Laboratory Investigation into the Effects of Silty Fines on Liquefaction Susceptibility of Chlef (Algeria) Sandy Soils

In a number of recent case studies, the liquefaction of silty sands has been reported. To investigate the undrained shear and deformation behaviour of Chlef sand–silt mixtures, a series of monotonic and stress-controlled cyclic triaxial tests were conducted on sand encountered at the site. The aim of this laboratory investigation is to study the influence of silt contents, expressed by means of the equivalent void ratio on undrained residual shear strength of loose, medium dense and dense sand–silt mixtures under monotonic loading and liquefaction potential under cyclic loading. After an earthquake event, the prediction of the post-liquefaction strength is becoming a challenging task in order to ensure the stability of different types of earth structures. Thus, the choice of the appropriate undrained residual shear strength of silty sandy soils that are prone to liquefaction to be used in engineering practice design should be established. To achieve this, a series of undrained triaxial tests were conducted on reconstituted saturated silty sand samples with different fines contents ranging from 0 to 40 %. In all tests, the confining pressure was held constant at 100 kPa. From the experimental results obtained, it is clear that the global void ratio cannot be used as a state parameter and may not characterize the actual behaviour of the soil as well. The equivalent void ratio expressing the fine particles participation in soil strength is then introduced. A linear relationship between the undrained shear residual shear strength and the equivalent void ratio has been obtained for the studied range of the fines contents. Cyclic test results confirm that the increase in the equivalent void ratio and the fines content accelerates the liquefaction phenomenon for the studied stress ratio and the liquefaction resistance decreases with the increase in either the equivalent void ratio or the loading amplitude level. These cyclic tests results confirm the obtained monotonic tests results.     

非塑性细粒对饱和砂土液化特性影响的试验研究

利用空心圆柱扭剪仪对含非塑性细粒的饱和砂土进行单调加载和循环扭剪试验,研究了不同细粒含量饱和砂土的液化特性。试验结果表明：（1）最大孔隙比与最小孔隙比均随着细粒含量的增加呈先减小后增大的趋势,分别在20%和40%时达到最小。（2）细粒含量从0%增加到20%,体积应变逐渐增加;细粒含量从20%增加到40%时,体积应变逐渐减小;之后随着细粒含量从40%增加到60%,体积应变再次增大;细粒含量超过60%以后,体积应变再次递减。（3）随着细粒含量的增加,土样的峰值强度随之降低,应力-应变关系从应变硬化特征发展为理想的弹塑性。相变角在细粒含量为30%时达到最小值。（4）细粒含量越大,达到液化所需的循环次数越小,液化时的应变越小。（5）抗液化强度曲线与抗液化应力比的变化趋势一致,在小于界限细粒含量（30%）时,随着细粒含量的增加而减小。在界限细粒含量附近（30%～50%）时,随着细粒含量的增加而增大。在细粒含量增加到60%时出现明显的骤减,之后再次随着细粒含量的增加而增大。界限细粒含量在40%左右。     

Cyclic liquefaction screening of sand with non-plastic fines:Critical state approach

There have been significant advances in the application of critical state,CS,in liquefaction potential assessment.This was done by comparing state parameter,j with estimated characteristic cyclic stress ratio,CSR due to an earthquake.A cyclic resistance ratio,CRR curve,which can be determined from cyclic liquefaction tests,separates historical liquefied and non-liquefied data points(j,CSR).On the other hand,the concepts of equivalent granular state parameter,j*,which was developed for sands with fines,can be used in lieu j to provide a unifying framework for characterizing the undrained response of sands with non/low plasticity fines,irrespective of fines content(fc).The present work combines these two propositions,and by merely substituting j*for j into the aforementioned CS approach to capture the influence of fc.A series of static and cyclic triaxial tests were conducted,separately and independently of the concept of j*,for sand with up to fc of 30%.The clean sand was collected from Sabarmati river belt at Ahmedabad city in India which was severely affected during the Bhuj earthquake,2001.The experimental data gave a single relation for CRR and j*which was then used to assess liquefaction potential for a SPT based case study,where fc varies along the depth.The prediction matched with the field observation.     

