基坑土体卸荷平面应变试验离散元数值分析

对基坑开挖影响范围内土体的应力路径进行平面应变试验离散元数值模拟,以研究结构性与卸荷形式对坑周土体宏微观力学特性的影响。首先,将一个描述土颗粒间胶结效应的简单三维胶结接触模型植入三维离散元软件PFC3D;其次,对初始K0固结状态的重塑土、结构性土试样分别进行常规三轴以及平面应变条件下4种不同卸荷应力路径的离散元模拟;最后,对经历不同卸荷形式的坑底土体单元进行再加荷模拟。模拟结果表明,在卸荷过程中,被动区土体峰值强度以及破坏时竖向应变随卸荷比增大而增大,且其强度小于主动区土体强度;在卸荷、再加荷过程中,被动区土体峰值强度随卸荷比增大而增大,但均小于不卸荷而直接加荷条件下的峰值强度;由于结构性的存在,土体由应变硬化向应变软化过渡,且强度增长;结构性与卸荷形式显著影响土体体积改变。在微观尺度,增大卸荷比或结构性均会增大垂直大主应力方向的平面上的法向接触力,进而提高其强度。     

构造物理模拟和PIV有限应变分析对构造裂缝预测的启示

油气储层中构造裂缝发育与有限应变状态关系密切,为了探索有限应变分析与构造裂缝预测的新技术方法,此次研究设计完成了一组单侧挤压收敛模型的物理模拟实验,并引入粒子图像测速(PIV,Particle Image Velocimetry)技术对实验过程进行了定量化分析。实验模型在垂向上为含粘性层的多层结构,实验结果形成了一个肉眼可见的箱状褶皱。通过PIV技术可以获取实验模型变形演化过程中各阶段的位移场数据,计算出各阶段的增量应变,实现从初始状态到褶皱形成之后整个变形过程的有限应变分析,探讨构造裂缝成因机制和分布规律,进行定量化裂缝预测。挤压变形过程初期,应变分布范围很广,有限应变较弱(约4%~8%),在挤压方向上的线应变表现为弱压应变,在垂向上的线应变表现为弱张应变,这种现象是褶皱和断层产生前平行层缩短和层增厚的纯剪变形结果,也是区域型张裂缝和剪裂缝形成的主要机制。褶皱和断层即将发育之时至发育之后,应变局限在断层发育的剪切带及附近区域,有限应变表现为较强(达20%)的剪切应变和剪切张应变,是断层面附近简单剪切变形作用的结果,也是局部型剪裂缝和张剪裂缝形成的主要机制。     

碎石堆小行星自旋演化过程的离散元数值模拟

近年的探测研究表明,由引力和范德华粘附力聚合成碎石堆结构可能是小行星的一种重要形成方式。为探究这类小行星的演化机制,文章基于离散元软件MatDEM进行二次开发,实现了单元的万有引力计算。建立直径为1.2 km“碎石堆”小行星数值模型,通过数值模拟分析了不同粘附强度和体积密度下,球形聚集体的旋转重塑和破坏过程。结果表明：球形聚集体受到其角速度初始增量的影响,以启动聚集体的重塑过程,导致其发生变形或者破坏;粘附强度越小,模型越容易被破坏,破坏前小行星会通过向扁圆状椭球体变形来保持自身构型的稳定性;对于粘附强度一定的小行星模型,体积密度的增大会有效增加其结构的稳定性,可以延后小行星破坏行为发生的时间,且颗粒脱落的数量随着密度的增大而减少;采用离散元分析可以较好地模拟小行星自旋演化过程。进一步与真实小行星数据相结合,有利于深入、全面地探索小行星自旋演化破坏机理。     

基于构造应变分析的裂缝预测方法及其应用

地层应变是构造裂缝产生的直接因素,根据构造应变大小可以预测构造裂缝发育的位置和强度,对研究区的主要裂缝发育区进行划分。文章以松辽盆地徐家围子断陷徐中地区营城组四段（简称营四段）为研究对象,在建立研究区精细的三维构造模型基础上,利用“构造恢复”方法实现研究区古构造恢复,通过开展有限应变值计算来预测构造裂缝的平面分布。研究表明：研究区营四段主要包括3个造缝期,即营城组末期、泉头组—青山口组时期、嫩江组时期,其中,营城组末期与泉头组—青山口组时期的构造变形较为强烈,是裂缝的主要形成时期。根据应变大小与试气产量的关系,将研究区划分为3类裂缝发育区,Ⅰ类裂缝发育区已钻井验证,表明利用构造应变对裂缝的预测结果可靠,Ⅱ类裂缝发育区可作为深层天然气的下一步挖潜的重要方向,Ⅲ类裂缝发育区产能较低,裂缝对储层的改造作用有限。     

大冶铁矿露天转地下开采的离散元数值模拟研究

离散单元方法特别适合于节理比较发育的岩体。对于模拟和分析矿山中因采矿引起的围岩松动和冒落等力学行为是比较有效的。用离散单元法研究了大冶铁矿狮子山采区露天转地下开采，采用充填法、崩落法采掘对露天边坡和围岩的影响及围岩变形规律，并得出了相应的结论，为设计和方案的选用提供了一些有益的根据。     

