Liquefaction and post-liquefaction properties of sand-silt mixtures and undisturbed silty sands

Severe damage to earth structures mainly consisting of fine sands containing non-plastic silt has often occurred due to the liquefaction of the fill materials. However, the changes in the liquefaction susceptibility and post-liquefaction undrained behaviour of fine sands associated with the increase in the amount of non-plastic silt have not been well understood under a constant degree of compaction which has been employed as the construction management index for these structures. To clarify this point, a series of undrained cyclic triaxial liquefaction tests, followed by monotonic loading, was conducted on seven sand-silt mixtures with fines contents ranging from 0 to 100% in the present study. Undrained triaxial compression tests without precedent cyclic loading were also performed on the same materials for comparison purposes. In these tests, cylindrical specimens with an initial degree of compaction of 95%, prepared by the wet tamping method, were employed to simulate the construction conditions of earth structures. The test results showed that: (1) the liquefaction resistance, and the static strengths of specimens with and without precedent cyclic loading history decreased with increasing fines content ranging from 0 to 50%; however, they increased afterward, and (2) sand-silt mixtures with fines contents of 30, 50, and 65% consistently exhibited extremely small post-liquefaction strengths without showing any significant changes in the excess pore water pressure generated in the precedent liquefaction tests, which may lead to the post-liquefaction flow failure of earth structures. From these results, the risk of employing a uniform degree of compaction irrespective of the fines content was presented. In the present study, similar triaxial liquefaction tests, with measurements of the shear wave velocities by means of bender elements, were also conducted on both undisturbed and reconstituted non-plastic medium-fine sands containing fines which were retrieved from road embankments damaged possibly due to liquefaction brought about by the 2011 off the Pacific coast of Tohoku earthquake. The soil ageing effects were briefly discussed from the test results.     

Effects of Clay Content on Liquefaction Characteristics of Gap-Graded Clayey Sands

A series of undrained, cyclic simple shear tests were performed on reconstituted specimens with various clay contents to study the effects of clay content on liquefaction characteristics of clayey sands based on a framework of an idealized binary packing model and intergrain state parameters. From observed liquefaction characteristics, clayey sands with different clay contents can be grouped as sand-like or clay-like soils depending on the clay content and the transitional fines content of the sand-clay mixture. A simple equation is derived and verified to correlate the transitional fines content with the void ratios of the clean sand and the pure clay consisting of the mixture. In addition, a new relationship for clay content correction is proposed based on the linear relationship between the cyclic resistance ratio and the clay content at the same intergranular void ratio. The cyclic resistance ratio of sand-like clayey sands can be divided into two components: (1) the resistance of the sand skeleton at the specific intergranular void ratio, and (2) the increment of cyclic resistance from clayey fines. The rate of increment for cyclic resistance varies with the properties of contained clay particles. Data from three independent studies have shown the proposed procedure is promising.     

黏粒和粉粒的共存对砂土静动力液化影响的试验研究

为了研究黏粒、粉粒与砂粒共存的土体中,黏粒和粉粒对砂土抗液化的影响规律,通过静力与动力三轴仪试验系统,对细粒(黏粒和粉粒)含量FC为5%和10%、三种不同细粒配比的试样进行静力与动力三轴试验。试验结果表明:相同细粒含量、不同细粒配比试样的抗液化强度不同;当细粒含量不同时,随着细粒中黏粒或粉粒含量的单调变化,试样所表现出的抗液化规律不同;FC=10%试样的抗液化强度整体高于FC=5%试样的相应强度。随着细粒含量及细粒中黏粒与粉粒相对含量的变化,黏粒与粉粒对砂粒的填充、黏结与骨架作用所占比例不同。     

The Effect of Fines on Critical State and Liquefaction Resistance Characteristics of Non-Plastic Silty Sands

