Influence of colloidal silica grout on liquefaction potential and cyclic undrained behavior of loose sand

Cyclic triaxial tests were performed to investigate the influence of colloidal silica grout on the deformation properties of saturated loose sand. Distinctly different deformation properties were observed between grouted and ungrouted samples. Untreated samples developed very little axial strain prior to the onset of liquefaction. However, once liquefaction was triggered, large strains occurred rapidly and the samples collapsed within a few additional loading cycles. In contrast, grouted sand samples experienced very little strain during cyclic loading. Additionally, the strain accumulated uniformly throughout loading rather than rapidly prior to collapse and the samples never collapsed. Cyclic triaxial tests were done on samples stabilized with colloidal silica at concentrations of 5, 10, 15, and 20%. In general, samples stabilized with higher concentrations of colloidal silica experienced very little strain during cyclic loading. Sands stabilized with lower concentrations tolerated cyclic loading well, but experienced slightly more strain. Thus, treatment with colloidal silica grout significantly increased the deformation resistance of loose sand to cyclic loading.     

Pore water pressure increment model for saturated Nanjing fine sand subject to cyclic loading

Three groups of dynamic triaxial tests were performed for saturated Nanjing fine sand subjected to uniform cyclic loading. The tested curves of the excess pore water pressure (EPWP) ratio variation with the ratio of the number of cycles are provided. The concept of the EPWP increment ratio is introduced and two new concepts of the effective dynamic shear stress ratio and the log decrement of effective stress are defined. It is found that the development of the EPWP increment ratio can be divided into three stages: descending, stable and ascending. Furthermore, at the stable and ascending stages, a satisfactory linear relationship is obtained between the accumulative EPWP increment ratio and natural logarithm of the effective dynamic shear stress ratio. Accordingly, the EPWP increment ratio at the number of cycles N has been deduced that is proportional to the log decrement of effective stress at the cycle number N-1, but is independent of the cyclic stress amplitude. Based on the analysis, a new EPWP increment model for saturated Nanjing fine sand is developed from tested data fitting, which provides a better prediction of the curves of EPWP generation, the number of cycles required for initial liquefaction and the liquefaction resistance.     

Simulation of torsional shear test results with neuro-fuzzy control system

In the first part of this study, a series of stress-controlled hollow cylinder cyclic torsional triaxial shear tests were conducted on loose to medium dense saturated samples of clean Toyoura sand to investigate its liquefaction behavior. A uniform cyclic sinusoidal loading at a 0.1 Hz frequency was applied to air-pluviated samples where confining pressure and relative density was varied. Cyclic shear stress–strain changes, the number of cycles to reach liquefaction and pore pressure variations were recorded. Results indicate that the liquefaction resistances of uniform sands are significantly affected by the method of sample preparation and initial conditions.     

含微生物气泡高饱和砂土循环三轴剪切CFD-DEM模拟

为探究砂土液化的微观机理,根据室内试验中微生物反硝化反应气泡的生成速率,建立数值模拟的时效性关系,分别制取微生物处理0天、2天、3天和5天的高饱和砂土试样,采用CFD-DEM耦合方法模拟不同工况下砂土试样的循环三轴不排水剪切试验。依据砂土试样的力链分布、抗液化振次、孔压比、轴向应变和力学配位数在加载过程中的变化情况,从宏微观角度分析砂土试样的抗液化能力。模拟结果表明:含微生物气泡高饱和砂土的抗液化强度较饱和砂土有所提升;随着微生物处理时间的增加,砂土试样的饱和度降低,孔压比和轴向应变的累积变慢,抗液化能力增强。     

