Behavior of Liquefied Sands Under Extremely Large Strain Levels in Cyclic Torsional Shear Tests

In order to study the cyclic behavior of liquefied sands at extremely large strain levels up to double amplitude shear strain of about 100%, a series of undrained cyclic torsional shear tests while keeping the specimen height constant was performed on saturated Toyoura sand under different densities, two kinds of in-situ frozen sandy samples and their reconstituted specimens. Due correction was made for the effect of membrane force on the measured shear stress. After exceeding a certain level of overall shear strain, the specimen deformation became non-uniform, which is called as “strain localization” in the present study. The initiation of such localization was associated with the changes in the cyclic amplitude of deviator stress and the increment of shear strain. In the case of Toyoura sand, the limiting value of shear strain to initiate strain localization was found to increase with decrease in the relative density, and such a trend was consistent with the empirical correlation of soil liquefaction when the relative density is higher than 30%. In the case of in-situ frozen sandy samples, their limiting shear strain values were smaller than those of the reconstituted specimens, suggesting that their soil structures were different from each other under different degrees of natural aging effects.     

Effects of Sample Disturbance on Small Strain Characteristics and Liquefaction Properties of Holocene and Pleistocene Sandy Soils

To investigate liquefaction properties of sandy soils, undrained cyclic loading tests are usually performed. However, it would be difficult to simulate fully the actual soil behaviour through the laboratory tests because the tested sample can be disturbed even though it is taken by in-situ freezing technique. In this study, by using three kinds of in-situ frozen sandy soils which were taken from Holocene and Pleistocene deposits and their reconstituted samples, their volume change properties were measured during freeze and thaw processes at different confining pressures of 30 kPa and 98 kPa. In order to investigate the effects of the possible sample disturbance on the liquefaction resistance, small strain characteristics were measured as well, which would reflect the soil structure. Decreases in the small strain characteristics and the liquefaction resistance were observed in case of the Holocene specimens that were thawed at the confining pressure that was lower than the in-situ overburden stress. On the other hand, in case of the Pleistocene specimens, the effects of the confining pressure during the thaw process on the small strain characteristics and the liquefaction resistance were small. Such contrastive feature between the Holocene and the Pleistocene samples could be linked with the difference in the types of their natural aging effects.     

Aging effects on liquefaction resistance of sand estimated from laboratory investigation

Liquefaction resistance is known to increase concomitantly with the increase in time after construction or sedimentation. Nevertheless, the mechanisms of its aging effect on liquefaction have not been completely elucidated. To clarify the mechanisms of aging in sandy soils, the liquefaction resistance (CRR), initial and secant shear moduli (G₀ and G_sec), and laboratory penetration resistance of long-term consolidated sand specimens were examined using cyclic undrained triaxial tests, local small strain (LSS) tests equipped with bender elements (BEs), and penetration index tests, respectively. Based on the existing reports, the CRR was inferred from G₀, G_sec, and the laboratory penetration resistance. In the case of Toyoura sand of D_r = 40%, the CRR increased by about 14% with a 360-day consolidated specimen in the cyclic undrained triaxial tests. However, increases in the CRR evaluated from G₀ and the laboratory penetration resistance were nothing and only 2%, respectively. G_sec started to degrade at greater shear strain in the long-term consolidated specimens. An increase in the CRR, evaluated from the G_sec of 0.01% shear strain, had a much better agreement with that obtained from the cyclic undrained triaxial tests.     

Effects of initial static shear on liquefaction and large deformation properties of loose saturated Toyoura sand in undrained cyclic torsional shear tests

This study focused on the role which static shear plays on the large deformation behavior of loose saturated sand during undrained cyclic loading. A series of undrained cyclic torsional shear tests was performed on saturated Toyoura sand specimens up to single amplitude shear strain exceeding 50%. Three types of cyclic loading patterns, i.e., stress reversal, intermediate and non-reversal, were employed by varying the initial static shear level and the cyclic shear stress amplitude. The observed types of failure could be distinguished into liquefaction (cyclic and rapid flow) and residual deformation by comparing both monotonic and cyclic undrained behavior. It was found that the presence of initial static shear does not always lead to an increase in the resistance to liquefaction or strain accumulation; they could either increase or decrease with an increasing initial static shear level depending on the type of loading pattern and failure behavior. In addition, according to the failure behavior which the specimens exhibited, three modes of development of large residual deformation were observed.     

