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1.
The quantitative analysis of the pore characteristics of granular materials has been often challenging due to arbitrarily shaped geometry of pores despite its significant implications. In this study, we investigate the size distribution and orientation of pores in dilative and contractive assemblies in the direct shear test by performing 3D discrete element simulations in conjunction with image processing of pore geometry. We quantitatively define unit pores by the Delaunay Tessellation followed by pore mergence and fitting them with ellipsoids. It is observed that the evolution of pore size distribution depends on the dilatancy of assemblies. Results also show that the direction of principal stresses governs the orientations of pores during shearing, with respect to the size of pores. This study highlights that the dominant factors of the pore characteristics upon shearing are stress anisotropy and particle mobilization to make the internal structure stable. 相似文献
2.
In this study, numerical simulations using the discrete element method (DEM) were performed to examine the evolution of pore characteristics in dense and loose samples subjected to a biaxial creep test. Sliding creep between particle contacts was incorporated in the DEM simulations, which displayed similar creep behavior found in experiments. The irregularly shaped pore geometry in the soil packing was quantified with a best-fitting ellipse with the aid of the region-based method. It has been found that the initial density of soils and the deviatoric stress values under which creep starts determine unique evolution of pore space. In addition, the weak pore structures, elongated along the horizontal direction (or perpendicular to the axial loading), collapse first and ultimately only those stable pore structures, elongated along the vertical direction (or parallel to the axial loading direction), survive and then dominate the entire soil structures as creep proceeds. The pore characteristics at last become more uniform and homogenized throughout the sample, and a more stable particle arching elongated along the loading direction is gradually produced during creep. 相似文献
3.
Yu Haizhen Zhao Wenguang Wang Ren Li Jianguo He Yang 《中国地质大学学报(英文版)》2005,16(4):340-344
INTRODUCTION Theexploitationofoffshorenaturalresourcesin thetropichaspresentedgeotechnicalengineerswith theproblemofdealingwithinsitucarbonatesedi mentsinharshconditionsforwhichfewengineering dataareavailable.Thecarbonatesedimentsaresub jectedtolargestaticstressesduetotheweightof foundationstructuresaswellascyclicstressesdueto waveaction,andinsomecases,earthquakeloading.Ithasbeenprovedthatthenatureofcyclicstresschangesoccurringintheseabeddepositduetowave loadinginvolvesacontinuousrotationo… 相似文献
4.
A simulation study of microstructure evolution inside the shear band in biaxial compression test 总被引：1，自引：0，他引：1
We study the development of microstructure inside the shear band in granular media consisting of elliptical‐shaped particles. Plane strain biaxial compression test was simulated using two‐dimensional distinct element method. The generation of large voids and concentration of excessive particle rotation inside a shear band are found in a quite similar manner to those observed in natural soils. Evolution of the microstructure inside and outside the shear band is studied. The magnitude and direction of particle rotation inside the shear band is influenced by orientation of long axes of elliptical particles. Because of such particle rotations inside the shear band, the preferred alignment of particles becomes horizontal in the residual state, which results in a more anisotropic contact normal distribution oriented along the major principal stress axis. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
5.
This study presents the correlations between quantified shape parameters and geotechnical properties for nine sand specimens. Four shape parameters, sphericity, convexity, elongation and slenderness, were quantified with two-dimensional microscopic images with the aid of image processing techniques. An instrumented oedometer cell is used to measure compressibility, thermal conductivity and shear wave velocity during loading, unloading and reloading stages. As the particle shape inherently determines the initial loose packing condition, initial void ratio and shape parameters are well correlated with compressibility. The applied stress in soils increases the interparticle contact area and contact quality; round particles tend to achieve higher thermal conductivity and shear wave velocity during stress-induced volume change. Multiple linear regression is implemented to capture the relative contributions of involved variables, revealing that the thermal evolution is governed by the initial packing density and particle shape. The experimental observations underscore the predominant effect that particle shape has on the geomechanical and physical properties upon stress-induced soil behavior. 相似文献
6.
