Strength and deformation behaviour of coarse-grained soil by true triaxial tests

In order to investigate the influence of intermediate principal stress on the stress-strain and strength behaviour of a coarse-grained soil, a series of true triaxial tests were performed. The tests were conducted in a recently developed true triaxial apparatus with constant minor principal stress σ3 and constant value of intermediate principal stress ratio b=（σ2-σ3）/（σ1-σ3） （al is the vertical stress, and % is the horizontal stress）. It is found that the intermediate principal strain, ε2, increases from negative to positive value with the increase of parameter b from zero to unity under a constant minor principal stress. The minor principal strain, ε3, is always negative. This implies that the specimen exhibits an evident anisotropy. The relationship between b and friction angle obtained from the tests is different from that predicted by LADE-DUNCAN and MATSUOKA-NAKAI criteria. Based on the test results, an empirical equation of g（b） that is the shape function of the failure surface on re-plane was presented. The proposed equation is verified to be reasonable by comparing the predicted results using the equation with true triaxial test results of soils, such as coarse-grained soils in this study, sands and gravels in other studies.

An anisotropic constitutive model of geomaterials based on true triaxial testing and its application

Series of testing on coarse grained soils were carried out with a true triaxial testing apparatus. The loads were applied from the major principal and minor principal directions, respectively, to simulate the construction and water impounding process of a rock fill dam. The stress and strain relationships induced by the different loading methods were investigated. A remarkable stress-induced anisotropy under complex stress state was observed. Contrary to popular assumptions in traditional numerical analysis and constitutive models, it was found that different elastic modulus and Poisson ratio exist in different principal directions in rock fill dams. From the testing results, an anisotropic constitutive model based on Duncan-Chang nonlinear model is presented to overcome the limitations of axi-symmetric assumptions in conventional triaxial experiments and constitutive models. Both models were then applied in FEM analysis of an under-construction earth core high rock soil filled dam with the focus on hydraulic fracturing. The study reveals the major biases that exist when numerical analysis and constitutive models do not give serious consideration to the intermediate principal stress and anisotropy effects in soil rock built structures.

Intermittent disturbance mechanical behavior and fractional deterioration mechanical model of rock under complex true triaxial stress paths

Mechanical excavation,blasting,adjacent rockburst and fracture slip that occur during mining excavation impose dynamic loads on the rock mass,leading to further fracture of damaged surrounding rock in three-dimensional high-stress and even causing disasters.Therefore,a novel complex true triaxial static-dynamic combined loading method reflecting underground excavation damage and then frequent intermittent disturbance failure is proposed.True triaxial static compression and intermittent disturbance tests are carried out on monzogabbro.The effects of intermediate principal stress and amplitude on the strength characteristics,deformation characteristics,failure characteristics,and precursors of monzogabbro are analyzed,intermediate principal stress and amplitude increase monzogabbro strength and tensile fracture mechanism.Rapid increases in microseismic parameters during rock loading can be precursors for intermittent rock disturbance.Based on the experimental result,the new damage fractional elements and method with considering crack initiation stress and crack unstable stress as initiation and acceleration condition of intermittent disturbance irreversible deformation are proposed.A novel three-dimensional disturbance fractional deterioration model considering the intermediate principal stress effect and intermittent disturbance damage effect is established,and the model predicted results align well with the experimental results.The sensitivity of stress states and model parameters is further explored,and the intermittent disturbance behaviors at different f are predicted.This study provides valuable theoretical bases for the stability analysis of deep mining engineering under dynamic loads.

Microdynamic mechanical properties and fracture evolution mechanism of monzogabbro with a true triaxial multilevel disturbance method

