南京粉砂三轴压缩过程中的三维孔隙结构演化特征

砂土三轴剪切过程中细观孔隙结构演化与其宏观力学性质之间存在着紧密联系。基于南京大学自主研发的微型三轴仪,对南京粉砂试样进行了三轴剪切试验,利用显微CT扫描,获得了加载过程中试样的三维孔隙结构演化特征。结果表明:试样表现出应变软化-硬化型的应力-应变关系曲线;端部摩擦阻碍试样体积膨胀,发生鼓形区状破坏;试样整体体孔隙率从初始的46.3%减小到40.9%,后增大到43.0%附近,且试样破坏区孔隙率显著增大;统计区状破坏区表征单元体孔隙参数,发现破坏前孔隙被压密消失,孔隙数量减少32.4%,平均孔隙半径减小48.8%,导致孔隙半径分布趋于集中,破坏后破坏区域内出现大量大半径的孔隙;颗粒间错动导致颗粒棱角破碎,使孔隙形状趋于规则。     

基于结构性及各向异性的软黏土变形性状试验

针对结构性和各向异性两大要素对天然软黏土变形性状的影响,对上海天然沉积软黏土进行一维固结试验、K0固结试验以及三轴不排水剪切试验。结果表明:上海天然沉积软黏土具有明显的结构性及各向异性特征;压缩特性在结构屈服破坏前后存在显著差别,具有结构性软黏土所特有的分段特征;不同初始固结应力下的等压及偏压固结不排水应力路径曲线最终都趋近于同一临界状态;在初始有效固结应力低于软黏土的结构屈服应力时,应力-应变曲线呈应变软化型,随着初始有效固结应力的增加,当其超过结构屈服应力后,应力-应变曲线逐渐呈现硬化型特征;在初始有效固结应力相同且低于结构屈服应力的前提下,偏压固结模式所对应的不排水剪切峰值强度高于等压固结模式,且屈服后的应变软化程度相较于等压固结模式更高;在不排水剪切强度达到峰值后,等压和偏压固结模式下的应力-应变曲线逐渐趋于重合;在初始有效固结应力相等的情况下,等压固结模式下产生的孔隙水压力较偏压模式下要高。     

地震前兆特征与岩样剪切应变率异常数值模拟

利用拉格朗日元法(FLAC)模拟了岩样的宏观力学行为、剪切应变率异常及演化。在峰值强度岩石的本构模型分别取线弹性模型及应变软化模型的条件下,得到如下结论：1)在弹性阶段之后,应力-时步曲线具有一定的周期性。与地震复发的周期性类似。在加载过程中,形变场由均匀逐渐向不均匀转变．最终形成交叉的剪切带网络。被该网络所包围的试样中部可视为背景空区．在空区之外,地震群比较活跃。在应变硬化阶段。可以从未来震源观测到前兆一剪切应变率异常条带。2)在中期阶段之后,空区内地震活动的开始增强明显要比空区外地震活动增强晚。地震的迁移是由外向内。3)短期阶段则相反：在硬化阶段．剪切带内外的剪切应变率的差别不很显著。试样处于相对平静状态。在软化阶段,若采用剪切应变率最大的位置作为震源．可确定震源在试样的中部。源兆和场兆都集中在老断层上。     

土工格室加筋正常固结粉质黏土应力应变响应

土工格室加筋正常固结粉质黏土对工程建设意义重大。基于土工格室与填土的相互作用机制和增量法,采用弹塑性理论、邓肯-张双曲线模型、修正剑桥屈服函数以及剪胀方程,推导了土工格室加筋正常固结黏土的应力应变响应计算模型,进行了土工格室加筋正常固结黏土的三轴固结排水剪切试验,实测不同围压下的应力应变关系,采用三轴试验结果验证了所提理论模型的正确性和可靠性。此外,采用所提理论模型进行了参数敏感分析,研究了填土的力学性质参数、格室的网格尺寸和力学性质参数对于加筋土应力应变响应的影响。研究结论表明,与砂砾料相似,土工格室约束作用对于正常固结黏土的内摩擦角影响较小,而黏聚力有所增加;随着轴向应变的增加或围压的减小,加筋效果逐渐明显;加筋正常固结黏土的强度和刚度随着非线性弹性常数k的增加、强度参数φ的增加和R_f的减小而增大,常数n对于加筋土应力应变响应影响较小。本研究成果可为工程建设提供理论借鉴。     

