堆石料级配缩尺方法对其室内试验结果的影响

对同一条现场级配曲线通过缩尺方法缩制成不同的试验模拟级配,进行了密度、力学和渗流特性的对比试验。试验结果显示：全采用等量替代法缩尺后由于小于5 mm含量保持不变,粗、细颗粒充填关系不理想,对应于密度和力学特性最差,渗透系数最大,随着混合法中相似级配法的使用,小于5 mm含量逐渐增加,粗、细颗粒充填关系得到明显改善,缩尺后的密度和力学特性逐渐增加,却带来渗透系数的逐渐减小。目前国内相关规程、规范对级配缩尺方法并没有做具体、明确的规定,有必要通过大量室内和现场对比试验,总结出室内科学的缩尺方法并建立反映缩尺效应的经验公式。     

间断级配砂砾石土的渗透变形试验研究

渗透变形是颗粒材料中细颗粒在渗流作用下发生重分布且导致土体的内部结构、水力及力学特性发生变化的现象,是导致砂砾石土地基及堤防结构破坏的主要原因之一。利用自主研发的刚性壁渗透仪对不同级配及细颗粒含量的间断级配砂砾石土在恒定水头渗流作用下进行渗透变形全过程试验,监测了渗流过程中的局部水力梯度空间分布以及竖向位移变化,分析了渗透试验结束后土体的颗粒级配空间分布变化。研究结果表明:土粒中细颗粒所处的欠填、满填及过填3种堆积状态决定了粗、细颗粒间不同的接触方式,影响其渗透性。渗透试验结束后细颗粒流失量沿试样高度的空间分布可以划分为3个区域,即顶部流失区、中部均匀区及底部流失区。局部水力梯度的快速下降伴随着竖向位移的突变,意味着渗透变形的开始;渗透变形启动时的局部水力梯度大于全局水力梯度,证实了采用大尺寸试验执行渗透试验的必要性。细颗粒处于过填状态的试样依然会发生渗透变形且导致强烈的沉降变形,值得进一步的研究。     

砂性土细颗粒起动临界水力坡降计算方法

地下水渗流作用下内部不稳定砂性土将发生潜蚀现象,潜蚀作用引起的土体渗透破坏会对土工建筑物或地基造成不良影响。考虑土体有效应力和细颗粒应力折减,建立渗流场中细颗粒受力模型,根据极限受力平衡状态得到潜蚀过程中砂性土细颗粒起动临界水力坡降计算公式,并通过DEM-CFD耦合方法以及现有试验数据进行验证。结果表明:砂性土中细颗粒以滚动方式起动,起动临界水力坡降受渗流水流、土体特性以及颗粒自身特性共同影响;砂性土表层细颗粒起动临界水力坡降受埋深影响较大,埋深1 cm的细颗粒最高、最低起动临界水力坡降相差10.169%,埋深10 cm时差异减少至1.061%。该计算方法与数值模拟和渗流试验结果的最大标准误差分别为6.038%、11.211%,可以较为准确地预测砂性土细颗粒起动临界水力坡降。     

细粒含量对冰碛土抗剪强度影响的实验研究

为研究细粒组颗粒在浸水/降雨条件下产生迁移对冰碛土抗剪强度的影响,从西藏林芝市帕隆藏布嘎隆寺沟流域内采取冰碛土样品,进行7种不同细粒（粒径小于2 mm）含量情况下的冰碛土比重和相对密度的测试,开展了不同围压下的大型饱和固结不排水三轴（CU）实验。结果表明：（1）细粒含量对干密度的影响较小,不同细粒含量的冰碛土孔隙比大致相同,细颗粒对孔隙具有改造作用,不同细粒含量会引起孔隙结构的差异,从而导致冰碛土的结构差异,最终导致抗剪强度的不同;（2）细颗粒迁移导致冰碛堤坡脚和内部一定深度细粒含量较高,达到一定的量值时,抗剪强度明显降低,冰碛堤容易发生剪切破坏;（3）细粒含量对抗剪强度参数的变化具有重要影响,从中还可以反映出冰碛土结构控制的变化：粗颗粒控制→粗细颗粒共同控制→细颗粒控制。研究结果对于评价冰碛堤的稳定性具有重要意义。     

Three-dimensional DEM modeling of the stress–strain behavior for the gap-graded soils subjected to internal erosion