The far-field microdynamic disturbance caused by the excavation of deep mineral resources and underground engineering can induce surrounding rock damage in high-stress conditions and even lead to disasters.However,the mechanical properties and damage/fracture evolution mechanisms of deep rock induced by microdynamic disturbance under three-dimensional stress states are unclear.Therefore,a true triaxial multilevel disturbance test method is proposed,which can completely simulate natural geostress,excavation stress redistribution(such as stress unloading,concentration and rotation),and subsequently the microdynamic disturbance triggering damaged rock failure.Based on a dynamic true triaxial test platform,true triaxial microdynamic disturbance tests under different frequency and amplitudes were carried out on monzogabbro.The results show that increasing amplitude or decreasing frequency diminishes the failure strength of monzogabbro.Deformation modulus gradually decreases during disturbance failure.As frequency and amplitude increase,the degradation rate of deformation modulus decreases slightly,disturbance dissipated energy increases significantly,and disturbance deformation anisotropy strengthens obviously.A damage model has been proposed to quantitatively characterize the disturbance-induced damage evolution at different frequency and amplitude under true triaxial stress.Before disturbance failure,the micro-tensile crack mechanism is dominant,and the micro-shear crack mechanism increases significantly at failure.With the increase of amplitude and frequency,the micro-shear crack mechanism increases.When approaching disturbance failure,the acoustic emission fractal dimension changes from a stable value to local large oscillation,and finally increases sharply to a high value at failure.Finally,the disturbance-induced failure mechanism of surrounding rock in deep engineering is clearly elucidated.

Experimental study of the dynamic mechanical responses and failure characteristics of coal under true triaxial confinements

Investigations on the dynamic mechanical properties and failure mechanisms of coal under in-situ stress is essential for the prevention of dynamic disasters in deep coal mines.Thus,a modified true triaxial Hopkinson bar was employed to explore the dynamic mechanical behaviors of coal at different confining pressures(0–20 MPa)and strain rates(40–220 s^(-1)).The results show that the dynamic peak stress is positively correlated with lateral static pre-stressσy andσz,but negatively correlated with axial static prestressσx.At approximate strain rates,increasing the lateral static pre-stress facilitates increasing the dynamic peak stress,but the minimum lateral static pre-stress is the primary factor limiting a significant increase in dynamic peak stress of coal.Furthermore,the dynamic differential stress is linearly related to the logarithm of strain rate,and the peak strain varies linearly with strain rate.However,there is no significant correlation between confining pressure and peak strain.Moreover,X-ray CT images and photographic fracture observations of coal samples show the failure patterns under uniaxial and triaxial conditions are splitting failure and shear failure,respectively.The device provides a viable approach for fully comprehending the dynamic mechanical behaviors of rock-like material in complex stress conditions.

New artificial neural networks for true triaxial stress state analysis and demonstration of intermediate principal stress effects on intact rock strength

Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stress on the intact rock strength are investigated and compared with laboratory results from the literature. To normalize differences in laboratory testing conditions, the stress state is used as the objective parameter in the artificial neural network model predictions. The variations of major principal stress of rock material with intermediate principal stress, minor principal stress and stress state are investigated. The artificial neural network simulations show that for the rock types examined, none were independent of intermediate principal stress effects. In addition, the results of the artificial neural network models, in general agreement with observations made by others, show （a） a general trend of strength increasing and reaching a peak at some intermediate stress state factor, followed by a decline in strength for most rock types; （b） a post-peak strength behavior dependent on the minor principal stress, with respect to rock type; （c） sensitivity to the stress state, and to the interaction between the stress state and uniaxial compressive strength of the test data by the artificial neural networks models （two-way analysis of variance; 95% confidence interval）. Artificial neural network modeling, a self-learning approach to polyaxial stress simulation, can thus complement the commonly observed difficult task of conducting true triaxial laboratory tests, and/or other methods that attempt to improve two-dimensional （2D） failure criteria by incorporating intermediate principal stress effects.

Study on fracture propagation behavior in ultra-heavy oil reservoirs based on true triaxial experiments

As the ultra-heavy oil reservoirs developed by steam assisted gravity drainage(SAGD)in the Fengcheng oilfield,Xinjiang have problems such as huge steam usage,long preheating period,low production,and inaccessible reserve in local parts.Based on the rock mechanics and porosity/permeability characteristics of heavy oil reservoir and interlayer,a series of true triaxial experiments and CT tests considering the fracturing fluid injection rate,viscosity,perforation density and location of fracture initiation were conducted to disclose the propagation behavior of micro-and macro-fractures in the reservoirs and mudstone interlayers.These experiments show that fracturing in the heavy oil reservoirs only generates microfractures that cannot break the interlayer.In contrast,when fracturing in the interlayer,the higher the injection rate(greater than 0.6 m^3/min),the lower the viscosity,the easier it is to form macro-fractures in the interlayers,and the further the fractures will propagate into the reservoirs.Also,increasing perforation density tends to create complex macro-fracture network in the interbedded reservoirs and mudstone interlayers.The findings of this study can provide scientific guidance for the selection of fracturing layer and the optimization of parameters in the interlayer fracturing of heavy oil reservoirs.