Ajanta Sachan 《Geotechnical and Geological Engineering》2013,31(1):213-228
The present paper evaluates the role of microfabric in strain localization patterns observed in soil specimens during its shear deformation in compression and extension triaxial testing. A series of compression and extension lubricated end triaxial tests are performed on Kaolin clay with extreme as well as intermediate microfabrics, which are obtained using slurry consolidation technique by varying calagon content from 0 to 3 %. Intermediate microfabric is the geometric arrangement of particles within the soil mass, which lies in between the particle orientation during two extreme microfabrics; flocculated and dispersed. Flocculated has random orientation of particles with face-to-edge particle contacts and dispersed has parallel orientation of particles with face-to-face particle contacts. When the specimen is subjected to large stress levels in triaxial testing, the particle orientation and geometric arrangement get affected due to the force acting on the clay platelets. In this experimental research, the variation in clay’s stress–strain and pore pressure response and initiation, propagation and formation of shear bands at different levels of compression and extension shearing is evaluated using digital image analysis setup associated with triaxial system. 相似文献
7.
In this paper, a lattice-type model to simulate the micro-mechanical behaviour of particulate/granular media is presented. In this numerical model, a particulate assembly is simulated as a lattice/truss. Nodes located at contacts between a particle and its neighbours are linked by bars to each other. Each particle is represented by a lattice within its microstructure and particle interact through load transfer at the nodes. Constraints are prescribed at the nodes to describe active, deactivated and reactivated contacts. When a particulate assembly develops into a mechanism (deformation with zero incremental load), further deformation is simulated through a framework that describes the kinematics of the particles (sliding, rolling and rotation of particles). This framework is formed by introducing nodes at the particle centroids and linking them with bars. Bars-linking particles with a non-sliding contact are assigned large stiffnesses relative to bars linking particles with a sliding contact. Numerical tests are conducted on two-dimensional assemblies of disks, arranged as very loose and very dense packing under simple shear loading conditions. The results concord with the results of numerical tests conducted using the discrete element method and with photoelastic experiments. Additionally, the model is applied to study the effects of initial imperfections caused by particles with low elastic modulus. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
8.
Revisiting the existence of an effective stress for wet granular soils with micromechanics 
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下载免费PDF全文 A possible effective stress variable for wet granular materials is numerically investigated based on an adapted discrete element method (DEM) model for an ideal three‐phase system. The DEM simulations consider granular materials made of nearly monodisperse spherical particles, in the pendular regime with the pore fluid mixture consisting of distinct water menisci bridging particle pairs. The contact force‐related stress contribution to the total stresses is isolated and tested as the effective stress candidate for dense or loose systems. It is first recalled that this contact stress tensor is indeed an adequate effective stress that describes stress limit states of wet samples with the same Mohr‐Coulomb criterion associated with their dry counterparts. As for constitutive relationships, it is demonstrated that the contact stress tensor used in conjunction with dry constitutive relations does describe the strains of wet samples during an initial strain regime but not beyond. Outside this so‐called quasi‐static strain regime, whose extent is much greater for dense than loose materials, dramatic changes in the contact network prevent macroscale contact stress‐strain relationships to apply in the same manner to dry and unsaturated conditions. The presented numerical results also reveal unexpected constitutive bifurcations for the loose material, related to stick‐slip macrobehavior. 相似文献
9.
为了研究粒状材料的各向异性力学行为与细观组构演化之间的关系,采用自主研发的双轴压缩试验系统,以圆形和椭圆形截面的金属棒状材料组成的二维堆积体为试验对象,对不同大主应力方向角?（沉积面与大主应力作用面的夹角）的试样进行了各向等压、常侧向压力、等p剪切3种应力路径试验,并通过分析试样在不同变形阶段的数字照片得到了其细观组构演化规律。发现对于椭圆形截面的试样存在一个卓越剪切方向,随剪应变增大,颗粒长轴呈现出向该方向偏转的趋势,并且在大变形条件下沿该方向形成剪切带;卓越剪切方向与沉积面方向关系不大,而与大主应力作用面方向夹角约为45°+ /2, 为残余内摩擦角;随卓越剪切方向与沉降方向夹角的不同,颗粒偏转程度的不同是导致剪胀特性和峰值强度各向异性的主要原因 相似文献
10.