Liquefaction and post-liquefaction resistance of sand reinforced with recycled geofibre

The present study provides an insight into the effect of recycled carpet fibre on the mechanical response of clean sand as backfill material subjected to monotonic loading and cyclic loading as well as post-liquefaction resistance of both unreinforced and carpet fibre reinforced soils. To achieve these goals, a series of multi-stage soil element tests under cyclic loading event resulting in liquefaction followed by undrained monotonic shearing without excess pore water pressure dissipation as well as a series of monotonic undrained shear test is conducted. All the specimens are isotropically consolidated under a constant effective confining stress of 100 kPa by considering the effect of cyclic stress ratio and carpet fibre content ranging from 0.25% to 0.75%. The obtained results revealed the efficiency of carpet fibre inclusion in increasing the secant shear modulus and ductility of clean sand under monotonic shearing without previous loading history. The impact of carpet fibre inclusion on the trend of cyclic excess pore water pressure generation and cyclic stiffness degradation was minimal. However, adding carpet fibre significantly improved both liquefaction and post-liquefaction resistances of clean sand. The liquefaction resistance of clean sand, at a constant 15 loading cycles, improved by 26.3% when the soil was reinforced with 0.75% recycled carpet fibre. In addition, the initial shear modulus of the liquefied specimen significantly increased by adding recycled carpet fibre.     

前期动载对低塑性粉土静态和动态强度的影响

 低塑性粉土广泛存在于世界范围内,在地震中容易产生液化现象,然而一些基础设施破坏不仅见于地震中也发生在地震后,这就决定了研究低塑性粉土震后行为的必要性。以美国中部密西西比河沿岸低塑性粉土为试验材料,研究动载对低塑性粉土静态和动态强度的影响。在动三轴仪上对试样施加动载引起超孔隙水压力,排水重固结后,分别对2组震动后试样进行静态和动态三轴强度试验。试验结果表明,当液化水平小于0.70时,前期动载对粉土的不排水剪切强度影响不大;相反地,只有当液化水平大于0.70,密西西比河沿岸粉土的震后重固结体积应变和不排水剪切强度才伴随着液化水平的提高显著增加,但相对于砂土而言,重固结体积应变在较低的液化水平时即有明显增加。与前期动载对不排水剪切强度的影响不同,当动载所引起的液化水平为0.35或轴向应变为0.2%时,抗液化强度达到最大值,若液化水平大于0.35,抗液化强度伴随液化水平提高而降低。如果前期荷载引起较大的压应变,在重固结后第二次动载循环中,轴向压力相比轴向拉力引起较小的超孔隙水压力。     

不排水循环荷载条件下胶结砂土宏微观力学性质离散元模拟研究

天然或人工胶结的存在能够提高砂土的抗液化能力,从宏微观尺度对其动力学性质进行研究具有重大意义.将已有的三维完整胶结接触模型引入到三维离散元程序中,对胶结砂土不排水循环三轴剪切试验进行三维离散元模拟,研究颗粒间胶结、循环应力比对离散元试样宏微观力学性质的影响.研究结果表明,胶结的存在能够抑制轴应变和孔压的发展,提高砂土的...     