Monotonic and cyclic triaxial tests were carried out on sand-silt mixtures for the investigation of the effect of fines content on their critical state and liquefaction resistance characteristics. Both the undrained and the drained monotonic tests produce a unique critical state line for each tested mixture, which moves downwards with increasing fines content up to a threshold value of 35% and then upwards. At a given void ratio and mean effective stress, the liquefaction resistance ratio decreases with increasing fines content up to the same threshold value of 35%, and increases thereafter with further increasing fines content. However, at a given intergranular void ratio, defined as the ratio of the volume of fines plus voids to that of sand particles, liquefaction resistance ratio increases with increasing fines content up to the threshold value. The threshold fines content value, which is an important parameter in determining the transition from the sand dominated to the silt dominated behaviour of sand-silt mixtures, is related to their particle packing. An expression is proposed for the estimation of the threshold fines content as a function of the mean diameter ratio, d₅₀/D₅₀, and the void ratio. The results, presented herein, also show that for each tested mixture the liquefaction resistance ratio is related to the state parameter and that this relation is influenced by the effective stress level and fines content. The results on the sand-silt mixtures are supported by similar results on natural silty sands.     

粉粒含量对砂土强度特性的影响

对含无塑性粉粒的砂土进行了三轴固结不排水试验,结果显示:粉粒含量通过颗粒组成和结构对粉砂强度和变形产生重要影响。在粉粒含量为6%,9%和12%时,松散试样在100 kPa围压下出现了静态液化现象,而粉粒含量增至15%时该现象消失,且随着围压的增大该现象也消失;在同一粉粒含量下,粉砂土残余内摩擦角高于峰值内摩擦角,但残余偏应力值很小,这是由于孔压增长很大,抵消了内摩擦角由?p′提高到?r′的有利影响;稳态线也受粉粒含量的影响,表现在随着粉粒含量的增加,稳态线逐渐向下移动;各粉细砂强度和变形对围压的敏感性强于纯净砂;粉粒含量对孔隙比的影响是含量超出一定值后才变得显著的,这说明粉粒加入到砂骨架中时,并未全部充填至砂粒孔隙中,而是以砂粒间接触点或面上为主,这样的接触结构导致了粉砂具有高体缩性,由此使得粉砂样表现出典型的应变软化特征。     

Characterizing the effects of fines on the liquefaction resistance of silty sands

Recent earthquakes in New Zealand and Japan indicate that evaluating the liquefaction potential of silty sands remains an area of difficulty and uncertainty in geotechnical engineering. This paper presents a comprehensive experimental study along with analysis and interpretation in the framework of critical state soil mechanics, with the aim to address the complicated effects of fines. Two series of sand-silt mixtures, formed by mixing two different base sands with the same type of non-plastic silt, were tested under a range of packing density, confining pressure and silt content, and a unified correlation was established between the cyclic resistance and the state parameter that collectively accounts for the effect of packing density and confining pressure. The proposed correlation is independent of packing density, confining pressure, fines content and base sand, and allows prediction of the cyclic resistance of silty sands under different states. Furthermore, the mechanism of the fines-content induced reduction of cyclic resistance and the mechanism of the base-sand effect observed from the tests are elaborated in the sound theoretical context. The present study suggests that the critical state soil mechanics is a rational and appealing framework for liquefaction analysis of both clean and silty sands.     

Modelling the Undrained Response of Fibre Reinforced Sands

Mixing a loose clean sand with random discrete flexible fibres has been found beneficial in decreasing the susceptibility to the phenomenon of liquefaction under monotonic loading. The addition of fibres can convert the strain softening response, typical of a loose unreinforced sand, into a strain hardening response by affecting the pore pressure generation and the effective stress path response. A new constitutive model based on the rule of mixtures has been used to simulate the undrained response of fibre reinforced sands. The model superimposes the individual contributions of the sand and the fibres according to their volumetric fraction. An apparent densification of the sand matrix induced by the presence of the fibres is accounted for in the model by assigning some of the void space to the fibres. This apparent densification is considered responsible for the observed strain hardening behaviour of reinforced sands. The proposed model is able to accommodate any distribution of fibre orientation: the orientation of fibres plays a key role in explaining the experimentally observed effective stress paths.     

