水泥砾质土三轴试验研究

对某水泥砾质土进行了三轴固结排水剪切试验,研究在不同水泥掺入比下水泥砾质土应力-应变关系和强度特性。结果表明：随水泥掺入比的增加,水泥砾质土试样在不同围压下应力-应变关系曲线均表现出不同程度的软化和剪胀现象,尤其在低围压（200 kPa）和高水泥掺入比（8%）下,剪胀性比较明显;在相同水泥掺入比下,峰值强度 随围压的增加近似呈线性增长;与不掺水泥砾质土相比,水泥砾质土的内摩擦角 因掺入不同量的水泥而得到不同程度的提高,黏聚力 随水泥掺入比的增加呈增长趋势,且增幅显著;割线模量 和 均随围压和水泥掺入比的增加而增加。不同高度的心墙堆石坝三维有限元算例的计算结果表明：水泥砾质土作为心墙材料可以显著降低土石坝心墙沉降。     

饱和砾性土不排水动力强度及变形特性试验

砾性土的动力强度和变形特性是分析砾性土地基及其构筑物地震永久变形的基础。本文采用大型动三轴仪对饱和砾性土进行不排水循环三轴试验,系统地研究了两种砾性土在循环加载条件下的不排水强度和变形特性,探讨了循环应力比、平均固结应力、固结应力比、初始孔隙比和循环次数对砾性土变形和超静孔压特性的影响。试验结果表明：砾性土的累积轴向应变和超静孔压比随循环应力比、平均固结应力和初始孔隙比的增大而增大,超静孔压比随固结应力比的增大而减小;当循环次数小于20时,固结应力比对累积轴向应变的影响较小,而当循环次数大于20时,累积轴向应变随固结应力比的增大而减小。根据试验结果,建立了考虑平均固结应力、初始孔隙比、循环次数和累积轴向应变影响的不排水动力强度和超静孔压经验模型,其计算值与试验结果比较吻合,说明该模型可以较为准确地描述循环荷载作用下砾性土的不排水动力强度、超静孔压和累积变形的变化规律。     

粗粒含量对砾类土直剪过程中强度与变形特性影响的离散元模拟研究

粗粒含量对砾类土的工程力学特性具有重要的影响。本文对4组不同粗粒含量的强风化玄武岩砾类土进行了大型直剪试验,并获取相关的强度与变形参数,基于离散单元法颗粒流理论,采用粒间作用为平行黏结模型的圆球模拟土颗粒,建立了4种不同粗粒含量砾类土直剪的离散单元模拟的计算模型,并进一步校正了颗粒单元细观参数,模拟了不同粗粒含量砾类土100kPa垂直压力时的应力应变关系、垂直变形以及剪切带上的土颗粒运动与颗粒间作用力传递的影响特性,分析了粗粒含量对砾类土宏观及细观力学性质的影响机理。结果表明:砾类土表现出的粗粒含量越大强度越高的本质是由于随粗粒含量增加时,土颗粒间平均刚度增加及颗粒间的咬合作用使得摩擦系数增加,采用平行黏结模型能较好的拟合峰值前剪应力-剪位移曲线,但是峰后曲线段尤其对软化现象的适应性不是十分理想; 垂直位移-剪位移模拟值与试验值存在一定的偏差; 随着粗粒含量的增加,最大剪应力时粗颗粒对力链的控制表现得愈明显,相应的剪切带厚度约为剪切盒高度的1/3～1/5,并随粗粒含量增加而增大。     

掺砾心墙料拉裂力学特性试验研究

研究掺砾心墙料的拉裂特性对深入研究高土石坝水力劈裂、坝顶裂缝以及坝肩横缝等问题至关重要,但目前已有的研究尚不够深入。基于自主研制的单向拉伸试验装置,对不同掺砾量下的心墙料进行了系列的单向拉伸试验,依据试验结果分析了掺砾心墙料拉裂破坏的机制。在此基础上得到以下结论：在试样各自最大干密度及最优含水率下,随着掺砾量的增加,心墙料的抗拉强度和拉应变呈线性递减关系;所有试样的拉应力?应变曲线呈分段指数关系,极限拉应力前后试验曲线可分别采用正负指数关系来描述;进行了系列三轴排水剪试验,分析各试样抗拉强度与强度指标的关系发现,对于所研究的掺砾心墙料,抗拉强度与其黏聚力呈较好的线性关系,在不具备试验条件的情况下,此关系可用来大致估算心墙料的抗拉强度。相关试验结果可为实际土心墙坝抗裂设计提供参照。     