主应力方向循环变化对饱和松砂不排水动力特性的影响

利用新研制的土工静力-动力液压三轴-扭转多功能剪切仪进行了主应力轴循环变化的多种模式竖向和扭转双向耦合循环剪切及普通循环扭剪试验。针对福建标准砂, 在均等固结条件下着重研究了振动过程中主应力方向的不同变化模式对饱和松砂不排水循环特性的影响。试验研究结果表明: 振动过程中主应力方向的变化方式对饱和砂土不排水动强度具有显著的影响, 在所采用的五种类型循环剪切应力路径中, 主应力方向连续旋转条件下的动强度最低。进一步的研究发现: 对于初始均等固结条件,分别采用初始平均有效固结压力和循环破坏次数归一化的循环孔隙水压力比与循环次数比之间的关系与振动过程中主应力方向变化方式无关, 这种归一化循环孔隙水压力比随广义剪应变的变化规律及累积广义剪应变与循环次数比之间关系均与振动过程中主应力方向变化方式无关。     

Undrained shear strength of desaturated loose sand under monotonic shearing

Previous studies have shown that desaturation can improve the liquefaction resistance of originally saturated sand. To apply this method in practice, we need to establish the relationship between undrained liquefaction resistance and the degree of saturation for sand. In this paper, previous studies on the undrained strength and liquefaction behavior of partly saturated sand are reviewed first. Then, experimental results of triaxial undrained compression and extension tests on loose sands are presented. The data are used to examine the relationships between undrained liquefaction resistance and degree of saturation of samples under different loading modes.     

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.     

Comparison of Liquefaction-Induced Ground Deformation Between Results from Undrained Cyclic Torsional Shear Tests and Observations From Previous Model Tests and Case Studies

In order to investigate liquefaction-induced ground displacement, we conducted a series of undrained cyclic torsional shear tests on saturated Toyoura sand using a modified torsional apparatus capable of applying and measuring double amplitude shear strain up to about 100%. The limiting value of double amplitude shear strain, at which strain localization appears during undrained cyclic loading tests, was evaluated from the test results with reference to the change in the deviator stress during liquefaction. The limiting strain values, which increase with a decrease in the relative density of the specimen, were found to be consistent with the maximum amounts of liquefaction-induced ground displacement observed in the previous shaking table model tests and most of the relevant case studies. This feature is reasonable considering the reduction in the mobilized cyclic shear stress in liquefied soil due to the degradation of the shear resistance. As long as the liquefied soil layer remains in uniform deformation, these limiting strain values may be used in estimating the maximum amount of liquefaction-induced ground displacement.     

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     

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.     

Geotechnical characterization of collapsible salty sands subjected to monotonic and cyclic loadings – A case study for areas with high seismicity

Collapsible soils are typically found in arid regions and often have an aeolian or alluvial origin. In their natural states, they may have a low moisture content and cemented structure that can contribute to high strength and stiffness; however, wetting or saturation can reduce the strength and stiffness due to loss of the cementation. This paper presents a geotechnical characterization of collapsible salty sands in the highly seismic southern coast of Peru, which makes the characterization of their dynamic properties and expected response to earthquake-induced demands (e.g., liquefaction) of primary importance. The geotechnical characterization was performed on intact and remolded samples utilizing various field and laboratory tests, including oedometer, direct shear, static triaxial, cyclic simple shear, torsional resonant column, plate loading, and MASW tests. The results revealed insights on the geotechnical properties and mechanical response of collapsible soils and the effects of salt cementation. The results indicated: 1) a decreasing brittle and collapsible behaviors with decreasing cementation while maintaining consistent post-collapse residual strength; 2) oedometer and in situ plate loading tests showed a sudden increase in deformations once cementation is broken; 3) higher dilation potential of collapsible soils as compared to natural sands; 4) decrease in the maximum shear modulus due to the loss of cementation; 5) increase in the stress dependence of the maximum shear modulus with loss of cementation; and 6) a higher resistance to liquefaction for the collapsible soils, even after washing, as compared to natural sands, which may be ascribed to the more pronounced dilation potential.     