Liquefaction resistance of fibre-reinforced silty sands under cyclic loading

Whether the so-called double porosity in soils with a loose and natural packing state is a concept with real-world implications is a fundamental yet controversial question in the study of cyclic undrained shear behaviour of fibre-reinforced silty sands. An attempt is made here to clarify the question by means of particle-level modelling combined with 41 undrained cyclic triaxial shear tests. The study shows that the initial Random Loose Packing changes to Random Close Packing and then Close Packing with silt content increments. The transition from random to close packing occurs at a threshold silt content which is relatively lower in coarser sands. For sands with <40% silt content, the rate of pore pressure growth with loading-unloading cycles increase with silt content increment. Reverse trend applies to silty sands with >40% silt content. Irrespective of fine content, fibres tend to sit deep into the silt pellets and encrust the macro-pore spaces. Generally, increasing fibre content leads to an increase in the average number of contacts per particle, dilation and easier dissipation of excess pore water pressure, a decrease in contact forces and improved liquefaction resistance. For sands with >40% silt content, effectiveness of fibre reinforcement diminishes with increasing sand median size.     

Aging Effects on Small Strain Shear Moduli and Liquefaction Properties of in-situ Frozen and Reconstituted Sandy Soils

In order to investigate the effects of different geological ages on liquefaction properties of sandy deposits, a series of undrained cyclic triaxial tests was performed on three kinds of in-situ frozen and their reconstituted samples which were retrieved from Holocene (Tone-river sand) and Pleistocene (Edo-river B and C sands) deposits. The specimens were subjected to isotropic consolidation at a specified confining stress which is equivalent to the in-situ overburden stress at the depth of sampling, and small strain shear moduli were measured before and during the undrained cyclic loading tests. The liquefaction properties and the small strain shear moduli were affected by not only the natural aging effect of the specimen but also the inter-locking effect that was enhanced by applying drained cyclic loading before the undrained cyclic loading tests. During liquefaction, different tendencies of degradation in the small strain shear moduli which would reflect the aging effects of the specimen were observed between Tone-river Holocene sand and Edo-river B and C Pleistocene sands. The applicability of reconstituted samples as substitutes for in-situ frozen samples was confirmed with Tone-river Holocene sand that has no cementation effect between soil particles, whereas it seems difficult to simulate fully the liquefaction behaviour of Edo-river B and C Pleistocene sands which have higher cementation effect.     

Stress-fractional soil model with reduced elastic region

The original stress-fractional plasticity approach was developed for the axisymmetric stress state. However, it cannot accurately capture the stress-strain behaviour, e.g., quasi-steady state flow, temporary stress peak and state-dependent dilatancy of soils under different loading conditions. In this study, a modified fractional-order plasticity model for soils under the general stress state is developed using an alternative yielding surface with a reduced elastic region at the dry side of the critical state line. The proposed model is then validated against the results of a series of drained and undrained tests conducted on different soils, including sand, rockfill, ballast and clay, subjected to different loading paths. It is observed that the proposed model can reasonably simulate the key features, such as the hardening/softening, contraction/dilation, liquefaction, quasi-steady state flow and steady state flow behaviour, of different soils.     

液化循环流动土体动剪切特性试验研究

循环流动特性是剪胀性砂土液化变形的典型特征,为研究液化循环流动土体的动力剪切特性,在骨架相对密实度分别为35%、50%和80%的砂土中添加不同含量的细颗粒,以改变液化流动土体的重度,通过循环扭剪试验研究不同骨架密度、具有不同细粒含量的液化流动土体在大变形阶段的剪切模量及阻尼比的变化规律。试验结果表明：液化循环流动土体在流动大变形阶段仍具有一定的模量,模量随着应变的增大而逐渐减小;流动变形阶段的模量大小与液化土体的重度基本无关;强度恢复阶段模量与细粒含量及骨架相对密实度密切相关;液化大变形阶段卸载模量趋于稳定,其稳定值约为初始卸载模量的35%;阻尼比随剪应变的增大而先增大,当土体达到初始液化以后,阻尼比随剪应变的发展呈减小的变化趋势;对于相同骨架密度的土体,相变角随着细粒含量的增加而减小,临界状态线的斜率随着细粒含量的而增加而增大。     

Characteristics of Soils with Low Plasticity: Intermediate Soil from Ishinomaki,Japan and Lean Clay From Drammen,Norway

