基于微观力学机制的各向异性结构性砂土的本构模型研究

在岩土破损力学基础上,基于微观破损机制,提出了考虑各向异性的结构性砂土本构理论。采用Lade-Duncan强度准则考虑中主应力对抗剪强度的影响;采用考虑颗粒排列组构的各向异性状态变量A反映各向异性对土体强度和变形的影响;通过相似扩大重塑土的屈服面反映结构性对土性的影响;通过引入非相关联流动法则考虑各向异性和结构性对土体塑性变形的影响。同时,将基于微观力学机制的损伤演化规律引入结构性土的硬化规律;该硬化规律同时考虑了塑性体积应变和剪切应变对各向异性结构性土强度的影响。然后将该模型用于模拟室内三轴压缩试验,初步验证了该模型的合理性和适用性。     

颗粒材料的微观临界状态理论模型

存在临界状态是颗粒材料的一个重要特性。基于孔隙胞元的颗粒离散元方法对二维颗粒体进行双轴加载数值试验,在详细分析数值模拟结果的基础上,从微观几何组构的角度揭示了临界状态的存在机制。基于剪胀性原理,提出了以接触价键表征的微观临界状态理论模型,得到了接触价键与塑性剪切应变的关系表达式,理论模型的结果和二维离散元数值模拟得到的结果吻合较好。通过比较不同情况下数值结果和理论模型中的参数,得到以下结论：表征微观临界状态的参数（临界接触价键和达到临界状态所需要的塑性剪切应变）依赖于颗粒体的微观特性,如颗粒形状、表面摩擦性质、颗粒体的围压和初始孔隙比。     

各向异性结构性砂土离散元分析

天然沉积砂土力学特性受各向异性及结构性影响明显,实际工程中不能忽视。为探究二者的影响,首先在二维离散元程序NS2D中采用椭圆颗粒模拟了重力场中颗粒长轴主方向为水平的各向异性净砂样,随后基于结构性砂土胶结厚度分布规律及室内试验提出了一个新的微观胶结接触模型并将其引入各向异性净砂样以模拟天然各向异性结构性砂土,最后对该离散元试样进行了双轴试验模拟,将模拟结果与室内试验结果对比以验证该模型的适用性,并对其微观力学特性变化进行研 究。分析结果表明,随着剪切进行,各向异性结构性砂土呈明显应变软化及剪胀现象;胶结接触逐渐减少,且主方向始终为竖向方向;胶结破坏速率及胶结破坏率变化情况与宏观力学响应较一致,且胶结物多为拉剪破坏;土颗粒排列主方向始终为水平向,且水平向排列颗粒所占比例略微增大。     

基于能量耗散机制的粗粒土圆度损伤特性分析

圆度损伤是研究粗粒土力学特性变化的重要指标,为探索剪切过程中粗粒土圆度损伤的演化过程,定性及定量从能量角度对损伤本质进行分析。根据能量耗散原理,分析了剪切系统及单个颗粒剪切过程中能量的耗散与传递。结合弹塑性力学与热力学定律,定义粗粒土圆度损伤因子Dr,建立了基于能量耗散下的圆度损伤模型。采用剪切试验研究不同圆度的花岗岩和卵石颗粒的强度特性,以观测效果较好的花岗岩颗粒为研究对象,研究不同法向应力下的圆度损伤演化过程,运用MATLAB对模型及试验数据分析。结果表明:圆度不同的颗粒(花岗岩、卵石)随圆度减小,传递到颗粒内部积聚的塑性势能增大,更易在颗粒边界棱角处产生微裂纹使颗粒磨损,从而增大粗颗粒圆度,强度特性减小;剪切试验粗粒土圆度损伤演化过程存在2次损伤拐点,可分为圆度损伤响应、圆度损伤演化、圆度损伤稳定3个阶段;同一类型颗粒法向应力增大,达到损伤临界状态圆度损伤所需能量增大,圆度损伤因子减小。基于能量耗散下建立的圆度损伤模型阐述了剪切过程中的圆度损伤,试验结果能合理地反应圆度损伤演化过程中颗粒力学特性响应。     

考虑转动阻抗的粗粒土离散元模拟

转动阻抗被定义为作用颗粒接触上的一对对称力偶,用来抵抗颗粒之间的相互转动。将转动阻抗引入到离散元模拟中是对传统离散单元法的重要改进。开发出考虑颗粒转动阻抗的接触模型,并将其嵌入到PFC2D中,利用该模型进行粗粒土的双轴剪切数值模型试验,研究剪切过程中转动阻抗对粗粒土的宏细观力学性质的影响。结果显示,在宏观方面,颗粒转动阻抗对粗粒土的宏观力学行为（应力-应变及体应变-轴应变行为）有重要的影响,随着转动阻抗的增加,粗粒土的剪切强度和最大摩擦角随之增加,这与已有的研究结果一致,证明所建模型是可靠的;在微观方面,考察转动阻抗对粗粒土内部微观结构的影响发现,随着转动阻抗的增加,粗粒土的内部的接触数目减少,而粗粒土的剪切强度增加,表明转动阻抗能够提高粗粒土力链网络的稳定性,同时发现随着转动阻抗的增加,粗粒土的各向异性增加主要是强力链各向异性的增加,说明转动阻抗增强了强力链的传递力的能力以及抵抗力链屈曲破坏能力。数值模拟结果表明,增加颗粒转动阻抗,粗粒土出现组构与轴应变非共轴的现象。     