Geotechnical reconnaissance and liquefaction analyses of a liquefaction site with silty fine sand in Southern Taiwan

Geotechnical reconnaissance of a recurrent liquefaction site at a Quaternary alluvial deposit in southern Taiwan was conducted to establish a comprehensive case history for liquefaction on silty fine sand with high fines content. The liquefaction occurred at a silty fine sand layer with D₅₀ = 0.09 mm and fines content greater than 35% and was triggered by a M_w = 6.4 earthquake on March 4, 2010, which induced maximum horizontal acceleration up to 0.189 g at the site. In situ subsurface characterizations, including standard penetration test, cone penetration test, and shear wave velocity measurement, were performed as well as cyclic simple shear tests on undisturbed specimens retrieved by a modified hydraulic piston sampler. Comparisons of cyclic resistance ratios (CRRs) indicate that CPT sounding with standard penetration rate could overestimate the resistance ratio and drainage conditions during penetration should be considered for high fines content soil in the liquefaction analysis. Additionally, variations of CRRs from different in situ tests indicate that correlations among in situ tests and CRR could be soil specific and precautions should be taken when using these curves on silty fine sands.     

Influence of plastic fines content on the liquefaction susceptibility of sands: monotonic loading

Acta Geotechnica - The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand–fines mixtures under...     

复杂加载条件下珊瑚砂抗液化强度试验研究

循环加载方式与应力路径对砂土的抗液化强度有很大的影响。利用GDS空心圆柱扭剪仪对南海珊瑚砂进行了一系列复杂加载条件下均等固结不排水循环试验,探讨了90°突变应力路径下主应力方向角对珊瑚砂抗液化强度的影响。试验结果发现：以循环应力比（CSR）作为应力水平指标,当不控制中主应力系数b的变化时,主应力方向角 对珊瑚砂的抗液化强度并无显著影响;当控制b始终保持0.5时,珊瑚砂的抗液化强度随着 的增加呈现出先减小后增大的趋势,且在 45°时的抗液化强度最低。基于分析循环荷载引起的土单元大、小循环主应力 、 变化,定义了单元体循环应力比（USR）作为一个新的物理指标,发现不同循环加载方式与应力路径条件下施加于珊瑚砂试样的USR与引起液化所需的循环次数NL存在事实上的唯一性关系。通过引自文献的4种无黏性土原始试验数据的再处理,独立地验证了以USR表征砂类土液化强度的适用性。     

Undrained cyclic behavior of bentonite–sand mixtures and factors affecting it

In this work, the cyclic behavior of bentonite–sand mixtures and factors affecting it were studied by means of a ring-shear apparatus and a scanning electron microscope. It was found that bentonite content had a significant influence on the liquefaction potential of the studied soils. A small amount of bentonite in the mixtures would cause the formation of “loose” microstructures, resulting in the occurrence of rapid liquefaction under cyclic loading, while a high bentonite content would cause the formation of clay matrixes, thus raising the soil resistance to liquefaction. In addition, the effect of pore water chemistry on the cyclic behavior of a high plasticity bentonite–sand mixture was carefully examined. It was also found that the presence of ions in pore water would change the clay microfabric, making it more open and thus more vulnerable to liquefaction. Finally, the effects of loading frequency on the cyclic behavior of mixtures with different amounts of bentonite were investigated. It was found that as the bentonite content increased, the influence became more pronounced.     

The liquefaction of clayey soils under cyclic loading

This paper seeks to investigate the liquefaction of clayey soils, a phenomenon that has been the trigger for many natural disasters in the last few decades, including landslides. Research was conducted on artificial clay-sand mixtures and natural clayey soils collected from the sliding surfaces of earthquake-induced landslides. The undrained response of normally consolidated clayey soils to cyclic loading was studied by means of a ring-shear apparatus. For the artificial clay-sand mixtures, it was found that the presence of a small amount of bentonite (≤ 7%) would cause rapid liquefaction, while a further increase in bentonite content (≥ 11%) produced the opposite effect of raising soil resistance to liquefaction by a significant degree. It was demonstrated that the bentonite-sand mixture was considerably more resistant to liquefaction than the kaolin-, and illite-mixtures, given the same clay content. The test results of plastic soils revealed the significant influence of plasticity on the liquefaction resistance of soil. The microfabric of clayey soil was investigated by means of a scanning electron microscope. The analysis showed that the liquefaction potential of soil was strongly related to certain particle arrangements. For example, soil vulnerable to liquefaction had an open microfabric in which clay aggregations generally gathered at the sand particle contact points, forming low-strength “clay bridges” that were destroyed easily during cyclic loading. On the other hand, the microfabric of soil that was resistant to liquefaction appeared to be more compact, with the clay producing a matrix that prevented sand grains from liquefying. In the case of the natural soils, the obtained results indicated that their cyclic behavior was similarly influenced by factors such as clay content, clay mineralogy and plasticity. The relation between the liquefaction potential of natural soil and its microfabric was thus also established. On the basis of the obtained results, the authors posited an explanation on the mechanism of liquefaction for clayey soil.     

