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.     

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。     

红砂岩应力脆性跌落系数的试验研究

岩石的应力—应变全过程曲线以峰值应力为界,通常分为破坏前区和破坏后区两部分。由于岩石破坏后区的力学特性对诸多岩石工程,如地下巷道、矿柱和岩爆等具有重要的工程意义,因此,此方面的研究已受到理论界和工程界的重视。岩石的破坏后区一般处于非稳定状态,当软化速率较大时,其力学响应用经典的应变软化模型来模拟计算难以收敛,而可以考虑采用脆塑性模型。使用RMT-150B型岩石力学试验系统,在轴向应变率保持为常数的条件下,对贵溪红砂岩进行了常规三轴压缩破坏试验,得到了不同围压下岩石的应力—应变全过程曲线。利用试验得到的应力—应变全过程曲线,得到了红砂岩的应力脆性跌落系数与围压的关系表达式,为采用非理想脆塑性模型对红砂岩介质及其中的构筑物进行数值分析提供了依据。     

不同围压下灰岩力学特性及破坏力学模型研究

为了研究围压对灰岩力学特性及破坏力学模型的影响,采用MTS815对完整灰岩岩样进行三轴压缩试验。基于岩样宏观破坏形式,建立张拉剪切破坏模型,构建峰值强度与围压关系表达式;讨论张拉剪切模型与纯剪切模型对岩石剪切强度参数值（粘聚力与内摩擦角）的影响。研究结果表明：岩样弹性模量、抗压强度及残余强度均随围压升高而增大,且残余强度与围压呈非线性相关;当岩样最终破坏时,环向应变与轴向应变比值随围压升高呈负指数降低;一定围压范围内,岩石抗压强度受剪切强度参数和抗拉强度影响;张拉剪切模型确定岩石剪切强度参数值随破裂角增大而增大,与纯剪切模型相比,数值均较小。因此,低围压时,考虑岩石破坏模型具有一定的实际意义。     

Strength properties and evolution laws of cracked sandstone samples in re-loading tests

To study the strength properties and evolution laws of cracked sandstone samples in re-loading tests,strength and damage evolution properties of intact sandstone samples were first analyzed through the triaxial compression tests carried on TAW-2000 microcomputer control electro-hydraulic servo rock triaxial test system. Damage evolution models were established based on dilatancy properties realizing the real-time and quantitative evaluation of samples damage state in loading process. On this basis, samples with different damage were obtained by pre-peak, peak point, post-peak and residual strength stage unloading tests in the loading process of intact samples. The characteristics of the stress-strain curves and strength evolution laws were studied through the re-loading tests of samples with different damage under different stress states. The experimental results showed that the slope of stress-strain curves, peak strength and residual strength of cracked samples increased linearly with confining pressure and decreased linearly with damage. The equivalent cohesion decreased with damage in the exponential decay curves. The mechanics properties of samples transformed from strain softening to strain hardening with damage.     

孔隙压力对岩样全部变形特征的影响

目的 研究了孔隙压力对剪切带图案及岩样全部变形特征的影响．方法 利用FLAC内嵌语言编制的FISH函数计算平面应变压缩岩样轴向、侧向、体积应变及泊松比．在峰前及峰后，岩石的本构模型分别取为线弹性模型及莫尔库仑剪破坏与拉破坏复合的应变软化模型．结果 随着孔隙压力的增加，岩样的破坏区域越来越广泛；剪切带倾角都接近于Arthur倾角；峰值强度及所对应的轴向、体积应变及侧向应变的大小均降低．当孔隙压力较低时，峰后应力-轴向应变曲线及应力-侧向应变曲线软化段斜率基本保持不变，根据单轴压缩条件下的解析解，这是由于岩样的破坏模式不随孔隙压力的增加而改变．结论当孔隙压力较高时，大量的单元发生破坏将消耗较多的能量，这使应力-轴向应变及侧向应变曲线软化段变平缓；岩样在轴向应变较低时就可获得较高的侧向变形量及泊松比，甚至负的体积应变．岩样失稳破坏的前兆的明显程度不随孔隙压力的改变而改变．     

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.     

孔隙压力作用下泥岩三轴蠕变实验研究

在具有孔隙压力系统的岩石三轴蠕变仪上,对泥岩进行了考虑孔隙压力作用的三轴蠕变实验.实验中不仅考虑了轴压σ₁和围压σ₃对蠕变的影响,而且考虑了孔隙压力p在泥岩蠕变过程中产生的重要影响.通过不同应力水平下的蠕变实验,对比含孔隙压力和不含孔隙压力的蠕变实验曲线,表明当轴压σ₁和围压σ₃一定时,随着孔隙压力p的增加,蠕变变形量将会减小,在稳态蠕变阶段的应变率也会减小,在一定程度上会延长蠕变三个阶段的蠕变时间.并采用经典蠕变模型对实验结果进行分析.     