复杂应力条件下粗粒料的剪胀性研究进展

粗粒料的剪胀性是其强度和变形研究的重要问题之一,它取决于颗粒排列、粒间压力等细微观作用;在宏观层面,受相对密度与应力状态的影响。围绕复杂应力条件下粗粒料的剪胀问题,从粗粒料力学特性的试验研究、粗粒料的剪胀性强度准则以及临界状态与剪胀性方面,对国内外的研究进展进行了系统论述。在此基础上,提出该领域呈现如下发展态势:①通过大型真三轴试验开展粗粒料剪胀率的试验研究与理论分析,能够促进粗粒料本构理论的发展进步;②结合应力路径试验中的应力变形规律研究,可实现从常规三轴条件下向复杂应力条件下的剪胀方程的进一步发展,从而体现强度与变形问题的统一;③采用状态参量去评价土体的松密状态与剪胀性,能够耦合应力状态与孔隙比,尽量避免“一种材料,多组参数”。研究结果表明状态参量和应力路径相结合的方法,使得粗粒料的力学特性研究更加具有科学性和系统性。

A generalized nonlinear three-dimensional Hoek‒Brown failure criterion

To understand the strengths of rocks under complex stress states,a generalized nonlinear threedimensional(3D)Hoek‒Brown failure(NGHB)criterion was proposed in this study.This criterion shares the same parameters with the generalized HB(GHB)criterion and inherits the parameter advantages of GHB.Two new parameters,b,and n,were introduced into the NGHB criterion that primarily controls the deviatoric plane shape of the NGHB criterion under triaxial tension and compression,respectively.The NGHB criterion can consider the influence of intermediate principal stress(IPS),where the deviatoric plane shape satisfies the smoothness requirements,while the HB criterion not.This criterion can degenerate into the two modified 3D HB criteria,the Priest criterion under triaxial compression condition and the HB criterion under triaxial compression and tension condition.This criterion was verified using true triaxial test data for different parameters,six types of rocks,and two kinds of in situ rock masses.For comparison,three existing 3D HB criteria were selected for performance comparison research.The result showed that the NGHB criterion gave better prediction performance than other criteria.The prediction errors of the strength of six types of rocks and two kinds of in situ rock masses were in the range of 2.0724%-3.5091%and 1.0144%-3.2321%,respectively.The proposed criterion lays a preliminary theoretical foundation for prediction of engineering rock mass strength under complex in situ stress conditions.

基于真三轴试验装置的岩石渗流密封方式改进及实验研究

国内现有的非常规油气储层的渗透性测试大多采用圆柱试样,通过假三轴试验装置进行测试,而真三轴条件下立方体试件棱边大多存在棱边串流与端面密封不严的缺陷。基于现有真三轴试验装置,以及大量的实验经验,针对现有的立方体试件单橡胶套渗流密封方式进行改进。介绍了嵌套式“上下密封套”结构特征,特别对棱边区的密封进行了设计,提出了密封套顶-底端面凹陷型内嵌“单孔网槽导流板”结构设计,并对顶部传压板的尺寸进行了讨论。对100 mm×100 mm×100 mm立方体煤样进行了真三轴渗流试验分析,得到了不同入口渗透压力真三轴围压条件下的渗透率。该密封方法的改进有助于提高科研单位及测试中心对原位地应力条件下非常规油气储层渗透性测试精度,并提高高端科研人才的实践创新能力与科研素养。