Liquefaction behavior of dredged silty-fine sands under cyclic loading for land reclamation: laboratory experiment and numerical simulation

Medium-coarse sands (CS) were dredged and exhausted in land reclamation. However, the remaining silty-fine sands (FS) were wasted. The liquefaction behavior of dredged silty-FS and the possibility of utilizing the remaining silty-FS as dredger fill source for land reclamation should be investigated. Cyclic consolidation-undrained triaxial tests were performed to investigate the liquefaction resistance of dredged silty-FS under different influencing factors. The cyclic stress ratio (CSR) of dredged silty-FS increased with the increase in initial relative density and consolidation stress ratio and decreased with the increase in silt content and consolidation stress. The CSR first decreased with the increase in clay content up to a threshold value and increased with the increase in clay content. A regression model was created to estimate the relationship between CSR and silt content, clay content, initial relative density, consolidation stress, consolidation stress ratio, and cyclic resistance ratio. Response surface methodology (RSM) was employed to investigate the mutual influence among the five independent variables. On the basis of cyclic triaxial tests, particle flow code models were introduced to investigate the microscopic internal fabric changes of dredged silty-FS and the influence of extended factors on liquefaction. The average microscopic contact force and coordination number between particles controlled the macroscopic mechanical behavior of sands. Sand liquefaction was due to the cumulative loss of coordination number under cyclic loading. The average contact force between particles was linearly decreased to 0 and the coordination number sharply decreased when the sample reached initial liquefaction. On the basis of numerical tests, CSR increased with the increase in D₅₀ and vibration frequency. The influence of vibration frequency was relatively small. In addition, the CS–FS and CS–FS–CS combination layers showed greater liquefaction resistance than the FS layer. In the filling process, the interbed of FS and CS improved the liquefaction resistance of dredged silty-FS to a certain extent.     

Influence of plastic fines content on the liquefaction susceptibility of sands: cyclic loading

Acta Geotechnica - The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand-fines mixtures under cyclic...     

Prediction of flow liquefaction instability of clean and silty sands

Adding a small amount of non-plastic silt to clean sands may lead to dramatic loss of shear strength and a noteworthy tendency toward contraction when the mechanical behavior of the mixture is compared with that of the clean host sand. Thus, simulation of the behavior of silty sands with varying fines content is still a challenging subject in geomechanics. A unified constitutive model for clean and silty sands is presented in this paper. To eliminate the factitious decrease of void ratio associated with inactive silt particles in various silty sand mixtures, the concept of equivalent void ratio is used in the model formulation instead of the global void ratio. In addition, the instantaneous soil state is expressed in terms of intergranular state parameter taking into account the combined influence of intergranular void ratio, mean principal effective stress and fines content. Then, dilatancy and plastic hardening modulus are directly linked to the intergranular state parameter. To improve the model capacity in simulation of cyclic tests, new features are added to the plastic hardening modulus. It is shown that the proposed model can reasonably reproduce the mechanical behavior as well as the onset of flow liquefaction instability of clean and silty sands using a unique set of parameters.     

细粒含量对尾矿材料液化特性的影响

为了模拟沉积分选后颗粒组成的变化对尾矿材料的动力特性的影响,尾矿筛分后按不同的细粒（颗粒直径小于 0.074 mm）含量制备尾矿试样,并进行了大量的土动力学试验。分析了细颗粒含量对尾矿材料的动力液化特性的影响规律。研究表明,对于铜矿类尾矿坝的尾矿材料,当细颗粒含量占到总量的35 %时其抗液化性能最佳。根据尾矿材料动力特性试验的研究结果,并结合现场的标准贯入试验成果,提出了适用于尾矿材料的细粒含量对标准贯入击数的修正式。该式可以分析不同细粒含量的尾矿材料的抗液化强度,提高尾矿坝液化判别方法的判别准确程度。     