P. Seethalakshmi 《Geomechanics and Geoengineering》2020,15(2):123-139
ABSTRACTMicaceous soil is believed to be detrimental for civil engineering constructions due to the effect of high compressibility, low compacted density and low shear strength. Individual mica particle has numerous intact mica flakes foliated over each other making it flexible upon loading and rebound upon unloading due to its low hardness and resilient nature. Hence, micaceous soils with mica content more than 10% are considered undesirable for highway pavements, embankments and railway track constructions. When platy mica particles are sufficiently numerous to interact with spherical sand particles, bridging and ordering phenomena are augmented within the soil mass creating unique sand-mica particle orientation (MS microstructure) unlike sand-sand particle orientation (PS microstructure). The current experimental research was conducted to evaluate the variation in stress–strain, pore pressure and effective stress path response of Sabarmati sand under the influence of mica (sand with 30% mica and pure sand) with MS and PS microstructure respectively. Effect of particle crushing on stress–strain and pore pressure response was also studied on Sabarmati sand with MS and PS microstructure. Distinctive macroscopic response was observed in Sabarmati sand with MS microstructure under the influence of mica as well as mica particle crushing. 相似文献
11.
考虑循环应力比和频率影响的动荷载下软土微观结构研究 总被引：3，自引：0，他引：3
以杭州原状软土为研究对象,借助扫描电镜和PCAS微观定量测试技术,采用分形理论对波浪荷载下的饱和软土微观结构进行研究,探讨不同循环应力比和不同频率条件下孔隙的分布特征及其变化规律。结果表明：循环应力比越大或频率越低,试样破坏所需的循环次数越少;试样临界破坏应变水平随循环应力比的增加呈升高趋势,而不同频率下的临界破坏应变水平则基本一致;孔隙的尺度、排列、形态等特征均随循环应力比的变化呈规律性变化,但随频率变化没有明显的规律性;循环剪应力作用下,孔隙破碎的同时兼并生长,这是剪切带上褶皱现象的微观本质,并且孔隙排列方向与剪切带方向基本一致。通过循环加载前、后微观结构特征参数的变化规律,揭示了波浪荷载下循环应力比和频率对土体微观结构的影响以及土体宏观变形的微观机制。 相似文献
12.
《Geomechanics and Geoengineering》2013,8(2):77-86
The use of discrete-element modelling (DEM) to simulate the behaviour of a highly idealized bituminous mixture under uniaxial and triaxial compressive creep tests is investigated in this paper. The idealized mixture comprises single-sized spherical particles (sand) mixed with bitumen and was chosen so that the packing characteristics are known and the behaviour of the mixture is dominated by the bitumen. The bitumen is represented as shear and normal (tensile and compressive) contact stiffnesses. Numerical sample preparation procedures for specimens containing spherical particles or clumps have been developed to ensure that the final specimen is isotropic and has the correct volumetric proportions. An elastic contact was used for the compressive normal contact stiffness and a viscoelastic contact was used for shear and tensile normal contact stiffness. Simulation results show that the idealized mixture is found to dilate when the ratio of compressive to tensile contact stiffness increases as a function of loading time. Uniaxial and triaxial viscoelastic simulations have been performed to investigate the effect of stress ratio on dilation and the numerical results have been verified with experimental data. The effects of introducing a proportion of frictional contacts and a more complex particle shape (clump) on dilation have been examined. 相似文献
13.