Liquefaction characteristics of gap-graded gravelly soils in K0 condition

A series of undrained cyclic direct simple shear tests, which used a soil container with a membrane reinforced with stack rings to maintain the K₀ condition and integrated bender elements for shear wave velocity measurement, were performed to study the liquefaction characteristics of gap-graded gravelly soils with no fines content. The intergrain state concept was employed to categorize gap-graded sand–gravel mixtures as sand-like, gravel-like, and in-transition soils, which show different liquefaction characteristics. The testing results reveal that a linear relationship exists between the shear wave velocity and the minor fraction content for sand–gravel mixtures at a given skeleton void ratio of the major fraction particles. For gap-graded gravelly sand, the gravel content has a small effect on the liquefaction resistance, and the cyclic resistance ratio (CRR) of gap-graded gravelly sands can be evaluated using current techniques for sands with gravel content corrections. In addition, the results indicate that the current shear wave velocity (V_s) based correlation underestimates the liquefaction resistance for V_s values less than 160 m/s, and different correlations should be proposed for sand-like and gravel-like gravelly soils. Preliminary modifications to the correlations used in current evaluations of liquefaction resistance have thus been proposed.     

Undrained monotonic and cyclic behavior of sand-ground rubber mixtures

In this study the stress–strain characteristics of sand-ground rubber mixtures are investigated in the sandlike zone,at different confining pressures,using hollow cylinder specimens subjected to torsional monotonic and cyclic loading.Under monotonic loading a mixture of sand-ground rubber with 10% and 25% rubber content show more contraction behaviour than that observed in a pure sand specimen.Phase transformation point in these mixtures are located on a larger shear strain.As expected,the shear strength of specimens decreases with increase of ground rubber content.However,with increasing of effective confining pressure,the loss in shear strength of the mixture is decreased.In addition,a mixture with 25% ground rubber shows a smaller loss in shear strength compared to a mixture with 10% ground rubber mixture.Under cyclic loading mixtures with 10% and 25% ground rubber have similar liquefaction resistance,especially at confining pressures of 110 k Pa and 260 k Pa.Therefore,by using of the mixture with 25% ground rubber,a larger volume of scrap tires could be recycled.The addition of ground rubber to sand would affect the shear strain variation and excess pore water pressure trends,and this effect was further intensified with increasing ground rubber percentage.     

复杂应力条件下饱和松砂单调与循环剪切特性的比较研究

本文利用大连理工大学新引进与开发的“土工静力-动力液压-三轴扭转多功能剪切仪”,针对福建标准砂,在不排水条件下同时进行了单调剪切试验与循环剪切试验,进而对其进行了对比分析。通过比较表明,应力-应变关系的应变软化和硬化特性与流滑变形和循环流动特性密切相关,当循环剪切应力水平高于单调剪切过程中应变软化阶段最小强度时将会发生流滑变形。无论在单调剪切中,还是在循环剪切中,稳定状态时的有效偏应力比随着大主应力方向与竖向之间夹角的增大而减小,在中主应力系数相同的条件下,循环剪切中呈现显著剪胀时的有效偏应力比和最终稳定状态时的有效偏应力比峰值分别与单调剪切中达到相变状态时的有效偏应力比和最终稳定有效偏应力比基本上一致。然而不排水条件下单调与循环剪切过程中孔隙水压力的增长特性却并不相同,循环剪切中的最大孔隙水压力随着初始主应力方向角的增大而减小,单调剪切中的最大孔隙水压力却随着主应力方向角的增大而增大。     

Influence of a cyclic and dynamic loading history on dynamic properties of dry sand, part I: cyclic and dynamic torsional prestraining

Initial fabric of a soil induced by its cyclic strain history is an important parameter together with the void ratio, state of stress and amplitude in respect to further accumulation of deformations under drained cyclic loading. It is of importance for the further deformation prediction to determine the initial fabric of the grain skeleton or the cyclic loading history of the soil. An attempt is made within this paper to correlate small strain stiffness of non-cohesive soil with its cyclic loading history. The results of performed cyclic and dynamic torsional tests show that small strain shear modulus is only moderately affected by cyclic prestraining even if high amplitudes are applied. A signature of prestraining history is observed in the tests since the sand memorizes its prestraining amplitude and the number of applied cycles.     

