石灰岩细观力学特性的颗粒流模拟

岩体地区地质灾害的发生和发展取决于岩石细观组分的运动学行为。研究岩石运动学行为时通常将岩石作为整体研究对象较多,而直接以细观组分为对象的研究较少。以石灰岩为例,根据室内试验获得的岩石力学性质指标,使用基于非连续介质理论的颗粒流方法,将材料离散成刚性颗粒组成的模型,把颗粒细观变化与宏观力学特性联系起来,建立了石灰岩的细观结构模型,获得了颗粒接触力、颗粒接触模量、接触连接强度和连接刚度比等细观力学参数。由于文中直接以细观成分为研究对象、反映了岩石和岩体组成的本质特点,所得结论不仅对含裂隙岩石本构关系研究具有广阔的应用前景,而且对岩体工程性质和地质灾害机制研究也具有重要的理论意义。     

Particle simulation of thermally-induced rock damage with consideration of temperature-dependent elastic modulus and strength

Based on the particle simulation method, a thermo-mechanical coupling particle model is proposed for simulating thermally-induced rock damage. In this model, rock material is simulated as an assembly of particles, which are connected to each other through their bonds, in the case of simulating mechanical deformation, but connected to each other through thermal pipes in the case of simulating heat conduction. The main advantages of using this model are that: (1) microscopic parameters of this model can be directly determined from the related macroscopic ones; (2) the temperature-dependent elastic modulus and strength are considered in an explicit manner, so that thermally-induced rock damage can be realistically simulated in a thermo-mechanical coupling problem. The related simulation results from an application example have demonstrated that: (1) the proposed model can produce similar behaviors to those observed in experiments; (2) the final failure is initiated from the outer surface of the testing sample and propagates toward the borehole; (3) microscopic crack initiation and propagation processes can be reasonably simulated at the cooling stage.     

低围压下颗粒形态对软黏土抗剪强度影响的离散元分析

黏土颗粒形态不仅反映黏土的矿物组分,更是影响其物理力学性质的重要因素之一。为了研究物质组成对软黏土微宏观性质的影响,采用离散元方法对不同颗粒形态的软黏土试样进行三轴压缩模拟试验。首先,基于扫描电镜图像量化颗粒形态,对天然状态下黏土颗粒的方向角和凹凸度进行统计,引入球度和凹凸度作为颗粒形态的特征参数;然后,基于原生矿物的单粒结构和黏土矿物的片状结构特征,构造球体单粒及圆柱体、正方体、长方体的片状簇体;最后,基于三轴试验离散元模拟方法,分析软黏土颗粒形态对其宏观力学及微观特性的影响。结果表明:片状颗粒试样比球体颗粒试样的初始模量高,抗剪强度大,随加载其排列趋于水平向分布;加载初期,颗粒球度对初始弹性模量影响较明显,初始弹性模量随着球度增大而逐渐减小;加载后期,颗粒凹凸度对抗剪强度指标影响作用逐渐凸显,试样内摩擦角和黏聚力随着凹凸度增大而逐渐减小;微观结构上,颗粒形状对颗粒位移和旋转也有较大影响。     

岩石化学风化时效效应的离散元模拟

微观上,岩石由晶粒和晶粒之间的联接组成。在岩石的风化过程中,晶粒之间的联接会被削弱,导致岩石微观缺陷的发展,从而表现为显著的时效效应。在已有胶结模型的基础上,将岩石的化学风化过程等效为颗粒之间胶结物质不断溶蚀进而影响颗粒之间胶结强度和刚度的过程,基于此建立了考虑化学风化的微观接触模型。随后将模型植入离散元软件,运用离散元方法（DEM）,模拟和对比分析了室内腐蚀试验与DEM模拟化学风化试验,结果表明：该模型可以反映岩石强度的化学风化时效效应,即随着反应时间增加,强度减小;可以反映单轴压缩试验中裂隙压密阶段以后变形的时效效应,即随着反应时间的增加,弹性模量、破坏时应变减小;粒间胶结的破坏,即微观缺陷的发育,是导致岩石化学风化时效效应的微观原因。同时DEM模拟风化与加载耦合的试验现象表明,施工过程中的施工速度会影响岩石的强度,施工应该选择合适的速率。     