纤维加筋砂土抗液化试验与数值模拟

砂土液化是导致重大地震灾害的主要原因之一。本研究探讨了天然纤维加筋砂土在循环荷载作用下的抗液化性能。在不排水条件下,对具有不同纤维含量的加筋砂土试样进行了一系列循环三轴试验,研究了饱和砂土的液化特性以及循环剪应变幅值、纤维含量对饱和砂土抗液化性能的影响。此外,通过模拟已完成的循环三轴试验,建立了二维有限元数值模型,并对具有不同纤维含量的加筋砂土进行了参数标定。研究结果表明:(1)增加循环剪应变幅值将促进超孔压累积,使得滞回曲线斜率和平均有效应力减小速度加快;(2)纤维的存在能够减缓超孔压的累积,随着纤维含量增加,加筋砂土抗液化能力得到明显提高;(3)标定后的本构模型参数能可靠地用于模拟纤维加筋砂土的液化响应。研究结果为饱和砂土抗液化问题与纤维加筋砂土的数值模拟提供了有价值的参考。     

饱和砂土液化判别与放大效应数值模拟研究

为研究地震作用下饱和砂土液化判别及地震放大效应的影响因素,采用边界面塑性模型框架内开发的砂土本构模型,基于开源有限元平台OpenSees建立了一维剪切梁土柱模型。以循环应力比CSR和循环抗力比CRR为控制指标,对比了不同液化判别方法的差异,分析了地震荷载类型和砂土相对密度对液化判别和放大效应的影响。研究表明：与数值模拟结果相比,Seed简化法计算的CSR更大,判断饱和砂土场地发生液化的可能性更高;冲击型地震波较振动型地震波更容易使饱和砂土场地发生液化,砂土相对密度越小场地越容易发生液化;放大系数随埋深的减小而增大,振动型地震波引起的放大效应整体大于冲击型,埋深较大时放大系数随砂土相对密度的增大而减小。     

Mechanical properties of cementitious sand and sand with small cyclic shear strain to assess aging effects on liquefaction

Liquefaction damage from earthquakes frequently indicates effects of sand aging on liquefaction resistance: Liquefaction damage in natural or aged reclaimed ground has been much less than that in young reclaimed ground. However, the mechanisms underlying aging effects remain unclear. Cementation and stress history of sand strongly influence aging effects: Cementation raises liquefaction resistance, whereas liquefaction history sometimes reduces liquefaction resistance. Small cyclic shear strain, from which the induced density change is almost negligible, was adopted as representing the stress history. To evaluate liquefaction resistance, initial shear modulus, and deformation characteristics of sand, we prepared specimens by adding cement and by applying a small cyclic shear strain. In cementitious sand, liquefaction resistance increased when cement contents exceeded 0.3% by mass. The initial shear modulus apparently increased at the same degree of cement addition as that which increased the liquefaction resistance. For sand with a small cyclic shear strain, the liquefaction resistance increased when the applied cyclic axial strain exceeded 0.01%. Application of small cyclic shear strain only slightly increased the initial shear modulus, but the linear elastic region tended to expand to greater shear strain. Shear properties of sand with small cyclic shear strain resembled those found for sand that had been consolidated for a long time.

     

Effect of Fines Type and Content of Sand on Correlation Between Shear Wave Velocity and Liquefaction Resistance

The use of shear wave velocity (V _s) measurements as an in situ test for evaluation of liquefaction potential has increased substantially due to its advantages. Relatively large numbers of studies have been performed to establish the correlation between V _s and liquefaction resistance (CRR) of clean sands. Usually, natural sands contain silt and/or clay, and previous studies have shown that both the amount of fines and their nature influence the values of CRR as well as V _s. Therefore, the CRR–V _s correlations may also be affected by fines content and type of sandy soils. However, effect of fines content and especially fines type of sandy soils on the correlation between V _s and CRR is inadequately addressed in the literature. In this study, cyclic triaxial and bender element tests were conducted on samples of sand containing various amounts of different types of fines, and the effects of fines on the values of CRR and V _s are investigated. The results show that G ₀ and CRR reduce even when small amounts of fines are added to sand. Therefore, use of plasticity index (PI) of the fines fraction is better than the PI of the overall soil when trying to assess the effects of fines. Using obtained experimental data as well as the established semiempirical CRR–V _s relationship, the CRR–V _s correlation was developed for all the tested soils, and the effect of fines type on the correlation is also examined. Based on the results obtained in this study, CRR–V _s correlation is affected by both the amount and the plasticity of the fines present in the sand, and this correlation is soil specific.     