土体稳定与承载特性的分析方法

结合研究团队多年的研究积累,分别从砂土与饱和软黏土两个方面阐述了在土体失稳与承载特性分析方法方面的理论研究成果。排水条件下密砂在到达峰值强度前可能会出现应变局部化失稳,而饱和松砂则可能出现分散性失稳;建立了基于三维非共轴塑性理论的变形分叉分析方法,分析了砂土平面应变试验和真三轴试验的应变局部化现象,进一步分析了各向异性砂土的强度特性和应变局部化现象,以及砂土的状态相关特性对应变局部化的影响;建立了基于状态相关本构模型的砂土应变局部化失稳弹塑性有限元分析方法,探讨了复合体理论和非局部塑性理论在消除有限元网格依赖性问题方面的可行性;建立了基于二阶功准则和状态相关本构模型的饱和砂土分散性失稳数值分析方法。提出了基于临界状态理论的饱和软黏土各向异性不排水强度理论公式,以及基于循环累积塑性应变的强度弱化公式,形成了适合低渗透性饱和软黏土的弹塑性静动力有限元分析方法;建立了非均质与各向异性软黏土地基稳定性问题的极限分析上限方法,进一步提出了不排水地基承载特性的虚拟加载上限分析方法,形成了分析地基承载特性循环弱化的弹塑性有限元法和虚拟加载上限法。所建立的分析方法将为砂土与饱和软黏土地基稳定性和承载特性的理论预测提供重要的技术手段。     

Evaluation of post-liquefaction volumetric strain of reconstituted samples based on soil compressibility

In this study, we performed undrained cyclic–torsional–shear tests on clean and silty sands to examine post-liquefaction volumetric strain. Accordingly, we proposed a novel density index—ultimate volumetric strain—to evaluate the density of clean sand and silty soils; the proposed density index is based on soil compressibility. We discussed the test results with respect to relative density, skeleton void ratio, and ultimate volumetric strain. Consequently, we confirmed that ultimate volumetric strain showed good correlation with post-liquefaction volumetric strain irrespective of soil type, whereas values for relative density and skeleton void ratio were discordant. Further, we observed that, compared to maximum shear strain, accumulated shear strain was a better index for representing the effect of loading history. From these results therefore, we proposed a chart to evaluate post-liquefaction volumetric strain based on ultimate volumetric strain and accumulated shear strain.     

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.     

循环荷载下砂土变形的细观数值模拟I:松砂试验结果

采用颗粒流方法模拟不同排水条件下砂土的双轴试验,研究了循环荷载作用下松砂渐进破坏过程中配位数、接触方向、粒间接触力的演化规律,应用表征上述量的组构参数研究了砂土的诱发各向异性,探讨了饱和砂土液化、状态转换面产生的微细观机理。研究表明:宏观的液化对应于细观组构上配位数的连续累积丧失和粒间接触力的不断减小,其根本原因在于循环荷载往返过程中,组构各向异性与应力各向异性发展的不匹配。研究成果对于揭示砂土变形的细观机理以及建立砂土的细观力学模型都具有意义。     

微生物处理砂土不排水循环三轴剪切CFD-DEM模拟

微生物诱导碳酸钙沉积是一种新型的地基处理技术,处理后的土体可以看成一种结构性土。首先,在已有三维含颗粒抗转动和抗扭转模型及三维胶结破坏准则的基础上,通过考虑颗粒碰撞接触过程中颗粒本身的塑性变形及率相关性的接触黏滞阻尼,建立考虑循环荷载作用下的三维胶结模型;然后,参考已有研究,建立了反硝化反应在加固砂土中的时效性关系。并引入CFD-DEM耦合程序,用以模拟分析不同胶结含量以及不同气泡含量下,微生物处理砂土在固结不排水循环剪切试验中的力学特性;最后,从宏微观角度分析生物胶结与生物气泡对砂土抗液化性能的影响及其作用机理。研究表明,胶结和气泡共同存在对抗液化能力的提升并没有起到“1+1=2”的效果;胶结的存在提高了非饱和砂土的抗液化能力,明显抑制孔压比和轴向应变的发展,力学配位数得到了提升;而气泡的存在却降低了胶结砂土的抗液化能力,使得胶结砂土达到初始液化的振次减少,轴向应变向受拉方向大幅增长,力学配位数下降明显。