考虑弹性模量变化的岩石统计损伤本构模型

为建立更为完善的岩石统计损伤本构模型,针对现有模型难以反映某些岩石在三轴试验条件下弹性模量随围压变化而改变的特性,从不同类型空隙对岩石变形性质的影响入手,深入分析了岩石弹性模量变化的机理,并在此基础上,依据弹性参数与岩石中未闭合裂隙数的关系建立了岩石弹性模量变化的分析方法。之后,引入统计损伤理论,考虑损伤阈值的影响和残余强度变形特征,建立了可模拟岩石变形全过程的统计损伤本构模型,并给出了基于三轴压缩试验曲线的模型参数确定方法。本文模型可反映岩石弹性模量随围压变化而改变的特性,且具备现有模型的优点,对岩石变形全过程模拟效果良好。最后,通过试验曲线与本文及同类模型理论曲线的对比分析,表明了本文模型的合理性与优越性。     

掘削卵砾石地层颗粒离散元模拟中的细观参数取值讨论

卵砾石地层被掘削时,其主要破坏模式并非一般的宏观整体剪切破坏,而是刀具切入卵砾石间的缝隙,将卵砾石颗粒剥离出原始地层,该过程较为契合颗粒离散元的物理力学假设。为确定掘削工况下的颗粒材料最优细观参数,对不同的颗粒接触法向刚度、接触刚度比、粒间摩擦系数、颗粒形状、颗粒尺寸等的颗粒材料开展了截割三维仿真试验,并分析研究了不同细观参数对掘削效果的影响。计算结果显示：颗粒接触法向刚度、接触刚度比的多种细观参数组合可以对应同一个宏观弹性模量值,接触刚度比大的细观参数组合的抗掘削能力更大。颗粒形状、粒间摩擦系数的多种细观参数组合可以对应同一个内摩擦角值,形状复杂的颗粒材料具有更大的抗掘削能力。在宏观参数相同的情况下,平均粒径更大的颗粒材料抗掘削能力更大。因此,利用颗粒离散元模拟掘削卵砾石地层问题时,除需保证虚拟颗粒材料的宏观整体剪切破坏响应与真实材料基本一致外,还需根据材料的抗掘削能力进一步标定细观参数,以获得更贴近实际情况的模拟效果。文章提出的方法可以为其他颗粒离散元掘削工况数值模拟中的参数标定提供参照依据。     

Dynamic analysis of an offshore structure on an elastic-plastic foundation-soil system

The effect of foundation stiffness upon the dynamic response of an offshore structure is investigated. The non-linear deformation behaviour of a stratified seabed is included in the finite element simulation. The seabed is represented by a simplified spring model. In the linear elastic analyses the stiffnesses are derived using elastic half-space theory. For the non-linear problems, elastic-plastic finite element analyses are used to generate equivalent properties.An example of the method is given as applied to a three towered concrete gravity platform. Variation of natural frequency is used to indicate the significance of changes in parameters such as soil shear modulus, structural stiffness and deck mass. The forced response behaviour to random wave excitation is also investigated using a linear elastic analysis. The effect of the interaction of the foundation — soil with the structure on the dynamic response is demonstrated by the results.     

岩石颗粒胶结方式对储层岩石弹性及渗流性质的影响

为研究岩石颗粒胶结方式对储层岩石弹性和渗流性质的影响,采用过程模拟法构建了三维数字岩心,在此基础上,分别利用有限元方法和格子玻尔兹曼方法研究了胶结物均匀生长、沿孔隙生长和沿喉道生长3种胶结方式对岩石弹性和渗流特性的影响规律.结果表明:岩石颗粒胶结方式会影响岩石刚性和孔隙连通性,引起岩石弹性模量和渗透率的变化.在相同孔隙度下,胶结物沿喉道生长形成的岩石抗压性最强,渗透率最小;沿孔隙生长形成的岩石抗压性最弱,渗透率最大.3种胶结方式下,岩石弹性模量随着胶结物含量增加而增大,变化率近似相等;岩石渗透率随着胶结物含量的增加而减小,岩石渗透性对颗粒胶结方式的变化更敏感.      

