,并验证模型曲线可以较好地模拟卸荷的应力–应变曲线。     

Evolution of poroelastic properties and permeability in damaged sandstone

This paper is devoted to experimental investigation of mechanical behavior, poroelastic properties and permeability in saturated sandstone. The emphasis is to study the evolution of Biot’s coefficients and permeability with the growth of microcracks. Basic mechanical responses are first investigated through triaxial compression tests, showing nonlinear stress–strain relations, volumetric dilatancy, pressure sensitivity, elastic modulus degradation and induced anisotropy. Original tests are then performed for the determination of Biot’s coefficients in the axial and radial directions at different levels of stress. It is shown that the evolution of Biot’s coefficient is clearly anisotropic in nature due to the oriented closure of initial microcracks and growth of induced microcracks. The rock permeability in axial direction is also measured for different values of stress; it decreases in the first stage with the closure of microcracks and then progressively increases due to the opening of induced microcracks. However, the permeability significantly increases only when the coalescence of microcracks occurs.     

基于盐岩压缩–扩容边界理论的盐岩储气库密闭性分析研究

 根据我国深部盐岩层中天然气储备的可行性研究,针对盐岩储气库密闭性分析开展试验研究和理论分析。首先对盐岩的压缩–扩容边界理论进行分析,根据应城纯盐岩的三轴压缩试验结果,建议采用双曲线函数拟合盐岩的压缩–扩容边界(CDB),对比不同研究者的试验结果表明,所提议的双曲线型较通常采用的抛物线型具有更好的适用性。在分析盐岩的CDB在主应力空间表达式的基础上,给出盐岩扩容接近度(DAI)的定义和表达式,提出采用基于盐岩的压缩–扩容边界理论来分析盐岩储气库的密闭性,并给出相应的算例。     

原状Q_3黄土湿陷特性的CT-三轴试验

利用应力控制式CT-三轴仪及CT-湿陷性三轴仪,在控制吸力的条件下,试验研究原状Q3黄土的浸水湿陷特性。做了2组共15个三轴试验,包括均压浸水试验和以双线法进行的侧向卸荷浸水试验,试验中利用CT机进行无损断面扫描,并用得到的CT数定量分析原状Q3黄土的结构性对湿陷的影响。结果表明:对于双线法湿陷试验,同一吸力和偏应力下,固结净围压大的试样湿陷变形大于固结净围压较小的试样湿陷变形,包括体应变、轴应变及偏应变量;均压浸水湿陷试验时,在试样饱和前,随着浸水量的增加CT数与浸水量之间基本呈线性关系;当试样趋于饱和时,CT数增大减缓;均压浸水湿陷试验中,在同一吸力下,固结净围压大的湿陷体应变较大,结构演化变量值也越大。