Fabric dilatancy and the plasticity modeling of granular media

A novel conceptual model of the mechanics of sands is developed within an elastic–plastic framework. Central to this model is the realization that volume changes in anisotropic granular materials occur as a result of two fundamentally different mechanisms. The first is purely kinematic, dilative, and is the result of the changes in anisotropic fabric. There is also a second volume change in granular media that occurs as a direct response to changes in stress as in a standard elastic/plastic continuum. The inclusion of the two sources of volume change results in three important datum states. When subjected to isotropic strains, the resulting stress state in granular materials is not isotropic but lies upon the kinematic normal consolidation line. There exists a state at which the fabric‐induced volumetric strain rate becomes equal to the stress‐induced volumetric strain rate making the total plastic volumetric strain rate equal to zero. Granular response changes from contractive to dilative at this phase transformation line. The third datum state is the one in which the stress‐induced volumetric strain rate is zero. The sand, however, continues to dilate at this state with the difference between stress and dilation ratio a constant as predicted by Taylor's stress–dilatancy rule. These predictions are shown in accordance with experimental data from a series of drained tests and undrained on Ottawa sand. Copyright © 2011 John Wiley & Sons, Ltd.     

Anisotropic Inhomogeneous Riemann S-Ellipsoids in Spheroidal Halos. I.

The classical Riemann S-ellipsoids are generalized to account for a Ferrers inhomogeneity in the mass distribution, a three-dimensional anisotropy of the velocity dispersions, and the gravitation of a spheroidal halo. The Ferrers inhomogeneity does not affect the equilibrium and stability conditions for the ellipsoids, but only changes the numerical coefficients in the equilibrium parameters. The anisotropy of the velocity dispersions changes both the equilibrium conditions and the criterion for stability of the ellipsoids, either extending or limiting the region in which the ellipsoids exist. A halo extends this region in all cases and stabilizes the ellipsoids. The observed of the velocity dispersions of stars in the neighborhood of the sun are obtained using the inhomgeneous model. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 613–632 (November 2005).     

混合型缓冲材料砌块渗透性及其各向异性研究

缓冲材料砌块具备良好的防渗性能,是保证高放废物处置库封闭性的关键。缓冲材料砌块经单轴加载挤压成型,块体尺寸大,测试与评价其渗透性能的均匀性和各向异性,对缓冲屏障设计具有重要价值。研究制备不同干密度的混合型缓冲材料砌块,定向切割获得砌块内部不同位置、不同方向的小试样,开展刚性壁和柔性壁渗透试验,综合评价了混合型缓冲砌块不同方向的水力传导系数。研究结果表明：砌块干密度越大,砌块的水力传导系数越低。所有砌块的水力传导系数均处于10?10 cm/s数量级,沿砌块z、?、r方向的水力传导系数基本相等,表明砌块渗透性具有良好的均匀性和各向同性,且满足高放缓冲材料防渗要求。两种试验方法获得的渗透结果差异较小,柔性壁渗透条件下砌块产生10%～20%的体积膨胀,导致其水力传导系数略大于刚性壁下的平行试样。引入CT试验,分析了渗透前后砌块的微观特性变化。试验证实,在饱和渗透之后,以CT值显示的砌块密度分布趋于均匀,砌块内部存在的微裂隙愈合消失。     

基于单三轴试验与直剪试验的层状软岩模拟

为探究模型试验中层状软岩模拟方法,依托九绵高速层状软岩隧道现场取样的室内试验结果,多次试验确定重晶石粉、石英砂、石膏粉、滑石粉和水的最优配比以模拟软岩基体,采用带孔薄膜模拟层理弱面的黏结作用,并通过直剪试验确定孔隙率,最后对不同层理角度与层理厚度的试样开展直剪与单、三轴试验以反映各向异性情况。结果表明：软岩基体模拟最优配比为0.55:0.15:0.07:0.06:0.17,重晶石粉对强度及破坏变形起决定作用,含量过低易压溃,含量过高产生上下贯通裂缝;采用30%孔洞率的薄膜模拟层理效果最佳;土样强度随层角呈U形变化,层厚减小（不小于2 cm）,强度减小。直剪试验中,45º层理剪切面朝层理方向倾斜,90º层理导致剪切面上下产生裂缝与碎裂,单三轴结果中,0º层理会产生小角度倾斜裂缝,45º层理产生垂直层理面的斜裂缝及二次破裂,90º层理产生顺层理的竖向劈裂;与现场结果对照后确定最优层厚为 3 cm,对层角与层厚的直剪与单三轴试验结果整体一致,揭示土样孔隙压密闭合−弹性−塑性胀裂破坏−徐变的过程。     

Modelling of anisotropic damage in brittle rocks under compression dominated stresses

A new model for describing induced anisotropic damage in brittle rocks is proposed. Although phenomenological, the model is based on physical grounds of micromechanical analysis. Induced damage is represented by a second rank tensor, which is related to the density and orientation of microcracks. Damage evolution is related to the propagation condition of microcracks. The onset of microcrack coalescence leading to softening behaviour is also considered. The effective elastic compliance of the damaged material is obtained from a specific form of Gibbs potential. Irreversible damage‐related strains due to residual opening of microcracks after unloading are also captured. All the model's parameters could be determined from conventional triaxial compression tests. The proposed model is applied to a typical brittle rock. Comparison between test data and numerical simulations shows an overall good agreement. The proposed model is able to describe the main features related to induced microcracks in brittle geomaterials. Copyright © 2002 John Wiley & Sons, Ltd.     

Extensive-Dilatancy Anisotropy (Eda) Inferred From Observations of Crustal Shear Waves Generated By A Refraction Experiment In Northern Scandinavia

Summary. Particle-motion plots of shear waves have been studied for the section FG of the FENNOLORA seismic experiment. Shear-wave splitting is observed on some records and the polarization of the first arriving shear waves show two peaks at about N35°W and N65°E. These results can be interpreted as being due to crack-induced anisotropy with the crack direction dominated by a (dominant) horizontal stress around N35°W. This is consistent with in situ stress measurements and focal mechanism studies in Scandinavia. the results show that seismic refraction experiments may be useful in providing evidence of crack-induced anisotropy in the stable continental crust.