A lattice type model for particulate media

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.     

用Monte Carlo方法模拟砂土的自然堆积过程

用MonteCarlo随机模拟方法模拟砂土颗粒在重力作用下,在圆柱与长方体容器中的自然堆积过程。首先用参考网格法生成一个砂土颗粒的松散结构,松散结构中粒子与粒子、粒子与边界间不存在任何接触;然后启动MonteCarlo随机模拟算法,即给处于松散结构中的每个粒子施加随机位移,得到新构形,如果新构形中粒子间或粒子与边界间发生重叠,则放弃这个构形;如果没有重叠存在,则判别粒子体系的势能变化,运用Metropolis准则来判别这个构形是否被接受,重复这个过程可以得到砂土的密集堆积结构。采用Schinner建议的接触发现算法判别粒子间是否存在重叠,同时详细介绍了粒子与边界间的接触发现算法。模拟结果表明,用MonteCarlo方法模拟砂土的自然堆积结构是非常有效的,可以为砂土的流动、压实等的数值模拟工作提供初始构形。     

陇西黄土三轴剪切过程微观变化研究

为了解黄土三轴剪切过程中的微观变化,采用PFC3D建立黄土三轴试验模型,模拟围压分别为0、50、150、300 kPa的三轴剪切试验,并与室内试验进行对比分析。分析结果表明：PFC3D能够较好地模拟出不同围压下从开始到破坏到残余变形整个过程中应力-应变的变化规律,且位移场及接触应力场的变化规律与室内三轴试验宏观现象较一致;发现弹性模量、泊松比及峰值强度与数值模型中微观参数有着密切的联系,如法向刚度kn控制试样宏观弹性模量,kn /ks值控制泊松比,摩擦系数控制峰值强度;通过体应变-轴向应变曲线发现,随着围压的增加应变能增大,试样呈现出由体积膨胀到体积减缩的变化规律。其研究结果为进一步探究黄土的应力-应变性状及抗剪强度特性提供参考。     

平行黏结模型宏细观力学参数相关性研究

采用变量控制法较全面地分析了各细观参数与宏观参数的定量关系,表明：弹性模量E随颗粒模量Ec、黏结模量 、平行黏结半径乘子 呈线性增长,随颗粒刚度比kn /ks、黏结刚度比 呈对数减小;泊松比则主要受kn /ks和 的影响,两者之间呈对数关系;颗粒键的黏结强度决定了材料的强度,室内材料黏聚力c和抗拉强度 主要受法向平行黏结强度 、平行黏结强度比 的影响,随 线性增长,随 对数减小;摩擦角 主要受颗粒摩擦系数u影响,两者呈对数关系。分析裂隙扩展特征,表明材料法向黏结强度 和切向黏结强度 的相对大小决定裂纹分布规律,随 增大,岩样的拉破坏区域减少,而压剪破坏区域增加,破坏面由剪切破坏向共轭破坏发展;材料的强度离散性越小,岩样破坏趋于集中,破坏面明显,强度均值标准差比值 >3.5为宜; 增加,宏观破坏形式向共轭破坏发展。细观参数的选取除了匹配强度参数,同时还需要考虑破坏形式的一致,考虑多参数相互影响,建立了宏细观参数之间的经验公式,对细观参数进行优化选择,并做了实例验证。室内试验和数值模拟获得的峰值荷载、变形参数、剪切强度等数值接近,应力-应变演化规律相同,破坏形态一致,表明细观参数结果是可靠的。     

Study on the particle breakage of ballast based on a GPU accelerated discrete element method