应变能-莫尔准则及其在岩石材料中的应用

本文应用应变能密度与三向应力圆的等价关系,分析修正了莫尔准则,提出用任意确定静水压力下不同洛德参数的极限莫尔圆包络曲线簇作准则线,即谓之为应变能-莫尔准则线,并引用岩土真三轴试验资料进行验证;最后还得出应变能-莫尔屈服条件,分析了屈服曲面和屈服轨迹的形状等。     

略论深基坑土体 c、φ值的某些影响因素

文章在分析直接剪切和三轴剪切试验结果的基础上,指出了土体的应力历史条件和应力路径是影响深基坑土体c、φ值的重要因素.     

黄土-三趾马红土滑坡滑带土剪切力学特性影响因素

中国西北地区黄土广泛沉积于三趾马红土之上形成"双层异质"接触层面,控制和影响黄土滑坡的形成。为研究黄土-三趾马红土界面剪切力学特性,研制可视化界面直剪仪,开展界面试样剪切试验,探讨齿面角度、含水率、干密度对其剪切破坏模式、强度与变形特性影响规律。结果表明:试样均沿界面产生剪切破坏,试样剪切破坏模式可分为齿间滑动、齿间滑动-齿面剪断、齿面剪断3种;界面试样剪切应力-位移曲线呈应变软化型,峰值强度后应变"跳跃"及"塑态"特征反映出界面的脆性与塑性剪断破坏特征,剪切刚度、剪切破坏位移演化规律均受界面接触条件影响;界面剪切过程存在明显剪胀效应,剪胀位移演化规律反映了剪切过程的齿面挤密、齿间滑动、齿面剪断与界面摩擦阶段;界面试样强度随界面接触条件均呈非线性变化,齿面角度越大,峰值强度与残余强度越高,试样剪切破坏模式越趋于脆性齿面剪断;含水率越高,界面峰值强度越低,试样剪切破坏模式越趋于塑性齿面剪断;界面试样干密度越大,峰值强度越大,残余强度越低,试样剪切破坏模式越趋于脆性的齿间滑动-齿面剪断。研究成果可为进一步研究黄土-三趾马红土界面强度准则,揭示黄土-三趾马红土复合型滑坡启动机制提供参考。     

渗流-应力耦合下侏罗系红砂岩力学及渗透特性试验研究

三峡库区广泛分布侏罗系红砂岩, 在水库运行期间红砂岩的渗流-应力耦合特性关乎库区内多数滑坡和岩质边坡的稳定性。借助岩石多场耦合三轴试验系统, 对三峡库区侏罗系红砂岩开展了不同围压、不同渗透压下的三轴压缩试验, 系统研究了红砂岩的三轴压缩力学特性和渗透率演化特征。研究结果表明: ①红砂岩的峰前应力-应变曲线可分为孔隙压密阶段、弹性变形阶段、微裂隙稳定发展阶段和非稳定发展阶段。红砂岩的力学参数与渗透压的关系呈负相关, 与围压呈正相关。②随着围压升高, 红砂岩的破坏模式由张拉破坏过渡到剪切破坏。③不同渗透压下, 渗透率曲线呈平稳发展→缓慢上升→快速上升3阶段演化规律; 不同围压下, 渗透率曲线先降低后升高。④从能量角度分析了渗透压和围压对岩石的作用, 验证了渗流对岩石的劣化效应以及围压对裂纹发展的抑制作用。本试验对鲜有报道的侏罗系红砂岩的强度、变形和渗透特性做了系统的研究, 对渗流-应力耦合课题有补充意义。其工况根据三峡库区边坡岩体的应力水平来确定, 试验结果对分析库区边坡稳定性具有指导意义。      