Two soils with low plasticity are investigated; intermediate soil from Ishinomaki, Japan and lean clay from Dram- men, Norway. Since both the soils were retrieved using the Japanese sampling method, the test results from these samples are comparable. Though they have the same order of plasticity index (I_p), there is a significant difference in the grain size distribution characteristics between these soils. Ishinomaki intermediate soil contains a lot of sand or silt sized particles, its I_p value being nearly proportional to its clay content. On the other hand, Drammen clay consists of a large proportion of rock flour, which contains little clay mineral. The study shows that the unconfined compression test significantly underestimates the undrained shear strength for both soils, and their residual effective stress (p'_r) is also very low. It has been found that to compensate for loss of p'_r, recompression tests are useful methods to evaluate the strength of such soils.     

循环荷载下饱和砂土的孔压触变性

将液化土体视为流体进行液化效应分析是一个前沿的技术思路。其中,合理描述液化土体的流体性质是一个关键问题。提出循环荷载下饱和砂土孔压触变性的概念和基本设想。采用点差法计算相变后饱和砂土流动性曲线各点曲率,给出了依据流动性曲线加速增长段的最大曲率确定初始流体状态的经验方法。基于饱和砂土不排水循环三轴试验,发现进入流体状态后的饱和砂土应力-应变率关系满足Cross型触变性流体状态方程,其内部结构参数与土中残余有效应力比具有正比例关系;此时Cross型触变性流体速率方程描述的物理实质即为土体内的孔压增长过程。试验结果印证了论文提出的基本设想,证明了循环荷载下进入流体状态后的饱和砂土具有孔压触变性流体特征。     

Steady state strength of sand:concepts and experiment

残余强度或稳态强度的确定是砂土液化研究中的重要课题。饱和砂土有３种典型的不排水剪切特征：稳态性状、准稳态性状、和加工硬化性状。在三轴不排水剪切中,大部分松砂表现出准稳态性状。新近的研究表明,“准稳态性状”不是砂土的固有性状,而是三轴试验中的边界条件所导致。本文通过试验研究发现,饱和砂土在三轴不排水剪切中通常表现出４个明显不同的阶段：初始阶段、坍塌阶段、临界状态应力阶段、和后破坏阶段。稳态强度只有在坍塌阶段中才会较好地表现出来。文中还对变形稳态的定义作了修正和补充,并根据修正的变形稳态定义给出了Ｕｎｉｍｉｎ砂的稳态强度及其它的一些试验性质。     

砂土的稳态强度:概念与试验

残余强度或稳态强度的稳定是砂土液化研究的重要课题,饱和砂土有３种典型的不排水剪切特征,稳态性状,准稳态性状,和加工硬化性状,在三轴不排水剪切中,大部分松砂表现出准稳态性状,新近的研究表明：“准稳态性状”不是砂土的固有性状,而是三轴试验中的边界条件所导致,本文通过试验研究发现,饱和砂土在三轴不排水剪切中通常表现出４个明显不同的阶段;初始阶段,坍塌阶段,临界状态应力阶段,和后破坏阶段,稳态强度只有在坍     

含细粒砂性土标贯液化判别方法改进研究

含细粒砂性土相对于纯净砂在自然界中分布更为广泛,但是对于其液化判别,一直都是作为纯净砂液化判别的附属成果,没有得到足够的重视。回顾了中国规范方法与NCEER推荐方法,结合1999年台湾集集地震液化土与非液化土数据,发现两个方法定义纯净砂的矛盾,通过进一步对比判别结果,建议对两个方法做如下改进:1对中国规范方法,首先取消粉砂黏粒含量取3%的规定,并且对黏粒含量不大于3%且细粒含量大于15%的粉砂及粉土,取ρc=Fc/4,否则取3%;2对NCEER方法,当黏粒含量不大于3%时,只针对细粒含量大于20%的土,才考虑调整(N1)60。改进之后,中国规范方法过于保守的问题得到解决,NCEER方法对于黏粒含量不大于3%且细粒含量大于5%的土判别结果也不再偏于危险。     

Linking inherent anisotropy with liquefaction phenomena of granular materials by means of DEM analysis