Breakage of particles will have greatly influence on mechanical behavior of granular material(GM)under external loads,such as ballast,rockfill and sand.The discrete element method(DEM)is one of the most popular methods for simulating GM as each particle is represented on its own.To study breakage mechanism of particle breakage,a cohesive contact mode is developed based on the GPU accelerated DEM code-Blaze-DEM.A database of the 3D geometry model of rock blocks is established based on the 3D scanning method.And an agglomerate describing the rock block with a series of non-overlapping spherical particles is used to build the DEM numerical model of a railway ballast sample,which is used to the DEM oedometric test to study the particles’breakage characteristics of the sample under external load.Furthermore,to obtain the meso-mechanical parameters used in DEM,a black-analysis method is used based on the laboratory tests of the rock sample.Based on the DEM numerical tests,the particle breakage process and mechanisms of the railway ballast are studied.All results show that the developed code can better used for large scale simulation of the particle breakage analysis of granular material.     

Size-dependent mechanical behavior of an intergranular bond revealed by an analytical model

The size of intergranular bonds significantly affects the macroscopic mechanical properties of geomaterials. A size-dependent bond contact model is desired in the distinct element method (DEM) for geomaterials formed by aggregates of bonded particles. This paper proposes an analytical solution of highly-precise stress fields of a biconcave bond between two identical disc-shaped particles under different loading paths based on Dvorkin’s solution. The Unified Strength theory is then introduced to obtain the initial failure domain in the bond. The proposed solution is consistent with results predicted by finite element simulations and experimental observations. The functions of bond stiffness with respect to all influencing parameters, i.e. bond width/thickness, particle radius and elastic parameters of bond material, are provided by the solution and empirically formulated by fitting a large number of analytical results. Additionally, the failure criterion or envelope under different combined loads is formulated for typical brittle bonds. The resulting failure criterion, approximated as an ellipsoid, depends on the size and material properties of the bonds. The proposed solution and equation can be implemented into a bond contact model used in DEM simulations of a geomaterial, where variation of bond sizes is significant and size-dependent contact model is important.     

Simulations of realistic granular soils in oedometer tests using physics engine

Discrete element method (DEM) has become a preeminent numerical tool for investigating the mechanical behavior of granular soils. However, traditional DEM uses sphere clusters to approximate realistic particles, which is computationally demanding when simulating many particles. This paper demonstrates the potential of using a physics engine technique to simulate realistic particles. The physics engines are originally developed for video games for simulating physical and mechanical processes that occur in the real world to produce realistic game experiences. The simulation accuracy and efficiency of physics engines have been significantly improved in the last two decades allowing them to be used as a scientific tool in many disciplines. This paper introduces modeling methodologies of physics engine including realistic particle representation and the contact model. Then, oedometer tests are simulated using realistic particles scanned by X-ray computed tomography (X-ray CT). The simulation results agree well with experimental results. This paper demonstrates that physics engines can output contact parameters for geotechnical analysis and force chains for visualization.     

三维梁-颗粒模型在岩石材料破坏模拟中的应用

用三维梁-颗粒模型BPM3D(beam-particlemodelinthreedimensions)对岩石类非均质脆性材料的力学性质和破坏过程进行了数值模拟。梁-颗粒模型是在离散单元法基础上,结合有限单元法中的网格模型提出的用于模拟岩石类材料损伤破坏过程的数值模型。在模型中,材料在细观层次上被离散为颗粒单元集合体,相邻颗粒单元由有限单元法中的弹脆性梁单元联结。梁单元的力学性质均按韦伯(Weibull)分布随机赋值,以模拟岩石类材料力学参数的空间变异性。材料内部裂纹通过断开梁单元来模拟。通过自动生成的非均质材料模型对岩石类材料的破坏机理进行研究。岩石类非均质脆性材料在单轴压缩状态下破坏过程细观数值模拟结果显示,岩石材料宏观破坏是由于其内部细观裂纹产生、扩展、贯通的结果。通过数值模拟结果之间的对比分析,揭示出岩石试样宏观破坏模式随细观层次上韦伯分布参数的变化而不同。与实际矿柱破坏形态的对比分析表明了模型的适用性。根据数值模拟结果对岩石类非均质材料的破坏机理进行了探讨。     