宽级配砾质土快速三轴固结排水剪试验影响因素研究

宽级配砾质土是由砾石料和黏土料按一定比例混合而成,其具有压缩性低、抗剪强度大等特点,目前常作为土石坝心墙料或路基填料而得到广泛应用。由于宽级配砾质土渗透系数较小,在常规三轴固结排水剪(CD)试验中固结排水较慢,导致其试验周期很长。为了提高固结排水的效率,可采用一种在试样中心加圆柱形砂芯的快速三轴CD试验方法。基于快速三轴CD试验,通过变化不同砂芯类别、不同砂芯直径、不同掺砾量等各因素,全面研究了各因素对宽级配砾质土快速三轴CD试验的影响。研究结果表明,快速三轴CD试验方法能够有效加快试样排水固结,从而加快整个试验进程;砂芯类别、砂芯直径、掺砾量等因素均对试验固结排水速度和剪切过程应力应变产生不同程度影响;砂芯直径越小,其试验成果与无砂芯试样越接近。     

路基砾类填料土动力特性影响因素试验分析

动应力-动应变关系作为反映路基填料土动力特性的重要参数,对其进行影响因素试验研究可为路基填料土的工程特性研究提供参考依据。本文借助动三轴试验手段进行砾类土动应力-动应变关系影响因素研究,采用应力控制的加载方式进行加载,分析了试验压力、土样含水率、土样压实度、荷载作用频率和初始静偏应力等5种条件和砾类土动力特性的作用规律,并总结砾类土动应力-动应变关系变化规律。研究结果表明：当动三轴试验的围压为30 kPa、土样压实度为96%、含水率为最佳含水率7.5%时,其动应力-动应变关系曲线接近应力轴,砾类土的动强度大;荷载作用频率为1、2 Hz变化时以及小于10 kPa的初始静偏应力对砾类土的动应力-应变关系影响较小;针对各影响因素下的动应力-动应变关系曲线,对其采用的双曲线模型拟合相关系数均大于0.96,说明该模型能够有效拟合其关系曲线;根据试验模型可知,试验中压力越大,参数a、b值越小,同理,土样的压实度越大,a、b值也越小。     

橡胶纤维-砂混合料力学特性的离散元三轴试验研究

橡胶砂作为一种轻质填料在岩土工程中具有良好的应用前景,为揭示橡胶纤维-砂混合料强度特性、偏应力-轴向应变曲线及微观结构的变化规律,开展了橡胶纤维-砂不同配比的三轴剪切试验,基于颗粒流理论和PFC3D程序,对橡胶纤维-砂混合料的三轴剪切试验进行了数值模拟,并与室内试验结果进行了比较。结果表明:随着橡胶含量的增加,试样的应力-应变关系由应变软化型向应变硬化型过渡,试样剪切过程中体变形式逐渐由体胀型变为体缩型;同一围压下橡胶纤维含量高的试样变形模量小,表明橡胶材料的掺入有助于增强试样的压缩性能。     

循环荷载下加筋砾性土填料的动三轴试验分析

为探究循环动载作用下加筋砾性土填料的动力特性,在不同加筋层数和围压下对加筋砾性土进行了固结不排水动三轴试验,研究加筋层数和围压对加筋砾性土动力特性的影响,并进一步分析了加筋砾性土轴向累积应变发展机制。研究表明:加筋层数增加时,轴向累积应变减小,回弹模量增大,且加筋作用的影响幅度逐渐衰减;增大围压时,土体轴向累积应变减小,回弹模量和动孔压均随之增大;随着加筋层数和振次的增加,滞回曲线逐渐向应力轴靠近,滞回圈面积逐渐减小,土体耗能作用减弱。基于安定理论和间接影响带理论,揭示了加筋作用对轴向累积应变发展的影响机制。建立了能够反映加筋层数的加筋砾性土轴向累积应变预测模型,其条件参数a、b、g与加筋层数呈线性关系,可有效预测循环荷载下加筋砾性土路基沉降变形规律。     

砾钢渣抗液化特性试验研究

陈化的钢渣作为土工回填材料是废弃钢渣循环利用的主要途径之一。按土的工程分类方法,将废弃钢渣划分为砾钢渣、粗钢渣和细钢渣。针对砾钢渣,考虑固结应力比、振动频率、围压和含砾量等影响因素开展动三轴试验研究。分析了砾钢渣的应力、应变和动孔隙水压力的特性,分析了砾钢渣试样的动强度与振动次数、动应变与振动次数、孔隙水压力与振动次数和动应力与动应变关系。采用Seed和Finn提出的饱和砂土动孔压计算模型分析砾钢渣的动孔压曲线类型,并与传统砂砾土的抗液化强度进行比较。得出砾钢渣的抗液化特性较好,工程中可以用砾钢渣替代传统的砂土、砂砾土、砂砾料和砂卵石作为回填料,解决砂砾资源日渐短缺的问题。     