The effects of surcharge on liquefaction resistance of silty sand

In the recent earthquakes, numerous cases of liquefaction occurred in sands containing both plastic and nonplastic fines that resulted in significant damage. Most previous research efforts have focused on clean sands regardless whether the sand contain fines or not made evident by widespread shallow foundation failures, numerous cases of settlement, and lateral displacement. Still, despite the amount of related research, results seem somewhat contradictory. This study directly demonstrates the beneficial or detrimental effect of uncertainty surcharges on seismic responses. Models were subjected to two destructive realistic events with similar PGA with various frequency contents and duration. In this paper, the dynamic analyses were conducted as fully non-linear elasto-plastic coupled stress-flow analyses with coupled liquefaction triggering which reasonably captured the actual behavior. The silty sand deposits underlying the surcharge are capable of liquefying and developing large shear deformations that can cause serious damage. Therefore, excess pore water pressure ratio cannot be enough by itself to evaluate liquefaction potential, so the deformations should be examined. Numerical results provide an estimate of the seismic performance liquefiable deposits underlying the surcharge; these results can be useful for a realistic practical engineering.     

Effects of initial static shear on cyclic resistance and pore pressure generation of saturated sand

In practical engineering, cyclic shear stresses induced by earthquakes, traffic, and waves are superimposed on the initial static shear stress in sand fills or deposits, leading to complex responses of soils such as their deformation characteristics, pore pressure generation, and susceptibility (or cyclic resistance) to liquefaction. To experimentally investigate the undrained cyclic response of saturated sand, a series of triaxial tests were performed, covering a broad range of initial static and cyclic deviatoric stress levels. The results indicate that different stress conditions lead to two types of cyclic behavior: cyclic mobility and residual deformation accumulation. The compressional static stress is beneficial to the cyclic resistance of the dense sand, whereas the extensional static stress is regarded as detrimental as it tended to reduce the cyclic strength. Moreover, by comparing the available test data obtained from the same sand with varying initial densities and confining pressures, the static shear effect on cyclic resistance was found to be dependent on the state of the sand. Compared to the interpretation made using the limiting pore pressure-based criterion, the conventional failure criterion using a cyclic axial strain of 5% may lead to a substantial overestimation of the cyclic resistance, thus resulting in unsafe assessment and design. Hence, by employing the pore pressure criterion, the pore pressure generated in the cyclic tests was investigated and was found to be significantly influenced by the static shear stress. A pore pressure generation model is proposed to obtain the pore pressure characteristics of sand under various static shear stress conditions.     

钙质砂与硅质砂液化特性对比试验研究

钙质砂颗粒具有形状不规则、多孔隙、强度低、易破碎等特点,较硅质砂表现出更为复杂的液化变形特性.本文对相同级配的钙质砂和硅质砂进行了物性试验、不排水循环三轴试验、轻型动力触探试验,研究两种砂在物理性质、抗液化能力和贯入阻力三方面的差异,分析实验结果得到结论如下:(1)钙质砂比硅质砂具有更大的孔隙比和内摩擦角,这与钙质砂颗...     

Investigation of monotonic and cyclic behavior of sand using a bounding surface plasticity model

Developing the pore water pressures in loose to medium sands below the water table may lead to liquefaction during earthquakes. The simulation of liquefaction (cyclic mobility and flow liquefaction) in sandy soils is one of the major challenges in constitutive modeling of soils. This paper presents the simulation of sand behavior using a critical state bounding surface plasticity model (Dafalias and Manzari’s model, 2004) during monotonic and cyclic loading. The drained, undrained, and cyclic triaxial tests were simulated using Dafalias and Manzari’s model. The simulation results showed that the model predicts behavior of sand, reasonably well. Also, for CSR?<?0.2, number of cycles for liquefaction is significantly increased. The residual strength of Babolsar sand is produced when it is deformed to an axial strain of 20 to 25%.