基于离散元数值方法的砂土小应变弹性特性探讨

土体在非常小应变（小于10?6）时表现为线弹性。以往大量试验研究表明,土体的弹性特性主要受到土体有效围压、孔隙比、颗粒级配和试样制备方法等影响。通过离散元方法模拟Duffy和Mindlin中的物理试验,建立球形颗粒规则排列的三维模型,继而进行小变形条件下的数值加载试验来确定弹性特性,并从细观层面揭示决定土体弹性特性的内在因素。模拟结果表明：粒状土的弹性模量取决于土体的颗粒接触配位数、法向接触力的大小和分布,跟宏观土体孔隙比、有效围压作用相对应。弹性模量应力指数n高于Hertz-Mindlin接触法则对应的1/3,主要是由于应力增加过程中配位数的增加和接触力趋于更均匀导致。同时,泊松比也随着配位数的增加而减小,并非保持不变,跟试验结果趋势一致。     

深基坑PCMW工法开挖过程离散元数值模拟分析

深基坑稳定性问题是重要的工程问题,其中,有效支护结构是保证基坑工程顺利实施的基础。本文引入离散元法,提出了基坑及支护结构建模和数值模拟方法。采用紧密堆积离散单元构建模型初始地层,引入了离散单元团簇模型来构建表面较为光滑的管桩。以南京某深基坑工程为例,采用三维离散元模拟软件MatDEM3D构建几何模型,并根据土体的力学性质设定相应单元的力学参数。数值模拟中,通过预平衡操作实现土层的压实,进而模拟了基坑开挖过程中土体和管桩的变形。将数值模拟结果与实测监测数据进行了对比分析,评价了基坑支护结构的有效性。这种基坑离散元数值模拟方法在基坑开挖过程的稳定性预判和支护结构的有效性评价上具有潜在的应用前景。     

土垫层缓冲落石冲击力特性离散元数值模拟分析

棚洞是我国西部山区防治崩塌落石灾害的主要工程措施之一。棚洞顶板以上通常铺设由砂或碎石组成的土垫层。土垫层的作用是避免落石直接冲击棚洞,缓冲落石的冲击力。长期以来,关于土垫层厚度对缓冲效果影响的研究较少,因此,还未形成统一的理论用于指导土垫层厚度的设计。文章运用离散单元方法建立落石冲击土垫层的数值模型,探究垫层厚度和落石下落高度对土垫层缓冲落石冲击力特性的影响。研究结果表明：落石冲击力峰值与落石下落高度呈幂函数关系,顶板中心力峰值与下落高度呈线性正相关关系;随着垫层厚度的增加,落石冲击力峰值减小,当垫层厚度增加到落石直径的1.0倍之后,落石冲击力峰值与垫层厚度无关;随垫层厚度的增大,顶板中心力峰值与落石冲击力峰值的比值减小,垫层缓冲效果增大,当垫层厚度增加到落石直径1.5倍之后,垫层缓冲效果增加不明显;垫层厚度建议取值为落石直径的1.5倍。     

基于离散元法的水力压裂数值模拟

不同条件下水压裂隙的发展特性对有效开采页岩气具有重要的指导作用。针对岩体在微观上为颗粒和孔隙的结构系统,提出离散元水力压裂数值模拟方法,离散元能量转化和能量守恒计算方法,建立了相应的三维离散元模型。采用自主研发的三维离散元模拟软件Mat DEM3D,通过控制模型的竖向应变与颗粒直径,来模拟地层中的应力与压裂速率的变化。模拟结果表明:(1)水力压裂产生裂隙的数量和方向受岩石的各向异性,压力状态和变化速率所影响。(2)裂隙在压缩波传播时发展,当水压力高速增加时,诱发的裂隙数量增多,并且有效能量(断裂热)百分比也随之增加,压裂作用也变得更明显。(3)当竖向应变为零时,50%的裂隙呈垂直状态,当竖向应变为-1×10-4时,裂隙趋于沿着最大压力方向发展,竖向裂隙的百分率增大。数值模拟和能量分析为定量地研究岩石水力压裂过程提供了一个新的方法。     

三峡永久船闸高边坡开挖三维离散元数值模拟

采用面－面接触的三维离散元刚性块体模型，从实测节现面中取出其中的三组，按照其倾向，倾角和节理间距将三峡永久船闸未开挖的区域划分为10^5个离散单元，通过施加力边界条件，给出了与实测初始地应力场接近的数值模拟结果；然后，分4步模拟了永久船闸的开挖过程。计算结果表明：开挖过程会引起节理面出现张开趋势，个别岩体还会沿着节理面滑移。岩体位移的不对称现象较为自然地说明了由节理引起的岩体各向异性特征。     

Application of Dimensional Analysis in Calibration of a Discrete Element Model for Rock Deformation and Fracture

Summary A discrete element approach was used in the simulation of rock fracture. The numerical synthetic material was made of rigid circular particles or cylinders that have interaction through normal and shear springs. The cylinders were bonded to each other at the contact points to withstand the applied loads. To characterize the microscopic properties of this synthetic material, a dimensional analysis approach was presented. It was shown that the dimensionless parameters and graphs obtained were useful tools for fast and efficient calibration of a synthetic material. This calibration method was employed for finding a numerical model for Pennsylvania Blue Sandstone. The numerical model could mimic many deformational and failure characteristics of the sandstone in both conventional and some non-conventional stress paths.     