钙质砂特征形状分析及不排水剪切强度研究

钙质砂的形状是影响其宏观力学性质的重要因素。人工挑选出不同特征形状(块状、片状、条状)的钙质砂,通过显微图像采集与处理技术,定义并构建了一个形状参数(偏离度α),对钙质砂的特征形状进行三维定量描述,配制3种不同形状掺量的钙质砂试样进行三轴固结不排水剪切试验,研究钙质砂的特征形状对其剪切特性的影响。研究结果表明：(1)偏离度α能够较好地对钙质砂的形状进行描述及区分,可作为一种量化钙质砂形状的有效手段;(2)钙质砂的不排水剪切强度与偏离度α有较好的相关性：若增加片状钙质砂的含量(α增加),则试样的不排水剪切强度降低,若增加条状钙质砂的含量(α减小),不排水剪切强度增大。研究成果不但加深了对钙质砂特征形状的理解,还为准确评估钙质砂的抗剪强度提供了新思路。     

含石量对土石混合体剪切特性的影响

为了探究不同含石量对土石混合体的抗剪强度及剪胀性的影响,利用先进的大型单剪试验仪进行了21组大型单剪试验。试验设计了从0%～80%含石量共7组试验样品,在100、200、300 kPa三种不同的法向压力下进行单剪试验。基于试验结果,分析了含石量对土石混合体的抗剪强度和剪胀、剪缩特性之间的关系。试验结果表明,在相同的法向压力下,随着含石量的增加,土石混合体的内摩擦角及黏聚力总体上有先增大后减小的趋势。当土石混合体在含石量为40%～50%之间时,其抗剪强度最大。研究表明：土石混合体抗剪强度受到土石混合体孔隙比的影响,同时随着含石量的增加,土石混合体中的结构形式及主导颗粒也相应的发生变化。当含石量在0%～20%之间时,细集料在土石混合体中占主导地位,土石混合体为悬浮密实结构,此时土石混合体的抗剪强度与基质颗粒的性质相近;当含石量在20%～50%时,土石混合体为骨架孔隙结构,随着含石量的增加,土石混合体的骨架逐渐形成,颗粒之间咬合力增加,使得黏聚力及内摩擦角都有明显提高;当含石量超过50%之后,土石混合体表现为骨架密实结构,孔隙率开始上升并且细粒料开始大幅减少,细集料不能充分填充块石之间的孔隙,于是土石混合体抗剪强度开始下降。     

基于离散元法的砂石混合体直剪试验结果分析

砂石混合体由力学性质以及结构相差极大的材料组成,其组成的重塑地层易发生塌陷等问题,因此对砂石混合体力学特性的研究具有重要的工程意义。砾石形状是砂砾石力学特性研究的重要属性参数,但采用规则图形对砾石进行描述不能反映出其真实的力学性质,采用数字图像处理技术构建的砾石数据库能反映砾石真实形状并可对特定形状参数进行具体分析。由于砂石混合体的粒径分布较广,采用特征粒径等无法描述整体粒度分布,故本文结合分形理论构建砂石混合体的二重分形结构模型,通过粒度分维值反演出完整的级配分布曲线。考虑到砂石混合体离散型的特点,采用离散元软件进行直剪试验数值模拟并对细观结构进行分析,研究结果表明,砂石混合体一般具有2个粒度分维值:砂粒度分维值和砾石粒度分维值,砂、砾石粒度分维值越接近,抗剪强度和内摩擦角越大;当两者相等时,砂石混合体具有一重分维,此时均一性最好,抗剪强度和内摩擦角最大;轴向系数是形容砾石形状的一个重要参数,随着轴向系数的增加,砾石显示出明显的条状性,在直剪试验中抗转动能力增强、周围接触数量增加,导致抗剪强度和内摩擦角不断增加。     

不同含石量条件下土石混合体剪切变形特征的试验研究

三峡库区库岸广泛分布宽级配的土石混合体,土石混合体的力学性状及在外部作用下的变形响应对库岸滑坡的启动或复活有重要的影响。基于此,选择典型土石混合体滑坡进行现场调查和取样,利用室内大型直剪仪,结合打孔-插丝-灌砂的方法监测剪切过程中的试样内部变形演化特征,开展了不同含石量条件下的土石混合体试样的剪切变形特性试验研究。研究结果表明:含石量对土石混合体的强度和变形特性具有明显的控制作用。当含石量小于20%时,块石对试样影响较小,细颗粒土占主导作用,剪切过程中表现为细颗粒间错动式的剪切变形,强度依赖于细颗粒的强度,试样呈应变硬化特性;随含石量增加(20% ~80%),块石开始接触并逐渐形成骨架结构,加之细颗粒土的嵌合作用,剪胀作用增强,剪切过程中表现为混合体间啃掘式的剪切变形,强度由块石和细颗粒共同作用,试样逐渐表现为应变软化特性;含石量大于80%后,试样内块石占主导作用,但由于细颗粒较少,块石间的骨架结构存在较多空隙,试样结构效应有所降低,强度和变形性能有所弱化。     