密实散粒体剪切破坏能量演化的离散元模拟

砂土等散粒体在剪切过程中的能量存储及耗散是其宏观力学响应的深层原因,但因量测难度较大而研究较少。将考虑抗转动的接触模型引入离散元软件PFC2D,基于热力学第一定律建立各种能量量测方法,并在平面应变双轴压缩试验中采用该方法统计密实散粒体在剪切过程中的能量演化规律。采取了4种耗散类型,即滑动-滚动（S-R）、滑动-非滚动（S-NR）、非滑动-滚动（NS-R）和非滑动-非滚动（NS-NR）。结果表明：密实散粒体加载时能量耗散以滑动摩擦为主;且小应变加载阶段,外力功主要转化为弹性应变能,但同时也存在均布于试样的耗散能;随着应变的增加,外力功的转化形式逐渐过渡为以耗散能为主,且集中分布在带状区域内;各个加载阶段的摩擦耗散均存在各向异性。     

颗粒破碎对钙质砂的应力-应变及强度影响研究

压力作用下颗粒发生破碎是引起砂土力学特性变化的重要因素之一, 对于钙质砂这种易破碎的材料更是如此。为进一步弄清颗粒破碎对钙质砂的应力-应变强度影响, 本文对钙质砂进行三轴固结排水剪切试验得到应力-应变曲线, 并筛分得到三轴试验前后钙质砂颗分曲线。通过引入Hardin定义的颗粒相对破碎率Br, 分析了相对密度、围压与颗粒破碎的关系及颗粒破碎对钙质砂应力-应变和抗剪强度的影响。结果表明:随围压的增大颗粒破碎增量逐渐减小, 直到破碎达到一个上限值, 此时围压和相对密度对颗粒破碎影响很小; 颗粒间的滑动标志着应力达到极限状态, 而颗粒破碎会阻碍应力达到极限状态, 在本实验中, 低围压时颗粒破碎少, 颗粒相对运动形式为滑移, 使应力-应变曲线为软化型, 高围压下颗粒破碎严重, 颗粒破碎在剪切过程中始终发生, 使应力-应变曲线呈应变硬化型; 颗粒破碎使体变从剪胀逐渐发展到剪缩, 且破碎越严重剪缩越严重; 在低围压下钙质砂强度主要由剪胀和咬合提供, 高围压下颗粒破碎严重, 剪胀消失, 咬合减小, 使峰值摩擦角减小, 抗剪强度降低。     

粗粒土的破碎耗能计算及影响因素

粗粒土的颗粒破碎直接改变了土体本身结构,对粗粒土的剪胀和内摩擦角都会产生影响。在土体剪切过程中,体积应力和剪切应力在体积应变和剪切应变上做功,这部分能量在剪切过程中转化为颗粒的弹性储能、颗粒间的摩擦耗能、颗粒剪胀时对外做功和颗粒破碎耗能4部分。准确计算剪切过程中粗粒土破碎耗能的目的是：从能量角度分析颗粒破碎对土体本构关系的影响,为建立考虑颗粒破碎的粗粒土本构关系创造条件。通过分析粗粒土的常规三轴试验数据,计算得到了剪切过程中的粗粒土破碎耗能。计算结果表明,常规三轴试验条件下粗粒土破碎耗能主要受固结应力、土体摩擦系数M等因素的影响。     

钙质砂颗粒破碎对临界状态影响的试验研究

钙质砂的颗粒易碎性是造成其变形和强度特性不同于石英砂的重要性质。本文基于临界状态理论,通过一系列试验定量地描述钙质砂临界状态线随颗粒破碎的演化规律。本文试验分两个阶段进行:第1阶段研究了60~2000 kPa围压条件下钙质砂的力学特性和颗粒破碎特征;第2阶段以不同破碎率的试样为母本重塑制样,在100~300 kPa围压条件下,剪切至破碎临界状态线。试验结果表明:在较小围压(<300 kPa)条件下,松砂和密砂均表现出明显的剪胀和应变软化特性;而高围压(>1 MPa)条件下,显著的颗粒破碎会造成试样的持续剪缩;颗粒破碎存在明显围压阈值,对于松砂而言,在围压小于300 kPa条件下,颗粒基本不发生破碎;在e-lg p'平面内,破碎临界状态线的截距Δe_Γ和斜率λ_c均会随着修正相对破碎率B_r*的增大而减小,即颗粒破碎会使临界状态线发生下移和逆时针转动;而在q-p'平面内,钙质砂的临界状态点落在同一条直线上,即存在唯一的临界状态应力比M_cr和临界摩擦角φ_cr。     