Cyclic and post-cyclic shear behavior of low-plasticity silt with varying clay content

This paper investigates the cyclic and post-cyclic shear behavior of low-plasticity silt and the impact of additional clay content. Bentonite clay was added to the low-plasticity Mississippi River Valley (MRV) silt (PI=6) to increase the clay content of the soil. A series of triaxial tests were conducted in the laboratory to examine the shear and pore pressure behavior during and after cyclic loading. As the bentonite content in the reconstituted specimens increased, the excess pore pressure developed at a slower rate and the total excess pore pressure decreased at the end of cyclic loading. In contrast to the MRV silt, the specimens modified with bentonite experienced cyclic softening rather than initial flow liquefaction. The cyclic shear strength increased with an increase in bentonite content. The post-cyclic reconsolidation behavior was a similar to a virgin compression process, and not recompression. Adding bentonite to the MRV silt results in changes in permeability, compressibility, undrained shear strength, and initial stiffness. Additionally, the cyclic loading had a marked effect on the shear behavior of low-plasticity soil with a PI<6, but not noticeable with a PI>6. This study suggests that the behavior of the Mississippi River Valley silt changes from contractive sand-like material to clay-like behavior at a PI≈6 due to the addition of clay.     

应用剪切波速判别砂土液化的研究综述

首先介绍了应用剪切波速判别饱和砂土振动液化的原理；然后从临界剪应变取值、砂土的抗液化能力与剪切波速相关关系的研究方法与设备、用剪切波速进行扰动评价、建立适用于不同土类的统一判别式四方面总结了国内外学者在该领域的一些研究成果及存在问题；最后针对以上问题提出作者的一些看法。     

Strain energy based evaluation of liquefaction and residual pore water pressure in sands using cyclic torsional shear experiments

In this study, cyclic hollow cylinder torsional tests were conducted on the reconstituted specimens of Toyoura sand in a practical range of initial density and stress states. The results were employed to evaluate the liquefaction resistance and residual pore water pressure of sand using the strain energy concept. A simple pore water pressure (PWP) model with two calibration parameters was developed for the prediction of residual pore pressure as a function of cumulative strain energy density and the capacity energy of sand. Capacity energy is defined as the cumulative strain energy that is required for liquefaction onset. Based on the results of the tests, an equation is then presented for the estimation of capacity energy in terms of relative density and initial effective confining pressure of sand. This equation is shown to work well as a state boundary curve, which can discriminate between the liquefied and non-liquefied field case histories. Several extra tests were also performed to investigate the effect of initial static shear stress on the proposed PWP model and capacity energy. The results show that initial shear stress has a minor effect on the trend of the proposed PWP model; however, it definitely affects the capacity energy. The final part of the paper aims to confirm reasonable performance of the proposed PWP model by the available observations of seismically induced pore water pressure in shaking table, centrifuge, and real site conditions.     

砂-锯末混合模型土的动力特性试验研究

为研究砂-锯末混合模型土的动力参数规律特性,采用动三轴试验,研究了此类土的滞回圈特性以及其动剪切模量和阻尼比随循环加载次数的变化规律。试验结果表明:分级循环加载过程中,随着荷载级别的增加,滞回圈的面积逐渐增大,形状从柳叶形向新月形发展;模型土的动剪切模量在各循环下基本为常量而阻尼比呈上下波动;而随荷载级别的增大,动剪切模量均值减小,阻尼比均值增大;与一般砂土相比,模型土的动力参数曲线分布形态类似,但动剪切模量比更小,阻尼比更大。     

Liquefaction potential and strain dependent dynamic properties of Kasai River sand