基于真三轴卸载试验不同倾角组合煤岩力学特性研究

为深入研究煤炭开采过程中顶板结构面倾角对煤岩力学特性的影响,通过自主研发的地声过程模拟试验系统,开展不同结构面倾角条件下组合煤岩真三轴加卸载试验,并利用声发射探测系统进行监测。研究结果表明:随着结构面倾角的增大,试样的破坏强度逐步下降,裂隙发展也逐渐减弱,其破坏形态从张拉剪切破坏逐渐向剪切破坏转变,直至倾角达到40°时,试样整体发生滑移破坏;在卸载破坏前,倾角<30°的试样发生充分的塑性变形,其承载力得到充分发挥,而倾角≥30°的试样达到临界破坏极限时迅速破坏,出现部分或完全滑移破坏;声发射信号集中于卸载破坏阶段,试样的结构面倾角越大,声发射累计计数越少,当倾角达到40°时发生滑移破坏,最大振铃计数大幅下降,对其累计计数略有影响。研究成果可为深入认识顶板结构面倾角引起的卸载破坏机制及矿山安全开采提供参考。

考虑中间主应力效应的修正Hoek-Brown真三轴强度准则

为改善Hoek-Brown强度准则未考虑中间主应力对岩石强度的影响,通过岩石真三轴试验结果分析了岩石强度的演化规律,引入中间主应力系数,量化分析主应力系数对岩石真三轴强度的影响,考虑Hoek-Brown强度准则中的参数和岩石应力水平间的关联性,提出了基于拉格朗日插值方法的修正岩石真三轴Hoek-Brown强度准则,分析了其空间包络特征。最后,利用7种岩石的真三轴试验数据与其它3种真三轴强度准则进行最优拟合误差分析,探讨了修正强度准则的合理性。研究结果表明:岩石强度随着最小主应力的增加逐渐增大,随着中间主应力的增加则呈现先增大后减小的变化规律,表现出显著的区间效应;修正强度准则不仅能够继承Hoek-Brown强度准则在子午面上的非线性优点,且能够表征岩石强度在应力空间中的基本特性;线性和非线性插值形式的修正强度准则空间包络面分别为非等边的六棱锥面以及能够满足拉压子午面区间连续光滑要求的圆锥面;修正强度准则能够较好地预测岩石真三轴试验强度,较为合理地反映中间主应力对岩石强度的影响;相比于线性插值形式强度准则,二次插值形式强度准则能够提高岩石强度的预测精度1.2~2.0倍;相较于其他真三轴强度准则,修正强度准则对不同硬脆性岩石的真三轴试验结果均具有良好的强度预测精度,体现了修正强度准则的适用性和可靠性。

弱动力扰动作用下岩石微裂隙演化特征及灾害防控

【目的】高应力叠加弱动力扰动是诱发冲击地压的关键因素,但不同扰动幅值、频率、卸载范围下的岩石微裂纹扩展特征和能量耗散规律尚不明确,无法为冲击地压防治提供技术支撑。【方法】基于真三轴卸载动力扰动试验,分析了不同扰动幅值(5、10 MPa)、频率(4、10 Hz)、三向应力卸载(0、12MPa)下深部围岩失稳破坏规律,并结合SEM扫描分析了岩石微裂隙特征。通过锚杆拉拔试验,优化了锚杆肋间距和肋高,提高了其吸能支护作用,提出了“吸能锚杆-低阻抗混凝土注浆-喷浆-挂网”组合支护技术。利用传感器对巷道进行长期监测,得到治理前后压力与振动数据。【结果和结论】研究表明:(1)随着扰动幅值和频率的增加,裂纹增加显著且不规则,岩石断口的方向分形维数降低。当扰动为10MPa、10Hz时,分形维数降至最低值0.62,孔隙方向角80°~120°孔隙定向频率达到最大值的52%,约为原始岩石的1.68倍。说明岩石受扰动后颗粒的应力不均匀,导致应力集中,断裂方向明显。(2)随着扰动幅值和频率的增加,SEM图像的微孔隙面积先快速增加,后缓慢增加且增加趋势越来越小。扰动频率每增加2 Hz,岩石微裂隙面积增加约24.13%。(3)现场测试表明随着锚杆肋间距和肋高增加,拉拔曲线形态由“弹塑性阶段-破坏失效阶段-残余阶段”逐渐过渡为“弹塑性阶段-微量屈服阶段-大量强化阶段-破坏失效阶段-残余阶段”,肋间距48mm、肋高2mm的螺纹钢锚杆吸能效果最好。经现场监测可将巷道压力稳定在36 N左右,峰值加速度控制在8000mm/s^(2)以内。研究揭示了卸载动力扰动作用下围岩破坏及能量释放规律,提出的“吸能锚杆-低阻抗混凝土注浆-喷浆-挂网”支护技术,可为类似深部工程提供理论指导。