Quantitative Prediction of Fracture Distribution of the Longmaxi Formation in the Dingshan Area,China using FEM Numerical Simulation

Fracture prediction is a technical issue in the field of petroleum exploration and production worldwide. Although there are many approaches to predict the distribution of cracks underground, these approaches have some limitations. To resolve these issues, we ascertained the relation between numerical simulations of tectonic stress and the predicted distribution of fractures from the perspective of geologic genesis, based on the characteristics of the shale reservoir in the Longmaxi Formation in Dingshan; the features of fracture development in this reservoir were considered. 3D finite element method (FEM) was applied in combination with rock mechanical parameters derived from the acoustic emissions. The paleotectonic stress field of the crack formation period was simulated for the Longmaxi Formation in the Dingshan area. The splitting factor in the study area was calculated based on the rock breaking criterion. The coefficient of fracture development was selected as the quantitative prediction classification criteria for the cracks. The results show that a higher coefficient of fracture development indicates a greater degree of fracture development. On the basis of the fracture development coefficient classification, a favorable area was identified for the development of fracture prediction in the study area. The prediction results indicat     

金江滑坡成因离散元模拟

在对金江巨型滑坡的工程环境条件及现今坡体物质组成、自身结构及变形破坏特征等方面调查研究的基础上,宏观判断滑坡为整体式滑移-弯曲破坏模式。为直观反映滑坡变形破坏的过程,根据对滑坡区基岩结构特征的分析,采用离散元软件2D_Block对滑坡成因机制进行模拟。根据滑坡普遍发育的地形地貌特征,恢复滑坡区原始斜坡地形,根据岩体结构划分模型计算单元,将地震效应考虑进2D_Block模型参数中。通过计算,展现了滑坡的发展演化过程,并将其分为三个阶段,即轻微隆起阶段、强烈隆起阶段、滑面贯通剪出阶段。     

Numerical Modeling of Natural Fracture Distributions in Shale

The production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures. This study presents a simulation of natural fractures in shale reservoirs, based on a discrete fracture network (DFN) method for hydraulic fracturing engineering. Fracture properties of the model are calculated from core fracture data, according to statistical mathematical analysis. The calculation results make full use of the quantitative information of core fracture orientation, density, opening and length, which constitute the direct and extensive data of mining engineering. The reliability and applicability of the model are analyzed with regard to model size and density, a calculation method for dominant size and density being proposed. Then, finite element analysis is applied to a hydraulic fracturing numerical simulation of a shale fractured reservoir in southeastern Chongqing. The hydraulic pressure distribution, fracture propagation, acoustic emission information and in situ stress changes during fracturing are analyzed. The results show the application of fracture statistics in fracture modeling and the influence of fracture distribution on hydraulic fracturing engineering. The present analysis may provide a reference for shale gas exploitation.     

Numerical Procedure for Dynamic Simulation of Discrete Fractures Due to Blasting

Summary  This paper describes development of a generic nonlinear, dynamic modelling technique to simulate discrete rock fractures due to blasting using the finite element method. The element elimination technique together with a brittle, Rankine failure-type material model are used as a means to simulate the initiation and growth of fractures in the rock under the effect of blast-induced dynamic pressure pulse. Dynamic loads representing ideal and non-ideal detonations are simulated and a new method, termed as optimised pressure profile, is proposed to approximate the pressure-time profile of the blast load to model the dynamic load. Comparison of numerical model results with previously reported observations from the literature reveals the ability of the model as a predictive tool and supports the validity of the developed modelling procedure. Author’s address: Hani S. Mitri, Department of Mining, Metals and Materials Engineering, McGill University, 3450 University Street, Montreal, Canada H3A 2A7     

Investigating the micro mechanics of cemented sand using DEM

The discrete element method has been used to investigate the micro mechanics of cemented sand. High‐pressure drained triaxial tests are modelled in 3D using a flexible membrane that allows the correct deformation to develop. Simulations with up to 12 MPa confining pressure are presented, which are compared with laboratory experiments on a sand with a range of cement contents. Cementation is modelled using ‘parallel bonds’, and various parameters and strength distributions are investigated. Varying levels of cementation are successfully modelled, with the correct qualitative behaviour observed and the separate effects of cementation and confining pressures demonstrated. The triaxial behaviour is found to be highly influenced by the distribution of bond strengths. Copyright © 2014 John Wiley & Sons, Ltd.     

遗传算法在边坡数值计算中的应用

改进了进化方向的遗传算法与有限元数值法，结合并研制了相应的软件。应用该软件对多类型岩土边坡进行弹性模量、内聚力、内摩擦角等参数反演分析，显示误差很小，收敛速度也很快，这说明改进进化方向遗传算法这种新型的优化算法在多类型岩土参数优化估计中具有独特的优势。