西天山特克斯达坂尔戈带金矿构造特征及其控矿作用

通过对西天山伊什基里克山东段的尔戈带金矿野外地质调查，结合室内显微构造和岩组分析得出，金矿赋矿层位属下石炭统大哈拉军山组上段岛弧火山碎屑岩。矿区在海西期发生了3次构造活动：早石炭世晚期，伊什基里克背斜核部产生的右行近东西向韧-脆性剪切带，无论沿走向还是顺倾向均呈舒缓波状，带内发育片理化带、S-C面理、A型褶皱、A型线理、石英亚颗粒、波状消光、旋转碎斑系和构造透镜体等，石英亚组构均具有小圆环带趋势和强极密部（c＋g）型式，开角约36°，σ1≈317°∠84°；晚石炭世，韧-脆性剪切带经历左行剪切，随着脆性程度的增加，近东西向左行压扭性断裂形成，应变椭球为三轴扁椭球状，平面压扁型应变；晚石炭世末期到早二叠世，北西向压扭性断裂带产生。尔戈带金矿体位于近东西向韧-脆性剪切带与近东西向、北西向断裂（带）的叠加部位，韧-脆性剪切带使部分金元素从矿源层分溢出来，产生初步聚集，其后压扭性断裂中局部张扭性部位是金沉淀、富集的有利场所，并在此形成金矿体。因此，早石炭世晚期到早二叠世，矿区内发育的韧-脆性剪切带和断裂（带）对尔戈带金矿有重要的控制作用。     

东北地区近期水平形变应变场研究

根据2011年和2013年东北地区GPS观测资料计算得到的地壳水平运动场和连续主应变场,研究了东北地区地壳运动特征和应变应力状态, 求解块体运动参数,计算东北主要断裂带的错动速率和断层应变应力状态,分析断裂带活动性。研究表明,2011～2013年期间,东北地区东部运动规律性相对较好,且东部运动场收敛于珲春-龙井一带,向东位移;中部和西部运动场一致性较弱,位移方向相对分散。主应变反映出,东北地区2011～2013年主压应力轴优势方向为NE55°。两个水平主应变轴都缩短的挤压区分布面积较小,两个主应变轴都伸长的拉张区次之,最小主应变轴缩短的区域和最大主应变轴伸长的区域所占比重最大。该时段Ms≥5.0地震集中在2013年,地震多发生在运动方向转折带附近及挤压-剪切活动区,最大的5.8级地震发生在不同运动区交界带和挤压作用相对强烈的构造带。     

Effect of Freeze-Thaw Cycles on Mechanical Properties and Permeability of Red Sandstone under Triaxial Compression

Geological disasters will happen in cold regions because of the effects of freeze-thaw cycles on rocks or soils, so studying the effects of these cycles on the mechanical characteristics and permeability properties of rocks is very important. In this study,red sandstone samples were frozen and thawed with 0,4, 8 and 12 cycles, each cycle including 12 h of freezing and 12 h of thawing. The P-wave velocities of these samples were measured, and the mechanical properties and evolution of the steady-state permeabilities were investigated in a series of uniaxial and triaxial compression tests. Experimental results show that, with the increasing of cyclic freeze-thaw times, the P-wave velocity of the red sandstone decreases. The number of freeze-thaw cycles has a significant influence on the uniaxial compressive strength, elastic modulus, cohesion, and angle of internal friction. The evolution of permeability of the rock samples after cycles of freeze-thaw in a complete stress-strain process under triaxial compression is closely related to the variation of the microstructure in the rock. There is a highly corresponding relationship between volumetric strain and permeability with axial strain in all stages of the stress-strain behaviour.     

Mechanical Properties of Methane Hydrate-Bearing Interlayered Sediments

The complex distribution of gas hydrate in sediments makes understanding the mechanical properties of hydrate-bearing sediments a challenging task. The mechanical behaviors of hydrate-bearing interlayered sediments are still poorly known. A series of triaxial shearing tests were conducted to investigate the strength parameters and deformation properties of methane hydrate-bearing interlayered sediments at the effective pressure of 1 MPa. The results indicate that the stress-strain curves of hydrate-bearing interlayered sediments are significantly different from that of hydrate-bearing sediments. The peak strength, Young's modulus, initial yielding modulus, and failure mode are deeply affected by the methane hydrate distribution. The failure behaviors and mechanism of strain softening and hardening patterns of the interlayered specimens are more complicated than those of the integrated specimens. This study compares the different mechanical behaviors between integrated and interlayered specimens containing gas hydrate, which can serve as a reference for the prediction and analysis of the deformation behaviors of natural gas hydrate reservoirs.     