In this work, 3D discrete element method modeling of drained shearing tests with gap-graded soils after internal erosion is carried out based on published experimental results. The erosion in the model is achieved by randomly deleting fine particles, mimicking the salt dissolving process in the experiments. The present model successfully simulates the stress–strain behavior of the physical test by employing the roll resistance and lateral membrane. The case without erosion shows a strain-softening and dilative response, while strain-hardening and contractive response starts to occur as the degree of erosion increases. The dilative to contractive transition is mainly caused by the increase in void ratio due to the loss of fine particles. The change from dilative behavior to contractive behavior is more abrupt for the specimen with larger fine particle percentage because the soil skeleton is mainly controlled by the fine particles instead of by the coarse soil particles. The transition from “fines in sand” to “sand in fines” might be associated with the rapid increasing in the contacts associated with fine particles in the specimen as the percentage of fine content increases. The erosion scenario based on the hydraulic gradient is also modeled by deleting the fine particles based on the ranking of the contact force. Compared with the scenario based on random deletion, the remaining fine particles for the erosion scenario based on the ranking of contact force are more dispersedly distributed, which might benefit the small strain stiffness but result in a smaller strength. This work provides some insights for better understanding the mechanism behind the internal erosion and the associated stress–strain behavior of soil. The gradient of the critical state line increases with more loss of fine particles denoting that the fine particles are helpful for holding the structure of the soils from larger deformation.

     

The critical state friction angle of granular materials: does it depend on grading?

Whether the critical state friction angle of granular materials depends on grading is a fundamental question of both academic and practical interest. The present study attempts to address this question through a specifically designed experimental program where the influence of particle grading was carefully isolated from other influencing factors. The laboratory experiments show that under otherwise similar conditions, the angle of friction at critical state is a constant independent of grading, but, for a given grading, the angle of friction at critical state is highly dependent on particle shape. This finding suggests that the commonly adopted practice of separately allowing for the effect of particle shape and the effect of grading on critical state friction angle is conceptually inappropriate and, hence, should be taken with caution in geotechnical design to avoid the risk of underestimating safety requirements. The study also reveals that varying particle gradation can impose a marked impact on liquefaction susceptibility of granular soils: Under the same post-consolidation state in terms of void ratio and confining pressure, a well-graded soil tends to be more susceptible to liquefaction than a uniformly graded soil. This variation of liquefaction susceptibility is shown to be consistent with the variation of location of the critical state locus in the compression space and is explainable by the critical state theory.     

堰塞坝体材料渗透特性及其稳定性研究

堰塞坝是由于崩塌、滑坡、泥石流等形成的天然坝体,不同于人工土石坝,堰塞坝坝体结构松散,颗粒级配不均匀,在较高水头作用下坝体可能发生渗透破坏而导致溃坝,严重威胁下游人民群众的生命及财产安全。由于堰塞坝存在较大粒径颗粒,常规的渗透试验装置难以满足要求,本文研制了直径为60cm的大直径渗透试验仪,进行了不同堰塞坝级配材料的渗透破坏试验,并探讨了堰塞坝体材料渗透特性的主要影响因素。研究发现:（1）堰塞坝材料的渗透破坏形式取决于材料级配,粗颗粒含量较多时为管涌破坏,细颗粒含量较多或粒径缺失时为流土破坏;（2）堰塞坝渗透系数随干密度的增大而减小,主要取决于细料填充粗料孔隙的程度,单独使用不均匀系数或曲率系数不适用于评价渗透系数的变化;（3）基于试验数据提出了用于堰塞坝渗流破坏形式的判别公式,并推导出堰塞坝管涌破坏的临界水力坡降计算公式。     

Grain size variations in ignimbrites and implications for the transport of pyroclastic flows