剪胀性是土特有的一种材料属性,而准确地描述砂土的剪胀性则是建立砂土本构模型的重要基础。大量常规三轴试验发现,在以相同加载条件下剪切时密砂和松砂会表现出完全不同的剪胀性和应力-应变关系特性,说明砂土的剪胀性不仅与其所处的应力状态有关,也与其物理状态相关。状态参量理论很好地解释了砂土所处应力状态和物理状态对剪胀性的共同作用。空心扭剪三轴试验仪可以实现不同主应力方向的单调剪切试验。试验结果表明,当砂土以不同主应力方向单调剪切时,即使处于相同初始应力条件和物理状态,砂土也会表现出不同的剪胀性,说明了主应力方向也是决定砂土剪胀性的重要条件。本文通过分析试验中主应力方向对砂土剪胀性的影响,提出了一个含有主应力方向的状态参量,并建立了相应的剪胀方程。通过与试验数据的对比,验证了该方法的正确性和准确性。 相似文献
14.
In this paper, an upper bound estimate of the limit load on non-associative coaxial granular materials is presented. The kinematic approach of the upper bound limit analysis has been utilised. The failure mechanism is assumed to coincide with the direction of the shear bands at every point throughout the body. The shear band orientation in non-associative coaxial materials, i.e. those with the same major principal stress and major principal strain increment directions, can be found based on the angle of dilation and the major principal stress direction. Therefore, having known the stress field at limiting equilibrium, the orientation of the shear bands and hence, the failure mechanism can be obtained. In this study, the stress field is first determined by the method of stress characteristics. Then, the finite element interpolation technique is used to interpolate the stress field and to find the orientation of the shear bands at every point within the field. Once the failure mechanism and the stress state at every point along velocity discontinuities have been found, the upper bound limit analysis has been performed to estimate the limit load. 相似文献
15.
Peijun Guo 《国际地质力学数值与分析法杂志》2013,37(4):353-373
By taking into account the continuity of stresses and pore pressure across the boundaries of a shear band, it is theoretically shown that incipient localization may take place in saturated loose contractive sand in the regime of decreasing deviator stress under undrained conditions. The undrained shear band orientation primarily depends on the Poisson's ratio and the dilatancy characteristics of the material. However, under strict isochoric constraint, localization is precluded in dense dilative sand with deviator stress increasing only. If any local volume change takes place with a certain mechanism, undrained shear band would be possible in dense sand, most likely in the regime of decreasing friction angle after the peak friction angle is mobilized. Numerical examples are given to demonstrate the variation of shear band inclinations with void ratios and initial consolidation pressures. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
16.
中主应力对砂土强度影响的细观机制研究 总被引：3，自引：1，他引：2
基于简化的颗粒单元体模型,分析总结了颗粒单元体的基本特点,针对中主应力对砂土强度的影响,重点分析了颗粒单元体之间的相互影响特点。对以往的真三轴试验结果进行了总结,得到了中主应力对砂土强度影响的规律,结合颗粒单元体的两种基本状态及其特点、大孔隙与颗粒单元体结构孔隙的特点以及颗粒单元体间相互影响的特点,揭示了中主应力对砂土强度影响规律的细观物理机制。这些规律包括：松砂的强度随着中主应力的增大是逐渐提高的;密砂随中主应力的增大,强度先提高,当中主应力接近大主应力时,却引起强度的降低;砂土越密实,强度平台段越长;在中主应力接近小主应力阶段,强度随中主应力的增大而提高的快慢与砂土的密实程度有关。 相似文献
17.
