Experimental investigation of the effect of grading characteristics on the liquefaction resistance of various graded sands

The liquefaction susceptibility of various graded fine to medium saturated sands are evaluated by stress controlled cyclic triaxial laboratory tests. Cyclic triaxial tests are performed on reconstituted specimens having global relative density of 60%. In all cyclic triaxial tests; loading pattern is selected as a sinusoidal wave form with 1.0 Hz frequency, and effective consolidation pressure is chosen to be 100 kPa. Liquefaction resistance is defined as the required cyclic stress ratio which caused initial liquefaction in 10 cycles during the cyclic triaxial test. The results are used to draw relationship between grading characteristics (e.g. coefficient of uniformity and coefficient of curvature) and the liquefaction resistance of various graded sands. It is found that a relationship between cyclic resistance and any of the size (i.e. D₁₀, D₃₀ or D₆₀) would be more realistic than to build a relation between grading characteristics and the cyclic resistance.     

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.     

Prediction of cyclic resistance ratio for silty sands and its applications in the simplified liquefaction analysis

Liquefaction resistance of granular soils is commonly characterized by the cyclic resistance ratio (CRR) in the simplified shear stress procedure of liquefaction potential assessment. This parameter is commonly estimated by cyclic tests on reconstituted samples or empirical correlations between liquefied/non-liquefied case histories. The current study employs results of cyclic triaxial tests on reconstituted soil specimens and presents a predictive equation for cyclic resistance ratio (CRR) of clean and silty sands. The CRR equation is a function of relative density, effective mean confining pressure, non-plastic fines content, number of harmonic cycles for liquefaction onset, and some other basic soil properties. It is demonstrated that the developed relationship obtains reasonable accuracy in the prediction of laboratory-based CRR. Based on the developed CRR model, new relationships are then presented for the coefficient of effective overburden pressure (K_σ) and magnitude scaling factor (MSF), two important modification factors in the simplified shear stress procedure. These new modification factors are then compared with those recommended by previous researchers. Finally, the possible application of the proposed CRR model in field condition is shown for a specific case. This study provides a preliminary insight into the liquefaction resistance of silty sands prior to the complementary laboratory studies.     

Unidirectional cyclic resistance of Ticino and Toyoura sands from centrifuge cone penetration tests

The evaluation of the undrained cyclic resistance of sandy deposits is required to forecast the soil behaviour during an earthquake (liquefaction, cyclic mobility); due to the difficulties in obtaining undisturbed samples of most liquefiable soils, it is usually deduced from field test results such as cone penetration tests. This paper proposes a methodology to evaluate the undrained cyclic resistance from normalised cone resistance of two well-studied silica sands (Ticino and Toyoura), with different mineralogy, one mainly composed of feldspar, the other of quartz. The determination of the cyclic resistance of Ticino and Toyoura sands was achieved through undrained cyclic triaxial tests on reconstituted specimens. The tip resistance was deduced from CPTs performed in centrifuge with a miniaturised piezocone on homogeneous reconstituted models. Both the undrained cyclic and tip resistances were correlated with the state parameter ψ. Results of centrifuge and triaxial tests were combined through ψ to deduce the cyclic resistance ratio CRR directly from the normalised cone resistance. The shape of the curve relating CRR to the normalised cone resistance resulted unusual respect to all the recognised curves widespread in the geotechnical literature. The aim of the proposed correlations is to provide a useful instrument to improve the actual knowledge on liquefaction and to give a contribution based on the critical state soil mechanics framework to the development of refined correlations between the cyclic resistance of a sand and the results of cone penetration tests.     

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.     

Effect of grading characteristics on the undrained shear strength of sand: review with new evidences

In this paper, the shear strength of saturated pure sand and sand–silt mixture is evaluated by monotonic undrained triaxial tests that were carried out on reconstituted specimens at same relative densities and a constant confining pressure (σ ₃?=?300 kPa). The test results were used to conclude on the effect of low non-plastic contents (0–20 %) and grading characteristics on the liquefaction resistance of the sand. The test results indicate that the undrained residual strength reduced with the increase of non-plastic fine content. Also, shear strength of gap-graded sand mixed with low non-plastic fine content increases with decrease in effective size (D ₅₀). In other words, in this state, we can use the D ₅₀ as a parameter to control of silty sand’s undrained resistance. Besides, the undrained residual strength of pure sand specimens with same effective size increases due to increase of coefficient of uniformity (C _u).     