基于离散元的冻结砂土三轴力学特性研究

采用离散元软件中的平行黏结模型模拟冻土中冰-土颗粒间相互作用。通过调整模型细观参数,模拟了特定围压不同试验温度和应变速率条件下冻结砂土的三轴试验规律。进一步对比分析计算和试验结果表明,模型的细观参数中的刚度系数和黏结强度对材料的宏观力学行为影响较大,设定的黏结强度参数与温度成反比而和应变速率成正比。在宏观上,峰值强度随温度降低而增大,随应变速率的增大而增大。进一步分析离散元中颗粒的运动产生的剪切带倾角表明：剪切带倾角与温度成反比,而应变速率对其的影响较小;根据已有数据结果,剪切带与内摩擦角间的统计关系符合经典土力学中的摩尔-库伦解。     

On the role of particle breakage in the shear failure behavior of granular soils by DEM

This article presents a fundamental study on the role of particle breakage on the shear behavior of granular soils using the three‐dimensional (3‐D) discrete element method. The effects of particle breakage on the stress ratio, volumetric strain, plastic deformation, and shear failure behavior of dense crushable specimens undergoing plane strain shearing conditions are thoroughly investigated through a variety of micromechanical analyses and mechanism demonstrations. The simulation of a granular specimen is based on the effective modeling of realistic fracture behavior of single soil particles, which is demonstrated by the qualitative agreement between the results from platen compression simulations and those from physical laboratory tests. The simulation results show that the major effects of particle breakage include the reduction of volumetric dilation and peak stress ratio and more importantly the plastic deformation mechanisms and the shear failure modes vary as a function of soil crushability. Consistent macro‐ and micromechanical evidence demonstrates that shear banding and massive volumetric contraction depict the two end failure modes of a dense specimen, which is dominated by particle rearrangement–induced dilation and particle crushing–induced compression, respectively, with a more general case being the combination and competition of the two failure modes in the medium range of soil crushability and confining stress. However, it is further shown that a highly crushable specimen will eventually develop a shear band at a large strain because of the continuous decay of particle breakage. Copyright © 2011 John Wiley & Sons, Ltd.     

Micromechanical inspection of incremental behaviour of crushable soils

In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle, those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear-enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge, and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood.

     

Macroscale friction of granular soils under monotonic and cyclic loading based upon micromechanical determination of dissipated energy

Macroscopic frictional behavior of granular materials is of great importance for studying several complex problems associated with fault slip and landslides. The main objective of this study is to model the macroscale frictional behavior of granular soils under monotonic and cyclic loadings based upon micromechanical determination of dissipated energy at particle contacts. This study is built on the general observation that the externally computed energy dissipation should be equal to the total internal energy dissipation derived from inter-particle sliding and rolling, energy losses from inter-particle collisions, and damping. For this purpose, the discrete element method is used to model a granular soil and determine the stored, dissipated, and damping energies associated with shear loading for applied monotonic and cyclic velocities. These energies are then related to the friction by an application of the Taylor-critical state power balance relationship. Also, the contributions of the different modes of energy dissipation (normal, shear, and rolling) to the total frictional resistance were studied. By changing the inter-particle friction, the simulations showed that the macroscopic friction was nearly constant, the slip friction increased almost linearly with increasing inter-particle friction, and the difference between the two was attributed to the non-energy dissipating dilatancy component. By providing a clear relationship between energy dissipated by micro-scale mechanisms versus the traditional engineering definition based on macro-scale (continuum) parameters, this study provides a means to develop a better understanding for the frictional behavior of granular media.

     

不同排水条件下饱和砂土快速大剪切力学特性

为探索滑坡灾害中土在复杂条件下的剪切力学特性,本文利用大型环剪试验机,通过进行各种排水条件下的连续大位移剪切试验,对不同法向应力、剪切速率和孔隙水压力等复杂条件下饱和砂土的力学特性及其变化机理进行了研究。结果表明：1）在连续快速剪切条件下,砂土剪切力学特性在干燥、不排水和排水等条件下呈现不同的变化形式。其中在不排水条件下,饱和砂出现一定的应变软化现象。2）在相同正应力和剪切速率的环剪试验中,饱和砂在不同排水条件下（上排水、下排水、上下排水）的抗剪强度出现显著差异。3）在排水环剪试验条件下,砂土剪切应力与强度的差异性变化不仅与土体内细土颗粒运移和结构变化有关,并且受到剪切过程中不同排水条件下孔隙水压力变化的影响和控制。4）排水环剪条件下,饱和砂孔隙水压力的消散变化不仅与不同排水方式下土体内所形成的排水通道顺畅程度有关,并且受到不同剪切速率和法向应力的影响作用。     