The paper provides an in-depth exploration of the role of particle crushing on particle kinematics and shear banding in sheared granular materials. As a two-dimensional approximation, a crushable granular material may be represented by an assembly of irregularly shaped polygons to include shape diversity of realistic granular materials. Particle assemblies are subjected to biaxial shearing under flexible boundary conditions. With increasing percentage of crushed particles, mesoscale deformation becomes increasingly unstable. Fragmented deformation patterns within the granular assemblies are unable to form stable and distinct shear bands. This is confirmed by the sparsity of large fluctuating velocities in highly crushable assemblies. Without generating distinct shear bands, deformation patterns and failure modes of a highly crushable assembly are similar to those of loose particle assemblies, which are regarded as diffuse deformation. High degrees of spatial association amongst the kinematical quantities confirm the key role that non-affine deformation and particle rotation play in the generation of shear bands. Therefore, particle kinematical quantities can be used to predict the onset and subsequent development of shear zones, which are generally marked by increased particle kinematic activity, such as intense particle rotation and high granular temperature. Our results indicate that shear band thickness increases, and its speed of development slows down, with increasing percentage of crushed particles. As particles crush, spatial force correlation becomes weaker, indicating a more diffuse nature of force transmission across particle contacts. 相似文献
18.
Stefano Ceriani Neil S. Mancktelow Giorgio Pennacchioni 《Journal of Structural Geology》2003,25(12):2005-2021
The rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear (Couette) flow was studied in 2D rock-analogue experiments. The influence of particle shape (elliptical vs. monoclinic), aspect ratio and the nature of the matrix/particle interface (lubricated vs. unlubricated) was investigated. Both matrix (PDMS) and lubricant (liquid soap) were linear viscous, with a viscosity ratio of ca. 104. Without lubricant, the rotational behaviour of all particles closely approximates the Jeffery theory. Lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position. Lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize. Weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation. In general for elliptical particles, rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle. For thick mantles (initially >10% of the volume of the particle), rotation rates are faster than Jeffery theory. For very thin mantles they are markedly slower compared with thick mantles, particularly when the long axis is nearly parallel to the shear direction. Rotation rates are never strictly zero, so true stabilization does not occur. However, for more elongate particles (axial RATIO=6) rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population. In experiments, the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain. In natural examples of porphyroclast systems, the weak mantle continually develops by recrystallization and/or cataclasis of the rigid clast core and a steady state between production and thinning could be attained, potentially leading to true stabilization for particles with a high axial ratio. 相似文献
19.
为研究复杂应力路径下冻土的强度与变形特征,采用冻土空心圆柱仪（FHCA-300）对不同负温状态下的饱和冻结黏土开展定向剪切试验,基于不同剪切方向下冻结黏土的轴向和扭剪分量的应力-应变关系,探讨土样的剪切变形特征、各向异性属性以及剪切带的演变规律,并考察温度、大主应力方向角、平均主应力以及中主应力系数等因素对冻结黏土强度的影响。结果表明：平均主应力p值对冻结黏土的应力-应变关系影响显著,尤其是p=4.5 MPa时具有较高的剪切强度,且该值可能为压融临界p值;大主应力方向变化会诱发冻结黏土的各向异性,随着大主应力方向角的增加,冻结黏土剪切强度逐渐降低,并有明显的剪切带产生;中主应力系数的增加使得轴向强度有逐渐降低的趋势,但对剪切强度影响不明显;随着温度的降低,冻结黏土强度逐渐增大,试样发生脆性破坏并出现剪切破裂面,其剪切强度主要取决于冰颗粒和土颗粒的胶结力。 相似文献
20.
Two types of stress path-controlled plane strain compression tests were performed on both loose and dense specimens of angular and sub-angular sands and two rounded glass beads with different particle sizes. Digital image correlation method was used to analyze local deformation developments, especially shear band patterns. The material behavior in response to shearing has been found to be dependent on the relative density, particle shape, and stress path. The results of analysis on local deformation developments showed that the onset of shear bands occurred prior to their peak strengths in both dense and loose specimens. The growth rates of local maximum shear strain along a shear band were approximately consistent with an increasing global axial strain after the onset of shear band. The shear band width was influenced by both the mean particle size and the particle shape. The measured shear band inclination angles were in between those estimated by Coulomb’s and Roscoe’s formulas. 相似文献