Ignimbrite flow units commonly show reverse grading of large pumice clasts and normal grading of large lithic clasts. Ignimbrites show coarse-tail grading, in which particles beneath a critical diameter, ranging from 64 to 2 mm, are ungraded. Above this size the larger the clast diameter the more pronounced the segregation. The grading is consistent with the theoretical settling rates of particles in a dispersion with a high particle concentration. Ignimbrite flow units show a reversely graded, fine grained basal layer which is attributed to the action of boundary forces during flow. Ignimbrites are commonly associated with cross-stratified pyroclastic surge deposits and fine ash fall deposits formed in the same eruption. The fine ash fall deposit is depleted in crystals and is thought to be the deposit of the fine turbulent cloud observed making up the upper parts of nuées ardentes. Pyroclastic flows are postulated to be dense, poorly expanded partly fluidized debris flows. Only its fine grained components can be fluidized by gas. Pyroclastic flows are believed to behave as a dispersion of larger clasts in a medium of fluidized fines, which acts as a lubricant similar to water in mud-flows. Poor sorting in ignimbrites is attributed to high particle concentrations not turbulence. Many pyroclastic flows may be laminar in their movement with apparent viscosities, deduced from the lateral grading of large lithic clasts, in the range 10¹?10³ poise.     

石灰岩发育土壤团聚体形成机制研究

土壤团聚体是土壤最基本的结构单元。为了解岩溶地区石灰岩发育土壤团聚体的形成机制,选择贵阳市花溪区花溪水库石灰岩所发育的土壤为材料,采用干、湿筛法,分析团聚体颗粒的分级情况,并比较拆分有机质与钙镁铁铝离子间连接后四种离子含量的变化以及团聚体各粒级含量的变化。结果显示:团聚体是由细小颗粒在胶结物质有机质与钙镁铁铝离子结合下形成微团聚体,进一步形成更大粒级的团聚体,最后在分子力的作用下粘结土粒或者其他团聚体形成完整的团聚体结构;有机质与钙离子的结合是团聚体形成的主要胶结物质,其次为有机质与镁铁离子的结合,有机制与铝离子的结合作用不大,这与石灰岩矿物主要是碳酸钙有关;研究区发育土壤大粒级水稳性团聚体含量相对较高,土壤抗蚀性好,抵抗侵蚀能力强。      

A computational procedure to assess the distribution of constriction sizes for an assembly of spheres

The effectiveness of filters to counteract internal erosion in earth structures is particularly related to their ability to capture fine particles moving under seepage flow through the porous material. More precisely, fine particles are likely to be trapped by the narrowest paths between pores: the constrictions. This paper proposes a methodology to compute the constriction size distribution of model granular filters taking into account the relative density of the material. The approach is based upon probabilistic methods which adopt stated simple geometric packing arrangements to represent the solid structure in the extreme density states. Two new models are proposed for the design of the constriction size distribution according to the type of filter grading: continuously graded or gap-graded materials. The models require the usual material characteristics: the grading curve, and the minimum and maximum void ratios for this material. Calibrated on the basis of statistical analyses over numerical assemblies of spheres generated by a discrete element method, the proposed new models constitute a promising tool to significantly improve the modeling of filtration processes in granular materials.     

Quickclays as products of glacial action: a new approach to their nature,geology, distribution and geotechnical properties

Essentially, quickclays are products of glaciation which accounts for their limited distribution; other modes of development being of lesser importance. Glacial grinding provides the fine silt and clay sizes required to constitute quickclays. Two major factors account for the geotechnical properties, a composition factor related to material comprising the soil system and a leaching factor, the effectiveness of which is related to the clay-mineral content of the soil.Soils can be divided into three types, based on criteria of particle size and type of interparticle bond. Among the latter are long-range active bonds, typical of true clay-mineral systems; and short-range inactive bonds such as are observed between two quartz particles. The three main soil types have: (a) small particles and long-range forces - the bond/weight ratioR is high; (b) small particles and short-range forces ? R > 1 ; and (c) large particles and short-range forces - i.e., the sands, andR < 1. Quickclays fall more conveniently into type (b) than into type (a) in as much as the most important property is a preponderance of inactive bonds in the soil system. This is achieved by a high content of non-clay mineral particles, and also may be assisted by leaching and cementation. If more than a critical proportion of long-range bonds are present, the required very high sensitivity does not occur.The initial failure of the soil might be treated as tensile using a volume failure criterion; the low settling velocity of the very small (≈ 1?2 μm) particles allows the solid-liquid transformation to occur after a modest shock loading. Observations of low thixotropy, lack of secondary settlement and sudden failure in compression tests can be explained by requirements of particle size and material.     