孔隙水压力与恒定时间对砂岩变形的实验研究

探讨三峡库水位在145-175 m之间涨落及蓄水时水压对岩（石）体的变形特性影响。以实验为基础,进行了不同轴向应力（1σ=55.54,34.18和12.82 MPa）条件下,周期孔隙水压力（Pmin=2 MPa,Pmax=6 MPa）与上、下限恒定时间（ΔT=120和240 s）作用下砂岩的变形特性实验。通过分析1ε-T与p-T关系曲线和p-1ε滞回曲线,可知：1ε-T砂岩曲线呈连续正弦波形演化,当孔隙水压力加载时,应变减小;上限恒定时,应变持续减小到谷值;卸载时,应变逐渐增大;下限恒定时,应变持续增大到峰值。砂岩p-1ε滞回曲线的2个阶段变化,一是微孔隙压密阶段,未形成明显的滞回曲线;二是孔隙水压力耦合阶段,形成了稳定的滞回曲线,表现形式由疏变密,并且稳定的滞回曲线呈逆时针演化。还对比了不同轴向应力和不同恒定时间条件下,每个p-1ε滞回曲线的四区段（加载段、卸载段、上限恒定段和下限恒定段）的Δε-n关系曲线。     

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。     

Numerical modeling of permeability evolution based on degradation approach during progressive failure of brittle rocks

The permeability evolution of rock during the progressive failure process is described.In combination with the strength degradation index,the degradation formulas of s and a,which are dependent on the plastic confining strain component,the material constants of Hoek–Brown failure criterion are presented,and a modified elemental scale elastic–brittle–plastic constitutive model of rock is established.The relationship between volumetric strain and permeability through tri-axial compression is investigated.Based on the above,a permeability evolution model is established.The model incorporates confining pressuredependent degradation of strength,dilatancy and corresponding permeability evolution.The model is implemented in FLAC by the FISH function method.The permeability evolution behavior of rock is investigated during the progressive failure process in a numerical case.The results show that the model is capable of reproducing,and allowing visualization of a range of hydro-mechanical responses of rock.The effects of confining pressure on degradation of strength,dilatancy and permeability evolution are also reflected.     

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.     

基于破坏类型的本溪灰岩本构关系研究

根据单轴和三轴条件下本溪灰岩的压缩试验和峰后循环加载试验,总结本溪灰岩的强度、变形随围压的变化规律,研究不同围压下本溪灰岩的破坏过程和重复加载过程,分析不同应力条件下本溪灰岩破坏的方式.结果表明:本溪灰岩在应力刚过峰值且未完全进入残余强度阶段,其弹性模量与峰前相同,此阶段进行循环加载时,新的峰值应力低于卸载点应力;在残余强度阶段,残余强度不再随重复加载发生明显变化;采用比较峰值时的环向弹性应变值与环向应变值的方法来判别本溪灰岩的破坏类型是可行的;不同围压下,本溪灰岩的破坏方式有张性破坏和剪切破坏2种类型,这2种破坏方式下本构模型的控制参数是不同的.分别选取了环向应变和剪切应变作为控制参数建立了本构模型,该模型很好地描述了本溪灰岩峰后阶段的应力脆性跌落现象及应力与应变的关系.     

Mechanical property and permeability of raw coal containing methane under unloading confining pressure

Based on domestic-developed triaxial servo-controlled seepage equipment for thermal–hydrological–mechanical coupling of coal containing methane, an experimental study was carried out to investigate mechanical property and gas permeability of raw coal, under the situation of conventional triaxial compression and unloading confining pressure tests in different gas pressure conditions. Triaxial unloading confining pressure process was reducing confining pressure while increasing axial pressure. The research results show that, compared with the peak intensity of conventional triaxial loading, the ultimate strength of coal samples of triaxial unloading confining pressure was lower, deformation under loading was far less than unloading, dilation caused by unloading was more obvious than loading. The change trend of volumetric strain would embody change of gas permeability of coal, the permeability first reduced along with volumetric strain increase, and then raised with volume strain decrease, furthermore, the change trends of permeability of coal before and after destruction were different in the stage of decreasing volume strain due to the effect of gas pressure. When gas pressure was greater, the effective confining pressure was smaller, and the radial deformation produced by unloading was greater. When the unloading failed confining pressure difference was smaller, coal would be easier to get unstable failure.     

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。     

Damage constitutive model for strain-softening rock based on normal distribution and its parameter determination

Firstly, using the damage model for rock based on Lemaitre hypothesis about strain equivalence, a new technique for measuring strength of rock micro-cells by adopting the Mohr-Coulomb criterion was developed, and a statistical damage evolution equation was established based on the property that strength of micro-cells is consistent with normal distribution function, through discussing the characteristics of random distributions for strength of micro-cells, then a statistical damage constitutive model that can simulate the full process of rock strain softening under specific confining pressure was set up. Secondly, a new method to determine the model parameters which can be applied to the situations under different confining pressures was proposed, by deeply studying the relations between the model parameters and characteristic parameters of the full stress-strain curve under different confining pressures. Therefore, a unified statistical damage constitutive model for rock softening which can reflect the effect of different confining pressures was set up. This model makes the physical property of model parameters explicit, contains only conventional mechanical parameters, and leads its application more convenient. Finally, the rationality of this model and its parameters-determining method were identified via comparative analyses between theoretical and experimental curves.     