The availability of efficient numerical techniques and high speed computation facilities for carrying out the nonlinear dynamic analysis of soil-structure interaction problems and the analysis of ground response due to earthquake loading increase the demand for proper estimation of dynamic properties of soil at small strain as well as at large strain levels. Accurate evaluation of strain dependent dynamic properties of soil such as shear modulus and damping characteristics along with the liquefaction potential are the most important criteria for the assessments of geotechnical problems involving dynamic loading. In this paper the results of resonant column tests and undrained cyclic triaxial tests are presented for Kasai River sand. A new correlation for dynamic shear damping (D_s) and maximum dynamic shear modulus (G_max) are proposed for the sand at small strain. The proposed relationships and the observed experimental data match quite well. The proposed relationships are also compared with the published relationships for other sands. The liquefaction potential of the sand is estimated at different relative densities and the damping characteristics at large strain level is also reported. An attempt has been made to correlate the G_max with the cyclic strength of the soil and also with the deviator stress (at 1% strain) from static triaxial tests.     

主应力轴持续旋转条件下饱和松砂的振动孔隙水压力特性

利用新研制的“土工静力-动力液压三轴-扭剪多功能剪切仪”,针对福建标准松砂,在三向非均等固结条件下,进行了能够模拟海洋波浪荷载作用下主应力轴连续旋转的循环耦合剪切试验。通过试验着重探讨了初始主应力方向、振动过程中主应力方向连续变化对不排水条件下砂土的振动孔隙水压力增长特性的影响。实验研究表明:在振动过程中主应力轴连续旋转的条件下,初始主应力方向对砂土的动孔压比与振次比之间关系具有显著的影响,随着初始大主应力与竖向之间夹角的增大,动孔压比的增长速度明显加快,具有较好的规律性;归一化孔压比与广义剪应变之间的关系基本上与初始主应力方向角和振动剪应力幅值无关。     

Evaluation for Earthquake Liquefaction of Loess Sites

ugh the sinusoid loading dynamic triaxial test, the liquefaction property of saturated loess and sand selected from a civil airport of Lanzhou, Gansu is examined. Based on the laboratory results, a comprehensive assessment on the earthquake liquefaction potential of the loess and sand is given, using the liquefaction resistance shear stress method and the results of seismic hazard assessment. It is found that under the effect of ground motion with exceedance probability of 10% within 50 years, the loess in the study is more susceptible to liquefaction than sand.     

Experimental investigation on fluid characteristics of medium dense saturated fine sand in pre- and post-liquefaction

Liquefaction of saturated loose sand is a major cause of extensive damage to buildings and infrastructures during large earthquakes. A better understanding of the behaviour of liquefied soil is becoming increasingly necessary to mitigate earthquake damage, and the fluid method has become an increasingly popular means to study the behaviour of liquefied soils. The purpose of this study is to determine the fluid characteristics of liquefied fine sand. In this paper, the apparent viscosity was measured as an index of fluid characteristics using the shaking table tests of pre-liquefaction behaviour of saturated fine sand at approximately 45 % relative density; the relationship of apparent viscosity and shear strain rate on liquefying fine sand was indicated as a power-law shear-thinning non-Newtonian fluid; and liquefying fine sand has the alternating behaviour of shear dilatancy and compressibility during cyclic loading. Additionally, a series of a monotonic axial compression loading tests in an undrained manner were performed to measure the shear stress and excess pore pressure ratio relationship on the post-liquefaction saturated fine sand at approximately 50 % relative density. The fluid characteristics of post-liquefaction fine sand exhibits rate dependence and can be described by a combined fluid model of time-independent and time-dependent power-law functions; the time-independent viscous resistance is not relevant to the excess pore pressure ratio; but the time-dependent frictional resistance is closely related to the excess pore pressure ratio. Furthermore, the results of the verification tests demonstrate that the proposed fluid model has good applicability for the fluid behaviour of the post-liquefaction fine sand.     