Small-Strain Shear Modulus and Damping Ratio of Sand-Rubber and Gravel-Rubber Mixtures

This study examines the small-strain dynamic properties of mixtures composed of sandy and gravelly soils with granulated tire rubber in terms of shear modulus (G_O), and damping ratio in shear (D_min). Torsional resonant column tests are performed on dry, dense specimens of soil-rubber mixtures in a range of soil to rubber particles size 5:1–1:10 and rubber content from 0 to 35% by mixture weight. The experimental results indicate that the response of the mixtures is significantly affected by the content of rubber and the relative size of rubber to soil particles. Concering the small-strain shear modulus, an equivalent void ratio is introduced that considers the volume of rubber particles as part of the total volume of voids. Based on a comprehensive set of test results a series of equations were developed that can be used to evaluate the shear modulus and damping ratio at small shear strain levels if the confining pressure, the content of rubber by mixture weight, the grain size of soil and rubber particles, and the dynamic and physical properties of the intact soil are known.     

Behavior of sand–rubber particle mixtures: experimental observations and numerical simulations

A series of laboratory experiments and numerical simulations are conducted to explore the characteristics of mixtures composed of sand and rubber particles of the same median diameter. The mixtures are prepared with different volumetric sand fractions (sf = V_sand/V_total). The experiment focuses on assessing the strain level on the characteristics of the mixture with the volume fraction of each component. Numerical simulations using the discrete element method are performed to obtain insight into the microscale behavior and internal mechanism of the mixtures. The experimental results show that the behavior of the mixtures is dependent on the relative sand and rubber particles composition with variation in the strain levels. The numerical simulation reveals the effect of the soft rubber particle inclusion in the mixture on the micromechanical parameters. In low sand fraction mixtures, a high shear stress along the contact is mobilized, and the stress state is driven to a more anisotropic condition because of the relatively high particle friction angle of the rubber. The rubber particles play different roles with the strain level in the mixture, including increasing the coordination number and controlling plasticity of the mixture in a small strain, preventing buckling of the force chain in an intermediate strain, and leading to contractive behavior in a large strain. Copyright © 2014 John Wiley & Sons, Ltd.     

主应力方向变化路径下等压固结粉土强度特性差异和能量评价方法研究

主应力轴方向旋转变化是地基在波浪、车辆等荷载作用下经受的一种复杂而典型动力路径特征,为研究粉土在不同主应力轴方向变换条件下的强度特性差异,以初始密实度为70％的长江入海口饱和粉土空心试样为对象开展主应力轴旋转、拉压交变动三轴和双幅扭剪三轴试验。试验发现,等压固结条件下3种路径均使试样出现小应变崩塌液化破坏,且崩塌应力状态,可在p’-q空间中用近似平行的一组准不稳定相态线归一。而在液化和崩塌振次上,以同剪应力峰值的动扭剪下最高,动三轴次之,主应力轴旋转最低。在此基础上,提出采用崩塌损耗能对不同路径下土体动强度进行评价,结果表明,崩塌损耗能与剪应力幅值无显著关系,而在上述3种动力路径中,试样崩塌损耗能依次降低,这与3种路径下得到崩塌时刻递增的有效围压特征相匹配。崩塌能强度标准的采用,克服了复杂动力路径下由于剪应力恒定或循环变化造成应力水平无法统一而干扰强度评价的不足     