基于离散元的颗粒材料中应变局部化形成与演化研究

岩土颗粒材料的应变局部化失效问题广泛存在于工程设计应用中,主要表现为介观尺度上的应变局部化现象和宏观尺度上的剪切带产生,目前在微观尺度上的形成机理尚不明确。为了系统研究颗粒集合体的应变局部化的形成与演化过程,通过使用离散单元法(Discrete Element Method,DEM)模拟了指定平面应变加载路径的真三轴试验,获取了宏观和微观尺度上的颗粒材料几何、运动以及力学信息。为了找出表征应变局部化特性的最佳特征量,比较了颗粒温度、波动位移和局部剪胀角等微观指标,发现波动位移在表征应变局部化方面与其他参数相比相关性更好,并选定其作为应变局部化表征变量。为了量化颗粒集合体从应变局部化开始产生到发育完成的具体应变区间,采用莫兰指数对波动位移的空间分布特征进行统计和分析,确定了颗粒集合体在弹塑性转换阶段的发育区间。进一步对颗粒集合体应变局部化发育区间内的波动位移空间分布进行探究,并统计不同空间区域内波动位移的概率密度函数,发现研究区域外部的局部塑性在过渡阶段停止演化,而内部塑性以逾渗模式发展。最终,通过波动位移的空间分布进行聚类分析,获得了描述介观尺度上应变局部化的团簇体模型。该模型可以将颗粒集合体宏观剪切带的形成与发育和单个颗粒的微观塑性发展相联系,刻画颗粒集合体从介观尺度上的应变局部化出现到宏观尺度上剪切带完全形成的演化过程。

单面卸载条件下深埋圆形洞室围岩岩爆的试验模拟研究

为深入研究深部洞室开挖诱发邻近既有洞室围岩岩爆的发生过程和机制,采用真三轴电液伺服诱变试验系统对具有强岩爆倾向性的花岗岩开展试验研究,模拟500 m的三维应力状态。通过卸载洞室围岩单侧荷载模拟邻近洞室开挖,在围岩应力重分布作用下最终诱发岩爆,试验中借助监测设备实时观测洞壁的破坏现象。试验结果表明:(1)在单面卸载条件下,岩爆过程可分为6个阶段,即卸载后的应力重分布、准备期、第1轮岩屑弹射和剥落、平静期、第2轮岩屑弹射和剥落、剧烈破坏;(2)在卸荷作用的影响下,洞壁岩爆呈现出明显的时空特征和非对称性,表现为卸荷后洞室顶底部率先出现明显裂纹,在随后的持续加载过程中,远离卸载面一侧发生剧烈岩爆并形成沿轴向贯通的V型槽;(3)基于定义的滞后效应系数可知,与三向加载岩爆相比,单面卸载下的洞室岩爆具有更强的突然性,且极易导致洞室的整体垮塌。为预防该类型岩爆,邻近洞室开挖时应及时对既有洞室远离卸载面一侧进行支护和加固处理。

含层理砂岩真三轴条件下力学特性研究

为了研究层状砂岩的力学特性与各向异性特征,开展了不同层理倾角与不同中间主应力下的砂岩力学试验。对7种不同倾角(0。、15。、30。、45。、60。、75。、90。)的砂岩进行了3个不同中间主应力水平(10、100、160 MPa)的真三轴试验研究,探讨了层理面倾角对砂岩变形、强度和破坏模式的影响。结果表明,随着中间主应力的增加,砂岩的各向异性逐渐减弱,并趋于各向同性;在高中间主应力的条件下,杨氏模量和破坏角均随各向异性角的增加逐渐增大;峰值强度也受到中间主应力和层面倾角耦合作用的影响;在中间主应力较低的条件下,峰值强度表现出典型的U型变化,随着中间主应力的增加,U型曲线逐渐变得平坦。该研究结果对于指导深埋硬岩工程的设计和施工具有一定的参考意义。