Cyclic and post-cyclic monotonic behavior of Adapazari soils

The August 17, 1999 Kocaeli earthquake affected the city of Adapazari, which is located in the northwest of Turkey, with severe liquefaction and bearing capacity failures causing tilting of buildings, excessive settlements and lateral displacements. To understand the stress–strain behavior and pore pressure behavior of undisturbed soils during the earthquake, the cyclic and post-cyclic shear strength tests have been conducted on soil samples obtained from Adapazari in a cyclic triaxial test system within the scope of this research. Cyclic tests have been conducted under stress controlled and undrained conditions. Post-cyclic monotonic tests have been conducted following cyclic tests. The strength curves obtained in the experiments showed that the dynamic resistance of silty sand was found to be 45% lower than those of high plasticity soils (MH). The strength of clayey soils with the plasticity index of PI=15–16% was lower compared to the strength of high plasticity soils. Also, it was observed that silty sand soils had the lowest strength. The dynamic strength of the soils increased with the increase in plasticity.     

基于OpenSees的砂土本构模型对比研究

饱和砂土液化是由地震引起的一种最常见的工程地质现象,也是造成重大地震灾害的主要原因之一。由于成因的复杂性和所造成灾害的严重性,饱和砂土液化一直是土动力学和岩土地震工程研究领域的重要课题。针对饱和砂土液化问题,基于开源地震工程数值计算平台OpenSees,对材料库中的4种砂土本构模型进行数值计算。采用二维u-p单元模拟土颗粒位移和孔隙水压力,分析和对比4种模型在循环动力荷载作用下的加速度、超孔隙水压力、位移、剪应力-剪应变和平均有效应力路径方面的响应结果。研究结果表明:(1)砂土对输入加速度表现出一定的放大效应,对于不同的模型,该放大效应存在一些差异;(2) Stress Density模型在循环动力荷载作用下易产生永久变形;(3)在循环动力荷载作用下,PDMY模型和CycLiqCPSP模型的强度逐渐降低,直到完全消失;(4) Stress Density模型和Manzari Dafalias模型在循环动力荷载下表现出明显的剪胀效应。研究成果对砂土液化的数值模拟问题具有重要的理论价值,可为饱和砂土的液化模拟和砂土本构模型的选取提供参考。     

黄土场地地震液化研究

通过往返加荷动三轴试验，对兰州某民用机场扩建工程场地的饱和黄土和砂土进行了液化试验。在试验结果的基础上，运用抗液化剪应力判别方法、地震危险性分析计算结果以及根据有限元一维模型计算求得的该场地的地震剪应力，对该场地饱和黄土和砂土的地震液化进行了综合判断。结果表明，在未来遭受到50年10％超越概率的地震作用时，该场地的饱和黄土比饱和砂土更容易发生液化。     

3rd Ishihara Lecture: An investigation into why liquefaction charts work: A necessary step toward integrating the states of art and practice

This paper is a systematic effort to clarify why field liquefaction charts based on Seed and Idriss׳ Simplified Procedure work so well. This is a necessary step toward integrating the states of the art (SOA) and practice (SOP) for evaluating liquefaction and its effects. The SOA relies mostly on laboratory measurements and correlations with void ratio and relative density of the sand. The SOP is based on field measurements of penetration resistance and shear wave velocity coupled with empirical or semi-empirical correlations. This gap slows down further progress in both SOP and SOA. The paper accomplishes its objective through: a literature review of relevant aspects of the SOA including factors influencing threshold shear strain and pore pressure buildup during cyclic strain-controlled tests; a discussion of factors influencing field penetration resistance and shear wave velocity; and a discussion of the meaning of the curves in the liquefaction charts separating liquefaction from no liquefaction, helped by recent full-scale and centrifuge results. It is concluded that the charts are curves of constant cyclic strain at the lower end (V_s1<160 m/s), with this strain being about 0.03–0.05% for earthquake magnitude, M_w≈7. It is also concluded, in a more speculative way, that the curves at the upper end probably correspond to a variable increasing cyclic strain and K_o, with this upper end controlled by overconsolidated and preshaken sands, and with cyclic strains needed to cause liquefaction being as high as 0.1–0.3%. These conclusions are validated by application to case histories corresponding to M_w≈7, mostly in the San Francisco Bay Area of California during the 1989 Loma Prieta earthquake.