围压作用下盐岩损伤恢复研究

为了解在围压作用下盐岩损伤恢复特性和盐岩损伤与强度之间的联系,设计了盐岩在不同围压、不同稳压时间之后盐岩强度测试试验。试验结果表明：加载初期,盐岩弹性模量随着稳压时间的增大而逐渐增大,且随着稳压时间的增大,盐岩峰值应力对应的轴向应变和横向应变均会逐渐增大;盐岩损伤与强度之间存在负相关联系;随着稳围压时间的增加,盐岩强度逐渐增大,最后保持在趋于相对稳定的状态;在围压作用下盐岩损伤愈合可以分为损伤快速恢复阶段和损伤缓慢恢复阶段,且对于不同稳压大小,盐岩损伤恢复两个阶段有明显的时间节点;在一定范围内,盐岩损伤快速恢复阶段的时间长短随着稳压阶段围压的增大而逐渐减小;增加稳围压阶段的围压大小可以一点程度上缩短盐岩损伤快速恢复阶段时间,但是增大围压可能会对盐岩造成新的损伤。     

不同围压下茅口灰岩渐进性破坏的试验研究

利用MTS815电液伺服控制刚性试验机进行不同围压下茅口灰岩三轴压缩试验,通过计算绘得相应裂隙体积应变图,分析得出裂纹起始应力、裂纹破坏应力。结果表明:随着围压的增大,应力门槛值均呈非线性增长态势,当围压超过17 MPa时,裂纹起始应力、裂纹破坏应力分别增加48.5%和20.1%,茅口灰岩延性开始增强;裂纹破坏应力为峰值强度的64%~75%,三轴压缩下茅口灰岩裂隙不稳定发展阶段较长;环向应变值随围压增大而增大,当轴力超过裂纹破坏应力进入裂隙不稳定发展阶段,环向应变增大2.7~3.2倍,用环向-轴向应力应变曲线图能较好的反映岩石应力门槛值。     

不同应力状态下北山花岗岩岩爆倾向性研究

为研究不同应力状态下北山花岗岩岩爆倾向性,对北山花岗岩试件进行不同围压下的三轴试验,通过轴向压力和横向变形协调控制加载,获得不同围压下岩石全应力应变曲线,研究岩石破坏前能量聚集及破坏后能量释放特征。结果表明：破坏时弹性能、总能量随着围压增加而增大,且破坏后的总能量变化量及弹性能释放量也随着围压增加而增大。分析岩石储能系数和能量释放指数对岩石破坏的影响,发现岩石储能系数和能量释放指数随围压增加而增大;综合能量储存系数和能量释放指数建立新的岩爆倾向性评价指标,该指标不仅表述了岩石破坏前后各能量的综合变化特征,且能研究不同应力状态下岩石岩爆倾向性;使用该指标研究不同应力状态对北山花岗岩的岩爆倾向性,结果表明：北山花岗岩具有较强的岩爆倾向性,且随着围压增大,岩爆倾向性增加;当围压低于10 MPa时,围压对北山花岗岩岩爆倾向性影响较小,当围压超过10 MPa时,围压对岩爆倾向性的影响突然增加,但随着围压进一步增加,其对岩爆倾向性影响逐渐减弱。     

轴压水压耦合作用下裂隙砂岩蠕变特性

在水利水电、水底隧道等岩土工程建设中,伴有初始损伤的岩石材料长期处于轴压水压耦合环境下,严重影响岩土工程的长期稳定性。为了保证处于轴压水压耦合环境下岩土工程的长期稳定性,以预加载制备的裂隙砂岩岩样为研究对象,利用特制的流固耦合多通道岩石流变系统,开展轴压水压耦合作用下裂隙砂岩单级加载蠕变实验及其蠕变模型研究,主要分析压力水与初始荷载对裂隙岩样蠕变特性的影响变化规律,并讨论新建立蠕变模型的准确性与合理性。蠕变实验研究结果表明：裂隙砂岩的轴向蠕变量随水压增大而减小;蠕变加速阶段的最大蠕变应变率随着水压升高而增大;初始阶段和破坏阶段的最大轴向蠕变应变率随初始荷载增大而增大。基于损伤理论,建立了考虑初始损伤的蠕变模型基本元件的非线性方程;根据幂函数方程,提出了新加速蠕变黏塑性模型;根据轴压水压耦合作用下裂隙砂岩的蠕变变形特征,建立了考虑时效损伤的一维非线性蠕变模型;基于增量理论,构建了轴压水压下考虑时效损伤的三维非线性蠕变模型。新蠕变模型预测的理论蠕变曲线与轴压水压下裂隙砂岩蠕变实验结果吻合度较高,这表明所建立的蠕变模型能较好的描述轴压水压耦合作用下裂隙岩石的蠕变变化特征,新蠕变模型合理,该成果为评价地应力与压力水耦合作用下岩土工程的长期稳定性提供理论依据。