A modification to dense sand dynamic simulation capability of Pastor–Zienkiewicz–Chan model

A modification to the nonlinear Pastor–Zienkiewicz–Chan (PZC) constitutive model without any change in the number of model parameters is introduced in order to simulate stiffness degradation of dense sands at dynamic loading. The PZC model is based on generalized plasticity and was verified by good prediction of liquefaction and undrained behavior of saturated sand. The PZC is a robust model that can predict drained dynamic behavior of sands, especially stiffness increase in loose sand at reloading of dynamic loading. Yet, this model does not show stiffness degradation of dense sand at reloading. The modification is made through modifying the stress memory factor, H _DM, which is multiplied by the plastic modulus, H _L. This modification does not influence reloading behavior of loose sand. The modified PZC model is verified via results of drained cyclic tests. Two cyclic triaxial tests on loose and dense specimens, along with two cyclic plane strain tests on dense sand are utilized for validation. The model simulation shows that the modified PZC model is able to predict the stiffness degradation of dense sand at reloading well.     

饱和层状砂土液化特性的动三轴试验研究

利用GDS动三轴试验系统采用等幅循环应变加载方式对含有不同厚度粉土的饱和层状砂土进行了液化强度试验。分析了均匀砂和含有不同粉粒层厚度的层状砂土在循环荷载作用下的变形和力学特性。试验分析表明：由于含粉粒夹层的层状土特殊的土体结构,其孔隙水压力发展规律与一般的无黏性砂土不同;饱和层状砂土的抗液化强度并不是随着粉粒层厚度的增加而单调增加的,而是存在一个临界点;液化临界剪应变的大小与液化判别标准和循环次数有很大关系。试验结果表明,粉粒夹层对层状砂土的液化特性有很大的影响,且更能模拟自然环境条件下的层状砂土地基液化特性。     

饱和砂土液化特性的可视化试验研究

针对目前散粒体液化细观机制研究的局限性,对C.K.C型动态三轴仪进行改进,研制了可用于研究散粒体细观组构特征的动三轴可视化试验系统。利用该系统对循环荷载作用下标准砂的颗粒运动规律及细观组构特征进行初步探讨,从颗粒的运动规律、定向性、配位数和孔隙率等细观角度分析了循环荷载作用下饱和砂土发生液化的细观机制,认为液化的发生是土体在循环荷载作用下颗粒不断运动、重新排列的结果,指出颗粒长轴方向、配位数和孔隙率等细观组构参量是反映循环荷载作用下饱和砂土液化宏观响应的重要参量     

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,这与钙质砂颗粒特点相匹配;(2)砂土抗液化能力随着相对密度的增大而增大,相同相对密度下钙质砂比硅质砂具有更高的抗液化能力和抗变形能力;(3)砂土贯入阻力随着相对密度的增大而增大,相同相对密度下钙质砂比硅质砂具有更高的贯入阻力。综合不排水循环三轴试验和轻型动力触探试验的结果,指出采用陆源硅质砂地基上经验数据建立的基于贯入阻力的液化判别方法直接用于钙质砂地基可能偏保守。     

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.     

厦门砂土的动力特性研究

为了探讨厦门砂土的动力特性,对厦门金秋豪园场地的中粗砂和细粉砂做了一系列振动三轴试验,并对同一场地的含泥中砂做了4个共振柱试验。根据试验结果,确定了中粗砂与细粉砂的液化曲线和动本构关系,提出了相应的动孔压模型,得到了含泥中砂在小应变条件下动剪切模量和阻尼比的变化规律。     

Effect of fines content and void ratio on the saturated hydraulic conductivity and undrained shear strength of sand–silt mixtures

The hydraulic conductivity represents an important indicator parameter in the generation and redistribution of excess pore pressure of sand–silt mixture soil deposits during earthquakes. This paper aims to determine the relationship between the undrained shear strength (liquefaction resistance) and the saturated hydraulic conductivity of the sand–silt mixtures and how much they are affected by the percentage of low plastic fines (finer than 0.074 mm) and void ratio of the soil. The results of flexible wall permeameter and undrained monotonic triaxial tests carried out on samples reconstituted from Chlef river sand with 0, 10, 20, 30, 40, and 50 % non-plastic silt at an effective confining pressure of 100 kPa and two initial relative densities (D _r = 20, 91 %) are presented and discussed. It was found that the undrained shear strength (liquefaction resistance) can be correlated to the fines content, intergranular void ratio and saturated hydraulic conductivity. The results obtained from this study reveal that the saturated hydraulic conductivity (k _sat) of the sand mixed with 50 % low plastic fines can be, in average, four orders of magnitude smaller than that of the clean sand. The results show also that the global void ratio could not be used as a pertinent parameter to explain the undrained shear strength and saturated hydraulic conductivity response of the sand–silt mixtures.     