真三轴条件下延安蟠龙地区致密砂岩力学特性试验

为了研究不同荷载作用下延安蟠龙地区致密砂岩的强度及变形特征,对致密砂岩开展真三轴试验,研究常规三轴应力下抗压强度与围压的关系,以及真三轴应力下σ_(3)和σ_(2)对致密砂岩强度和变形的影响。研究结果表明,真三轴应力下,σ_(3)和σ_(2)对致密砂岩的强度都起了一定的保护作用,使得岩石的承载能力增强;随着σ_(3)和σ_(2)的增大,岩样的弹性模量和峰值强度均不断增大,峰值应变逐渐减小。σ_(1)-ε_(1)峰值前后曲线陡升陡降,说明研究区致密砂岩具有明显的脆性特征,表明研究区在压裂过程中容易形成压裂缝,有利于非常规油气的开采。该研究对致密砂岩油气开采具有借鉴意义。

广义塑性本构模型三维化方法的研究与验证

在二维空间中基于p、q分量构建的广义塑性本构模型无法反映中主应力对土石料变形特性的影响,因此需要将其进行三维化以反映出一般应力状态下本构关系。研究了多种三维化方法,通过与真三轴试验对比,基于广义非线性强度理论(generalized nonlinear strength theory, GNST)的三维化方法可以使本构模型较好地反映土石料一般应力路径下的应力应变关系。对某面板堆石坝进行的数值计算,并与监测结果对比,表明三维化后的本构模型能更好地反映堆石坝的变形规律。因此,基于GNST的模型三维化方法可以应用于高土石坝设计中,并可以获得较好的预测结果。

真三轴应力状态下含预制裂隙煤系泥岩强度与破坏特征

对一系列含有预制缺陷的煤系泥岩试样进行了真三轴试验(TTT),即中间主应力(σ_(2))平行于预制缺陷的试样(TTT-Flaw-2)和最小主应力平行于预制缺陷的试样(σ_(3))(TTT-Flaw-3),研究σ_(2)加载方向和预制缺陷的位置对试样强度和破坏模式的影响。结果表明,完整试样和含预制缺陷试样的峰值强度随σ_(2)的增加先增大后减小,符合Mogi-Coulomb准则。在相同的应力条件下,完整试样的强度明显大于含预制缺陷试样的强度,且TTT-Flaw-2试样的强度最低。CT扫描结果显示,在常规三轴试验条件下,含预制缺陷试样中的裂纹并不总是沿着预制缺陷尖端形成,而是随机分布在试样内部;在TTT-Flaw-2试样中,反翼型裂纹从预制缺陷尖端附近开始形成,最终主要分布在σ_(2)加载方向上;而在TTT-Flaw-3试样中,在σ_(2)加载方向上出现剪切裂纹,而在σ_(3)加载方向上几乎没有观察到反翼型裂纹。

不同岩性与断面形状的深部硬岩隧洞板裂失稳试验

为探究岩性与断面形状对深部硬岩隧洞板裂失稳特性的影响,开展板裂失稳室内真三轴试验。首先,选择以大理岩和花岗岩为主的2种硬岩试样以及高边墙城门洞形和曲墙拱形2种实际应用中的典型断面形状试样;然后,从板裂失稳形态、剥落岩板(片)特征及板裂特征应力3方面入手,对比分析不同岩性与断面形状影响下的不同破坏特征;最后,利用数值模拟探究围岩板裂裂纹发育扩展过程中相应的位移及应力分布特征,进一步揭示深部硬岩隧洞的板裂失稳特性。结果表明:试验过程中,对于板裂失稳形态,与花岗岩相比,大理岩试样岩板剥落及张开破坏现象更为显著;与高边墙城门洞形相比,曲墙拱形试样板裂破坏涉及的断面轮廓范围更小。对于剥落岩板(片)特征,不同岩性产生不同的剥落岩板形态;与高边墙城门洞形相比,曲墙拱形试样对应的近外层剥落岩板更趋细长。对于板裂特征应力,与大理岩相比,花岗岩试样在板裂开始时的应力门槛值更高,板裂失稳演化进程更快;与曲墙拱形相比,高边墙城门洞形试样从开始出现板裂直至发生明显板裂破坏的特征应力均更高。数值模拟中的位移较大区域集中在孔洞侧壁处,距离侧壁越远,位移相对越小;切向应力集中是导致板裂发生的主要原因。