Cyclic Shearing Deformation Behavior of Saturated Clays

The apparatus for static and dynamic universal triaxial and torsional shear soil testing is employed to perform stress-controlled cyclic single-direction torsional shear tests and two-direction coupled shear tests under unconsolidated-undrained conditions. Through a series of tests on saturated clay, the effects of initial shear stress and stress reversal on the clay’s strain-stress behavior are examined, and the behavior of pore water pressure is studied. The experimental results indicate that the patterns of stress-strain relations are distinctly influenced by the initial shear stress in the cyclic single-direction shear tests. When the initial shear stress is large and no stress reversal occurs, the predominant deformation behavior is characterized by an accumulative effect. When the initial shear stress is zero and symmetrical cyclic stress occurs, the predominant deformation behavior is characterized by a cyclic effect. The pore water pressure fluctuates around the confining pressure with the increase of cycle number. It seems that the fluctuating amplitude increases with the increase of the cyclic stress. But a buildup of pore water pressure does not occur. The de- formations of clay samples under the complex initial and the cyclic coupled stress conditions include the normal deviatoric deforma- tion and horizontal shear deformation, the average deformation and cyclic deformation. A general strain failure criterion taking into account these deformations is recommended and is proved more stable and suitable compared to the strain failure criteria currently used.     

Unsaturated Creep Behaviors of Weak Intercalated Soils in Soft Rock of Badong Formation

The cutting slopes in soft rock of redbed appeared in Yichang-Badong highway often suffer from the instability along weak intercalations, so the creep behaviors of weak intercalated soils are crucially important for the stability of cutting slopes. Because the deformation of weak intercalated soils is significantly affected by water content due to the strong water sensitivity, it is necessary to study the influence of matric suction on the creep behaviors of weak intercalated soils. In order to find out the unsaturated creep characters of weak intercalated soils, a GDS unsaturated triaxial apparatus was used. Then the triaxial creep experiments on weak intercalated soil samples under varying matric suction were conducted to obtain the unsaturated creep curves. The results show that the weak intercalated soils have obvious creep behaviors, and the creep strain is in nonlinear relationship with stress and time. When the matric suction is constant, a larger deviator stress will lead to a larger creep strain; When the deviator stress is constant, a smaller matric suction will lead to a larger creep strain. Based on the Mesri creep model, an improved creep model for weak intercalated soils under varying matric suction was established, in which the relationship of stress-strain was expressed with a hyperbolic function, and the relationship of strain-time was expressed with power functions in stages. Then an unsaturated creep model including stress-matric suction-strain-time for weak intercalated soils was established based on the power function relationship between matric suction and Ed(a parameter of the improved creep model). The comparison of the calculated values of creep model and the experimental values shows that the creep behaviors of weak intercalated soils can be predicted by the unsaturated creep model by and large.     

基于Hoek-Brown准则的节理岩体能量参数估算

岩体变形和破坏可以看作能量耗散和释放的过程.由于节理岩体结构复杂且难以开展室内试验,因此无法通过试验直接求取其能量学参数.基于Hoek-Brown准则和岩石能量理论,提出了节理岩体在临界状态能量学参数的估算方法.针对含贯通节理(或层面)岩体,通过修正岩块单轴抗压强度以体现贯通节理的方向效应.采用PFC3D分别模拟小尺寸岩样(Φ50mm×100mm)和大尺寸岩体(Φ2m×2m)的三轴压缩试验,通过岩石三轴试验结果拟合岩石数值模拟的细观参数并应用于节理岩体的模拟.根据节理岩体模拟得到应力应变曲线和能量流,验证了Hoek-Brown准则对节理岩体能量参数估算的合理性.     