Numerical study of partially drained penetration and pore pressure dissipation in piezocone test

The piezocone penetration test (CPTU) is commonly used as a fast and economical tool to identify soil profile and to estimate relevant material properties in soils ranging from fine to coarse-grained. Moreover, in the case of fine-grained soils (clays and silts), the consolidation coefficient and the permeability can be estimated through the dissipation test. Undrained conditions are commonly assumed for the interpretation of CPTU in fine-grained soils, but in soils such as silts, penetration may occur in partially drained conditions. This aspect is often neglected in data interpretation thus leading to an inaccurate estimate of soil properties. This paper investigates numerically the effect of partial drainage during penetration on the measured tip resistance and the subsequent pore pressure dissipation response contributing to a more accurate interpretation of field data. A realistic simulation of the cone penetration is achieved with the two-phase Material Point Method, modelling the soil response with the modified Cam-Clay model. The approach takes into account large soil deformations induced by the advancing cone, soil–water, and soil–structure interactions, as well as nonlinear soil behavior.     

Analysis of mechanical behaviour and internal stability of granular materials using discrete element method

Granular materials like sand are widely used in civil engineering. They are composed of different sizes of grains, which generate a complex behaviour, difficult to assess experimentally. Internal instability of a granular material is its inability to prevent the loss of its fine particles under flow effect. It is geometrically possible if the fine particles can migrate through the pores of the coarse soil matrix and results in a change in its mechanical properties. This paper uses the three‐dimensional Particle Flow Code (PFC3D/DEM) to study the stability/instability of granular materials and their mechanical behaviour after suffusion. Stability properties of widely graded materials are analysed by simulating the transport of smaller particles through the constrictions formed by the coarse particles under the effect of a downward flow with uniform pressure gradient. A sample made by an initially stable material according to the Kenney & Lau geometrical criterion was divided into five equal layers. The classification of these layers by this criterion before and after the test shows that even stable granular materials can lose fine particles and present local instability. The failure criterion of eroded samples, in which erosion is simulated by progressive removal of fine particles, evolves in an unexpected way. Internal friction angle increases with the initial porosity, the rate of lost fine particles and the average diameter D₅₀. Copyright © 2016 John Wiley & Sons, Ltd.     

砂砾土渗流侵蚀中细粒迁移-沉积-滤通的物理水力临界条件

细粒迁移机制是理解砂砾土渗流侵蚀过程的基础与关键,对研究砂砾土斜坡雨水侵蚀过程的细观致灾机制具有重要意义。目前其运移模式及运移状态发生转变的临界条件并不清晰,不同物理水力条件下的细粒运动类型有所不同。为掌握砂砾土侵蚀过程中细粒的整体运动类型及其发生改变的临界条件,采用可视圆柱入渗试验和离散元数值模拟,分析了细粒迁移的影响因素和内部机理。结果表明：（1）细粒迁移受级配和水力梯度影响显著,而受初始孔隙率影响不显著,且级配的影响大于水力梯度;（2）水力作用下细粒整体运动状态可分为沉积和滤通2种模式,内部结构不稳定的砂砾土细粒运动处于滤通状态,内部结构稳定和稳定性过渡型砂砾土随水力梯度升高细粒的运动状态从整体沉积转变为整体滤通;（3）细粒运动状态在粒径比和水力梯度共同作用下存在明显界限,最终得到细粒沉积-滤通转变的临界条件为$ i = 3.4 - $$ 0.12{{\text{e}}^{\left( {{D_{15}}/{d_{85}}} \right)/1.5}} $。研究可为砂砾土斜坡渗蚀失稳防护提供理论指导。     

堆石料缩尺方法的分形特性及缩尺效应研究

采用6种不同缩尺方法将同一条现场级配曲线缩制成不同的数值模拟级配曲线,建立以不同粒径范围内颗粒数为测量数的分形模型,研究了颗粒级配分布的分形特性;基于颗粒流方法,生成与级配曲线相对应的6组数值试样进行双轴压缩试验,研究了缩尺方法对数值试样宏观力学特性及细观力学响应的影响;并讨论了颗粒级配分布的分形特性与数值试样力学特性之间的关系。结果表明：采用不同方法缩尺后数值试样的颗粒级配分布具有分形特性,分维数D为1.463～1.783;随着缩尺方法相似比尺M的增大,数值试样中细颗粒数量增多,粗细颗粒的充填关系得到改善,力学特性逐步提高;颗粒级配分布分维数D与数值试样力学特性指标之间存在较好的线性关系。     