考虑颗粒破碎的钙质砂修正邓肯-张E-B模型

在常规应力水平下颗粒发生破碎是钙质砂有别于其他砂土的重要性质之一,且由于颗粒破碎的存在,使用传统的本构模型无法很好地模拟钙质砂的力学行为。因此本文以最为普及的本构模型之一--邓肯-张E-B模型为基础,对其进行颗粒破碎方面的修正以得到一个能用于钙质砂的本构模型。具体方法为:首先本文采用Hardin提出的相对破碎Br这一指标来度量颗粒破碎的大小。之后研究分析得出了颗粒破碎对邓肯-张模型参数(内摩擦角φ、割线模量E50及体积模量B)的影响规律。然后通过颗粒破碎与输入能量之间的关系将各状态下无法直接确定的相对破碎Br与可确定的应力-应变状态联系起来。最终得到了一个考虑颗粒破碎的钙质砂修正邓肯-张E-B模型。为验证模型的准确性及适用性,本文还使用该模型对4种不同粒径范围且试验围压不同的钙质砂的三轴排水行为进行了模拟。结果表明拟合效果较好,模型能适用于各种不同粒径范围的钙质砂,并且在颗粒破碎较大的情况下明显优于传统邓肯-张模型。     

Experimental characterization and 3D DEM simulation of bond breakages in artificially cemented sands with different bond strengths when subjected to triaxial shearing

This paper describes the mechanical behavior of artificially cemented sands with strong, intermediate, and weak bond strengths, using experimentation and 3D discrete element method (DEM) simulation. The focus is on the features of bond breakage and the associated influences on the stress–strain responses. Under triaxial shearing, the acoustic emission rate captured in the experiment and the bond breakage rate recorded in the simulations show resemblance to the stress–strain response, especially for strongly and intermediately cemented samples, where a strain softening response is observed. The simulations further reveal the shear band formation coincides with the development of bond breakage locations due to the local weakness caused by the bond breakages. Strain softening and volumetric dilation are observed inside the shear band, while the region outside the shear band undergoes elastic unloading. The weakly cemented sample exhibits a strain hardening response instead; bond breakages and the associated local weaknesses are always randomly formed such that no persistent shear band is observed. Note that in the DEM simulation, the flexible membrane boundary is established by a network of bonded membrane particles; the membrane particle network is further partitioned into finite triangular elements. The associated algorithm can accurately distribute the applied confining pressure onto the membrane particles and determine the sample volume.     

南海钙质砂宏细观破碎力学特性

针对取自我国南部某海域的钙质砂样本,做了以下两方面工作：一是通过电子显微镜获取了钙质砂颗粒的几何投影图像,利用图像处理技术对图形进行黑白二值化处理,获取单元颗粒形状轮廓边界,使用圆度和粗糙度2个参数对钙质砂的颗粒形状进行定义和量化。二是通过不同围压下的三轴固结排水剪切试验及试验前后的颗分测量对比,研究了颗粒破碎对钙质砂的变形、强度、能量耗散等特性的影响。研究表明,大粒径钙质砂（粒径大于2.0 mm）和小粒径钙质砂（粒径小于0.5 mm）形态比较接近圆形、颗粒表面相对光滑;相比而言,中间粒径（粒径介于0.5～2.0 mm之间）钙质砂形状较不规则,表面棱角较多。钙质砂在三轴排水剪切过程中发生颗粒破碎,试样向着级配均匀的方向发展。随着初始围压的增大,颗粒破碎程度加大,土样整体剪胀趋势减小,而破碎引起的能量耗散增加。而在高围压（初始围压为600 kPa）剪切过程中,仅考虑摩擦耗散,以及同时考虑摩擦、体积耗散两种情况下,计算得到的最大颗粒破碎耗散分别可达土样总输入塑性功的25%和18%。     

剪切作用下钙质砂颗粒破碎试验研究

钙质砂是一种海洋沉积物,与陆源砂比起来,钙质砂受力后易产生颗粒破碎,从而使其力学性质发生变化。对取自南沙群岛永暑礁附近海域的钙质砂进行了不同围压、不同应变下的三轴剪切试验,对试验前后的试样进行了颗粒大小分析试验。试验结果表明,钙质砂在三轴剪切作用下颗粒破碎十分严重,同时用Hardin模型对其破碎进行了度量,并就围压、剪切应变与破碎之间的关系进行了分析。