泸定大渡河桥冰碛土的结构及现场剪切试验研究

泸定大渡河桥康定岸分布巨厚层冰碛土,为研究土的抗剪强度特性,在不同位置和深度进行了6组现场剪切试验,基于地质勘察和试验结果分析土的结构特征、剪切强度和变形特性及其与土的结构的关系。研究表明:冰碛土的颗粒以粗粒、巨粒粒组为主,骨架颗粒呈悬浮状,混杂、无序堆积,骨架间充填杂基,形成骨架悬浮密实结构。根据颗粒组成和骨架风化程度,划分为骨架悬浮密实结构、软化骨架悬浮密实结构、砂砾土富集结构、大块石包绕结构等4种细观结构类型。冰碛土剪切破坏主要有剪切破碎带、包绕块石边界、锯齿状剪切和切穿软化骨架等4种模式,剪切荷载作用下线弹性变形阶段明显,剪切刚度大,初始屈服历时短。颗粒骨架和杂基形成的悬浮密实结构,是冰碛土强度与变形特性的内在控制因素,剪切荷载作用下骨架颗粒与具有一定胶结的基质间相互作用,剪切破坏时表现为基质的压碎与骨架的变位。骨架颗粒强度、大骨架颗粒分布、基质胶结程度的不同,对冰碛土强度和变形特性都构成一定的影响。土的抗剪强度、剪切刚度和剪胀性随骨架强度和基质胶结程度的提高而增大,而延性随之变差。     

Investigating new materials in the context of analog-physical models

To broaden the availability of granular materials that are suitable for the analog modeling of upper crustal deformation, we investigated the mechanical behaviors of pure quartz sand and two sand mixtures (quartz sand–powdered barite and quartz sand mica crystals) using ring-shear tests and simple convergent sandbox experiments. The ring-shear test results indicate that the three materials have similar peak friction angles (between 39.25° and 36.02°), but the magnitude of the shear strain and the shear strength required to cause their failure are different. The differences between the analog models are identified by distinct fault kinematics and different grain flows, which are primarily related to differences in the plastic elasto-frictional rheology. We conclude that the use of the quartz–mica mixture, which showed the strongest distributed (plastic) deformation, can improve analog models where different materials are required to simulate crystalline basement (sand) and supracrustal rocks (sand mica mixture). This is a common situation in extension and inversion tectonics, such as, for example, in inversion tectonics, when a granitic basement block acts as a buttress.     

The Effect of Relative Density and Confining Stress on Shear Properties of Sands with Varying Grading

The research investigated the influence of grain size distribution on the shear behavior of sand specimen in loose, medium and dense states. The investigation aimed at understanding the extent or degree at which static shear strength of soil is affected by its density. A standard indicator (coefficient of uniformity) was used in constituting four (4) different sand specimens—narrowly graded (NAG), intermediately graded (ING), well graded (WG) and gap graded (GAG). Different normal stress values were applied during the testing program. The specimens were sheared using a ring shear apparatus until a residual value of shear stress was obtained. Result obtained from the analysis shows two remarkable stress paths. Medium dense to dense specimens show dilative stress path while loose (less dense) specimens show contractive stress path. At loose state, WG specimens show higher peak and residual shear strengths than poorly graded ones. Of importance was the fact that all the NAG specimens underwent complete liquefaction but WG specimens did not undergo complete liquefaction. This was interpreted as strong inter-particle contacts/forces binding the grains which enhanced high shear resistance. The research also showed that increase in relative density leads to higher shear strength, particle size distribution notwithstanding. Also, result of the analysis showed that WG specimens in medium-and dense states have higher peak but lower residual strength than ING and NAG specimens-this phenomenon was termed reverse behavior by the author(s). GAG specimens when sheared have the lowest peak and residual strength at any given value of relative density.     

Effect of Strain Rate on the Strength Characteristics of Soil–Lime Mixture

The effects of rate of strain on strength and deformation characteristics of soil–lime were investigated. Five strain rates (0.1, 0.8, 2.0, 4.0 and 7.0 %/min), five lime contents (0, 3, 6, 9 and 12 %) by dry soil weight and three cell pressures (100, 200 and 340 kN/m²) were carried. Triaxial tests, under unconsolidated condition, were used to study the effect of strain rate on strength and initial modulus of elasticity of soil and soil–lime mixture after two curing periods 7 and 21 days, respectively. A total of 405 triaxial specimens have been tested, where 225 specimens have been tested with first curing period (7 days). The testing program includes nine specimens for each strain rate, and each lime content was carried out, including natural soil with zero lime content. Another set of triaxial tests with a total of 180 specimens for the second curing period (21 days) was prepared at optimum moisture content, and the corresponding maximum dry density was also tested. The effects of strain rate and curing period on each of stress–strain behavior, type of failure, deviator stress at failure, cohesion and angle of internal friction and initial modulus of elasticity were studied thoroughly for the natural soil as well as soil–lime mixtures. For natural soil, the test results showed that the undrained shear strength, the initial modulus of elasticity and the cohesion increase significantly as the strain rate increase, while for soil–lime mixture at different curing periods, the undrained shear strength, initial modulus of elasticity and the cohesion increases to a maximum and then decreases as the strain rate and lime content increase. Also, the same variables and angle of internal friction increase with increasing curing period.