块石定向性特征对土-石混合体强度影响的数值模拟

为了探讨土-石混合体的力学强度受块石定向性特征的影响,开发了二维椭圆块石集合体随机模型(2D Elliptic Block Collection Stochastic Model,简称EBCS2D),并在此基础上进行了双轴压缩数值模拟。模拟结果显示土-石混合体的强度受块石定向性特征的影响,该影响规律与含石量显著关联。通过对各向异性屈服面曲线方程和数值模拟结果的拟合分析,验证了该屈服面曲线方程在表达土-石混合体强度方面的适应性。通过引入土-石相互作用单元的概念,阐明了在低、中、高3种含石量下,块石孤立作用和团簇作用交替主导,造成了块石定向性特征对土-石混合体力学强度的不同规律的影响。      

Particle breakage and the mobilized drained shear strengths of sand

This paper presents particle breakage and the mobilized drained shear strengths of sand with the purpose of clarifying the influence of particle breakage on the mobilized shear strengths of sand. Several drained triaxial tests were carried out on Silica sand No.5 under 3 MPa confining pressure to produce the pre-crushed sands in simulating the high-pressure shear process on soil to result in particle breakage, and then the pre-crushed sands were re-sheared in series of drained triaxial tests to investigate the mobilized strengths of the pre-crushed sands in detecting the influence of particle breakage. It was found that, by deteriorating strain-stress behavior, particle breakage resulted in change of stress-dilatancy behavior in translation and rotation of the relation of the dilatancy factor and the effective principal stress ratio. For a given initial void ratio, particle breakage resulted in impairment of dilatancy behavior of soil to be more contractive in deterioration of the mobilized friction angle and the mobilized dilatancy angle and reduction of void ratio. However, particle breakage resulted in increase of the mobilized basic friction angle especially before failure. In addition, the influence of particle breakage on the mobilized strengths was revealed to be influenced by the shear stress-strain state.     

Discrete numerical modeling of granular materials considering crushability

The aim of this study is to numerically investigate the influence of particle breakage on the mechanical behavior of granular materials using a discrete element method (DEM). To enable particle crushing, non-crushable elementary particles are boned together to represents the granular aggregates which can be crushed when the external force exceeds its strength. The flaw of the aggregate was also modeled by randomly distributed void. Single particle crushing tests were carried out to determine the distribution of particle strength. The results of single particle crushing tests illustrate that the simulated single particle fracture strength and pattern agree well with the Weibull’s distribution equation. Conventional oedometer tests, drained monotonic and cyclic triaxial tests were also carried out to investigate the crushing of the aggregates and the associated mechanical behaviors. The effect of confining pressure on the crushing of aggregates and the mechanical behavior was also analyzed. It was found that the peak stress and dilation decrease significantly when particle crushing was considered. The deformation behavior of the specimen is essentially controlled by two factors: particle rearrangement - induced dilation and particle crushing - induced contraction. The increase of permanent strain and the reduction of dilation were observed during cyclic loading and they tend to reach a stable state after a certain number of cycles. The crushing of aggregate is most significant in the first two cycles. The results also indicated that for the same axial strain the volumetric strain and the bound breakage in the cyclic loading tests are significantly larger than that in the monotonic loading tests, especially at high cyclic stress ratio.     

基于损伤分区与统一强度理论的岩石本构模型

自然界中岩石内部均存在一定程度的损伤,损伤的持续演化过程对岩体的稳定性极为不利。为了研究岩石的损伤演化过程,将岩石分为未损伤部分、闭合裂隙部分与张开裂隙部分,采用统一强度理论与统计损伤理论分析岩石微元强度的分布,通过分析变形协调条件下各部分的应力-应变特征得到岩石的损伤本构模型与损伤演化模型,与巴东组紫红色泥岩三轴压缩试验结果对比验证后将模型应用于某反倾层状岩质边坡的破坏深度分析。分析结果表明:提出的损伤本构模型可以较好地模拟巴东组紫红色泥岩的三轴压缩应力-应变特性,提出的损伤演化模型可以较好地分析巴东组紫红色泥岩的损伤演化过程,且模型参数具有明确的物理意义;此外,根据基于损伤演化模型的反倾层状岩质边坡破坏深度修正模型计算得到的结果偏保守,用于岩土工程设计偏安全。