随机分布贯穿裂隙岩体变形特性研究

洞室开挖后,其周边通常会产生许多随机分布的贯穿裂隙,直接影响洞室围岩稳定,研究随机分布贯穿裂隙岩体的变形及变形特性具有重要意义。基于线弹性理论和线性刚度理论计算岩石和裂隙的位移,用概率的方法建立了随机分布贯穿裂隙岩体变形的计算模型,给出了随机分布贯穿裂隙岩体的等效弹性模量和等效泊松比,研究了岩石和裂隙的材料参数和几何参数对岩体等效弹性模量和等效泊松比的影响。可得如下结论：等效弹性模量和等效泊松比随着岩石弹性模量的增大而增大;等效泊松比随着岩石泊松比的增大而增大;等效弹性模量和等效泊松比随着裂隙法向刚度的增大而增大;随着剪切刚度的增大,等效弹性模量逐渐增大,而等效泊松比则逐渐减少;随着裂隙平均间距的增大,等效弹性模量逐渐减小,等效泊松比在平均倾斜角较小时逐渐增大,在平均倾斜角较大时逐渐减小;随着裂隙平均倾斜角的增大,等效弹性模量先减小后增大,而等效泊松比先增大后减小。模型能较全面地考虑构成岩体的岩石和裂隙的材料参数与几何参数对岩体变形的影响,其结果对研究洞室围岩的变形和工程设计有一定的参考价值。     

Hydrothermal systems in peridotites of slow-spreading mid-oceanic ridges. Modeling phase transitions and material balance: Downwelling limb of a hydrothermal circulation cell

A model is development for the kinetic and thermodynamic simulation of the interaction of seawater and its metamorphosed derivatives with crustal rocks in slow-spreading ridges. The thermodynamic modulus of the model is based on the GEOCHEQ complex, which makes it possible to simulate equilibria in systems of aqueous solutions-minerals-gases. The calculating code was modified and adjusted for the thermodynamic-kinetic simulation of the passage of irreversible solution-rock reactions with time. The simulations were carried out for a simplified crustal vertical section of slow-spreading (Hess-type) ridges, which consist only of mantle peridotites (spinel harzburgites). The results of our simulations demonstrate that the degree of peridotite serpentinization under the effect of low-temperature seawater when the rocks are exposed at the seafloor surface remains very low even after 10000 years of interaction. Serpentinization becomes efficient only at temperatures of 130–150°C at crustal depths of 3.5–4.5 km. The results of our simulations allowed us to develop a thermodynamic model for the origin of hydrothermal systems in peridotites in slow-spreading ridges, with regard for the major stages in the material and tectonic evolution of the Hess crust.     

颗粒配比对岩石力学特征影响的数值模拟研究

岩石是矿物颗粒的集合体同时也是一种重要的非均质材料,了解它的力学特征对岩土工程及矿产开采都具有重要的指导作用。作为典型的颗粒材料,颗粒单元体的粒径分布配比必然影响着岩石的宏观力学表现。通过设置不同体积配比下的颗粒材料单元体,利用PCF2D软件模拟了相同颗粒材料单元体不同配比下岩石模型的力学特征。模拟结果表明颗粒单元体配比对岩石的力学特征有明显的影响。在模拟过程中大颗粒的配比显著影响着岩石的抗压强度,大颗粒含量相对越高,抗压强度越大。而细颗粒的配比影响着岩石的抗拉强度,细颗粒含量相对越高,抗拉强度越大,但是过多的细颗粒会降低岩石的抗拉强度。考虑岩石压缩过程中裂缝形态的影响。结果表明均匀分布、5:2:3、7:2:1的颗粒配比形成了贯穿裂缝,而1:2:7和3:2:5的颗粒配比未能形成贯穿裂缝,且细颗粒配比越高,裂缝数目出现高值的概率也越大。