饱和砂土的剪切波速与其抗液化强度关系研究

根据饱和砂土剪切波速与其抗液化强度的相关性原理,利用剪切波速与振动三轴联合实验装置,进行了控制饱和砂土初始剪切波速的振动液化实验,依据实验结果建立了剪切波速与抗液化强度的定量关系。最后用现场勘查数据对此定量关系进行验证,结果表明：该关系式对实际 66 个未液化地点的判别准确率达到 81.2 %;对 108 个实际液化地点的判别准确率达到 62.8 %;平均判别准确率达到 69.5 %。     

The influence of grading on the shear strength of loose sands in stress-controlled ring shear tests

Three silica sand samples—well graded, intermediately graded, and narrowly graded—having different uniformity coefficients, were constituted to allow the investigation of the influence of particle size distribution on their mechanical behavior. Using a ring shear apparatus, samples were tested under a wide range of laboratory conditions. Results of the tests clearly indicate that, for specimens confined under identical stress conditions, well-graded specimens have higher values of peak and steady state strengths than the rest of the specimens. A relationship between uniformity coefficient and shear strengths shows that the higher the uniformity coefficient, the higher the shear strength. On account of these, well-graded specimens have higher static liquefaction resistance than the poorly graded specimens. Within the range of normal stresses employed in the tests, results reveal that not only are poorly graded sands more likely to suffer higher postfailure strength reduction, but that their steady-state strengths are easily reduced to zero, the magnitude of the confining stress notwithstanding. This reduction of shear resistance to zero has been described as complete liquefaction in this paper. While almost all of the narrowly graded specimens suffered complete liquefaction, widely graded ones did not; an observation that seem to highlight the influence of grading on the mechanical behavior of the sands.     

南京砂强度特征与静态液化现象分析

在松散、中密和密实状态下,以南京粉细砂三轴固结不排水试验结果为基础,进行了强度、变形与静态液化特征的分析。松散南京砂强度特性表现出典型的应变软化,当轴向变形小于1 %时强度达到最大值,而后急剧降低;在50,100 kPa围压时发生了静态液化。但随着固结压力的增大,静态液化消失。与南京砂具有相同土骨架的松散纯净砂却在低围压下未出现静态液化,其形成机制是：粉粒的存在未使土体孔隙比发生较大变化,却引起更大的体缩性;中密和密实南京粉细砂表现出加工硬化的强度特征,临界应力状态线倾角高达55°,具有较高的抗静态液化能力。     

Laboratory assessment of saturation and sample molding effects on shear resistance and mechanical characteristics of sandy soil

In order to evaluate shear resistance characteristics of sand, which has previously experienced liquefaction, two series of drained and undrained monotonic triaxial compression tests on medium dense sand were carried out. In the first test series, the influence of the specimen preparation method and confining pressure has been studied. It was found that there was a marked difference in the behavior even though the density and stress conditions were identical. The conclusion was that the soil fabric was responsible for this result. In the second series of tests, the saturation influence on the shear resistance of the sand was examined. The results showed that the decrease in Skempton's pore pressure coefficient B improves the shear resistance and increases the friction angle of the sand.     

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。     

Simplified DMT-based methods for evaluating liquefaction resistance of soils

This paper presents simplified dilatometer test (DMT)-based methods for evaluation of liquefaction resistance of soils, which is expressed in terms of cyclic resistance ratio (CRR). Two DMT parameters, horizontal stress index (K_D) and dilatometer modulus (E_D), are used as an index for assessing liquefaction resistance of soils. Specifically, CRR–K_D and CRR–E_D boundary curves are established based on the existing boundary curves that have already been developed based on standard penetration test (SPT) and cone penetration test (CPT). One key element in the development of CRR–K_D and CRR–E_D boundary curves is the correlations between K_D (or E_D) and the blow count (N) in the SPT or cone tip resistance (q_c) from the CPT. In this study, these correlations are established through regression analysis of the test results of SPT, CPT, and DMT conducted side-by-side at each of five sites selected. The validity of the developed CRR–K_D and CRR–E_D curves for evaluating liquefaction resistance is examined with published liquefaction case histories. The results of the study show that the developed DMT-based models are quite promising as a tool for evaluating liquefaction resistance of soils.     

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.