流变物质对软土流变参数影响的蠕变试验

流变性是软土的一种长期变形特性,对软土地基的沉降产生重要影响。现有研究表明,软土中高黏性的流变物质是产生其流变特性的主要物质因素,当流变物质分布具有一定均匀性时其流变特性与流变物质的分布形态无关,仅与流变物质本身性质和含量比例相关。通过蠕变试验研究了流变物质对软土流变参数的影响,视强亲水性的膨润土微小颗粒为具有高黏性的流变物质,均匀地把其掺入到人工土试样中进行一维压缩蠕变试验,引入基于邓肯非线性弹性的流变元件模型对蠕变试验曲线进行拟合分析,根据流变物质含量确定流变模型参数的变化规律,建立了模型等效黏滞系数、等效弹性模量和邓肯非线性弹性模量与膨润土（流变物质）含量的关系。模型拟合分析发现,基于邓肯非线性弹性的软土流变元件模型与试验结果具有良好一致性,模型参数与流变物质含量之间关系为双曲线方程。     

恒定渗流作用下泥石流起动过程冲刷试验分析

渗流是泥石流水动力条件主要来源之一,不同渗流流量具有不同的渗流力和冲刷力,从而可引起不同规模泥石流。通过开展室内水槽试验,利用测压管量测渗流过程中的孔隙水压力,并结合高清摄像技术从微观角度记录堆积土体内部细颗粒的运移、骨架颗粒的坍塌现象,以此分析研究土体渗透破坏、起动形成泥石流过程中的渗流和冲刷作用。在此基础上设定水槽坡度为7°,调节恒定渗流流量分别为120、170、265、320 ml/s,分析不同恒定渗流流量对固体堆积物失稳、泥石流起动过程中流态变化的影响。分析结果表明,在恒定渗流流量作用下,堆积土体内部细颗粒迁移、骨架颗粒坍塌造成土体颗粒重排列、孔隙水压力上升进而导致土体抗力降低是泥石流土体颗粒失稳、起动、冲刷的重要原因;随着渗流流量增加,流速迅速上升,土体内孔隙水压力逐步增大,骨架颗粒的失稳、移动主要受渗流及水流冲刷两方面共同作用,堆积土体颗粒的移动分别表现出缓慢小幅滑动后稳定、过渡型滑动和快速流滑现象。     

Effects of Soil Aging on Mechanical and Hydraulic Properties of a Silty Soil

The aging phenomenon, which produces changes in material state over time, is associated with significant modification of mechanical and physical soil properties. This change should be accounted for during geotechnical design. Although soils sometimes improve with aging, the opposite effect is occasionally observed. This paper describes a study performed to investigate the effect of aging on the mechanical behavior and the permeability of a silty soil. Undrained unconsolidated triaxial shear tests and triaxial permeability tests were performed on disturbed and compacted samples. Upon conclusion of these tests, the samples were sealed from air and moisture. The results show an important increase in both the undrained shear strength and the deformation modulus caused by silt rigidification during the aging process. These changes cause an over estimation of laboratory measured shear strength. For instance, the increase in the deformation modulus and undrained cohesion can approach 100 % for an approximate 328 day storage period. Sample permeability was found to decrease with aging. This reduction can be ascribed to several causes including micro-organisms growth, secondary sample consolidation and progressive filling caused by the migration of very fine particles. These phenomena might have negated the expected increase in permeability with aging time reported in the literature.     

降雨作用下砾石土斜坡坡面形态及临界特性

砾石土因其级配宽、不均匀系数大、透水性强等特点在工程防渗中得到广泛的应用,其强度和力学性质受粗细粒含量和粒间咬合程度、黏接状态等影响。通过人工降雨原型试验、模型试验、室内试验及理论分析,研究松散砾石土斜坡在降雨作用下坡面土体的形态特征和土体性状的变化以及斜坡稳定性和该过程中可能出现的临界状态,探索坡面松散砾石土触变液化的过程和机理。试验结果表明,砾石土斜坡在降雨过程中,坡面土体形态、坡面径流泥沙含量具有阶段性特性;各典型现象土体含水量分布具有区段性;土体强度和斜坡稳定性随着土体含水量的增加均存在明显的临界特征。