The liquefaction phenomena of sands have been studied by many researchers to date. Laboratory element tests have revealed key factors that govern liquefaction phenomena, such as relative density, particle size distribution, and grain shape. However, challenges remain in quantifying inherent anisotropy and in evaluating its impact on liquefaction phenomena. This contribution explores the effect of inherent anisotropy on the mechanical response of granular materials using the discrete element method. Samples composed of spherical particles are prepared which have approximately the same void ratio and mean coordination number (CN), but varying degrees of inherent anisotropy in terms of contact normals. Their mechanical responses are compared under drained and undrained triaxial monotonic loading as well as under undrained cyclic loading. The simulation results reveal that cyclic instability followed by liquefaction can be observed for loose samples having a large degree of inherent anisotropy. Since a sample having initial anisotropy tends to deform more in its weaker direction, leading to lower liquefaction resistance, a sample having an isotropic fabric potentially exhibits the greatest liquefaction resistance. Moreover, the effective stress path during undrained cyclic loading is found to follow the instability and failure lines observed for static liquefaction under undrained monotonic loading. From a micromechanical perspective, the recovery of effective stress during liquefaction can be observed when a threshold CN develops along with the evolving induced anisotropy. Realising that the conventional index of the anisotropic degree (a) is not effective when the CN drops to almost zero during cyclic liquefaction, this contribution proposes an alternative index, effective anisotropy (a×CN), with which the evolution of induced anisotropy can be tracked effectively, and common upper and lower bounds can be defined for both undrained monotonic and cyclic loading tests.     

深圳海积软土基于CU试验的修正剑桥模型

通过室内试验测定了深圳地区某典型淤泥质粘土的修正剑桥模型参数,基于固结不排水剪切(CU)试验的有效应力路径建立了所述软土的应力应变本构方程,利用4/5阶龙格库塔法求解了微分方程的数值解,并通过计算值与试验值进行对比,验证了修正剑桥模型对于深圳海积软土的适用性。临界状态有效应力比M值通过CU试验测得,为得到CU试验的有效应力路径方程,将孔隙水压力随偏应力的变化关系采用二次多项式拟合,拟合效果较好。从土的本构关系理论出发,通过室内试验与结果分析,测定了深圳地区某典型软土的修正剑桥模型参数,并通过对照组的试验予以预测和验证得到了较理想的结果。     

循环荷载下黄土特性模拟

在不排水的条件下，黄土在单调荷载下会出现软化，在循环荷载下会出现液化（或循环移动）。对此给出了试验结果，并尝试用边界面及广义塑性模型来预测黄土的软化及液化现象。结果表明：循环荷载下塑性应变累积速度要比单调荷载下软化阶段塑性应变累积速度慢，广义塑性模型可较好地预测黄土软化、液化现象。讨论并分析了模型参数对预测结果的影响。     

Static liquefaction of fibre reinforced sand under monotonic loading

This paper explores the possibility of improving the monotonic undrained response of a loose clean sand which normally appears susceptible to the phenomenon of static liquefaction by mixing the sand with discrete flexible fibres. It is shown that the reinforcement inclusions reduce the potential for the occurrence of liquefaction in both compression and extension triaxial loadings and convert a strain softening response (typical for a loose unreinforced sand) into a strain hardening response. Fibre orientation distribution and the apparent sand matrix densification due to the presence of fibres in the voids appear important for the fibre reinforced soil behaviour. Normalisation of the effective stress paths with the mean effective stress at the end of consolidation shows a common path once the characteristic state is reached irrespective of the fibre concentration. The mobilised angles of friction coming from the slopes of the stress paths at large strains are very different for compression and extension and this is a consequence of the anisotropic nature of the distribution of fibre orientations. When full liquefaction of reinforced specimens is induced by strain reversal, the lateral spreading of soil seems to be prevented. Analytical developments including the key aspect of fibre orientation distribution, have shown that once the tensile contribution of fibres has been taken out of the composite stresses, the experimental data in the stress plane for all the tests at large shear strains nicely collapse onto a unique line corresponding very closely to the mobilised steady state or critical state angle of friction of the tested sand.