A compression model for sand–silt mixtures based on the concept of active and inactive voids

A compression model for sand–silt mixtures is needed in geotechnical engineering, for example in the analysis and prediction of deformation of levees and embankments due to internal erosion. In this paper, we introduce a novel concept of dividing the voids of a granular material into two hypothetical fractions: active and inactive voids. The active voids are kinematically available to the compression process. The inactive voids are kinematically unavailable to the compression process. The volume of active voids is dependent on the initial density and effective stress level. The volume of inactive voids is dependent on the amount of fines in the mixture. The current paper considers 1-D and isotropic compression behavior of sand–silt mixtures at stress levels lower than 2 MPa, so no substantial particle breakage is expected to occur. To successfully predict the void ratio for a sand–silt mixture during compression loading, we need (1) a mathematical expression for the evolution of the active void ratio during compression and (2) a relationship between the inactive void ratio and fines content of the mixture. For sand–silt mixtures with any amount of fines, the proposed model requires five material parameters, which are determined from two compression tests, and four minimum void ratio tests on sand–silt mixtures with different fines content. The performance of the proposed model is verified for six different types of sand–silt mixture with various fines contents, by comparing the predicted void ratios with the measured data from the experiments. The comparisons show a good agreement between the predictions and the measured data and prove the suitability of the proposed model for the prediction of compressibility of sand–silt mixtures with any amount of fines.     

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.     

Compressional behavior of clayey sand and transition fines content

Clayey sand can be considered as a composite matrix of coarse and fine grains. The interaction between coarser and finer grain matrices affects the overall stress–strain behavior of these soils. Intergranular void ratio, e_s (which is the void ratio of the coarser grain matrix) can be utilized as an alternative parameter to express the compressive response of such soils. Oedometer tests conducted on reconstituted kaolinite–sand mixtures indicate that initial conditions, percentage of fines, and stress conditions influence the compression characteristics evidently. Tests showed that, up to a fraction of fines, which is named as transition fines content (FC_t), compression behavior of the mixtures is mainly controlled by the sand grains. When concentration of fines exceeds FC_t, kaolinite controls the compression. It was found that FC_t varies between 19% and 34% depending on the above mentioned factors. This range of fines content is also consistent with various values reported in literature regarding the strength alteration. Performed direct shear tests revealed that there is also a close relationship between transition fines content and shear strength, which is harmonic with the oedometer test results.     

Undrained shear strength response under monotonic loading of Chlef (Algeria) sandy soil

During the last mid-century, the Chlef area was strongly affected by two earthquakes. From the geological context, there were numerous ejections onto the ground level of great masses of sandy soils and large displacements of various forms of some building foundations. These damages are due to soil liquefaction problem. This loss of shear strength can be attributed to many factors. History of recent cases indicates that sand deposited with silt content is much more liquefiable than clean sand. Therefore, a deep understanding of silty sand behavior is needed for the liquefaction assessment of silty sandy soils. Moreover, during seismic shaking, the post-liquefaction behavior of silty sand and, consequently, the stability of structures founded on liquefied soil depend on the steady-state shear strength of soil. The objective of this laboratory investigation is to show the effect of silt contents and the relative density on the mechanical behavior of such soils in monotonic loading. In this context, a series of undrained triaxial tests were performed on reconstituted saturated silty sand samples with different fines content ranging from 0% to 40%. In all tests, the confining pressure was held constant to 100 kPa. The fines content and the global void ratio are expressed by means of the equivalent void ratio. Linear correlations relating the undrained residual shear strength of loose, medium dense, and dense (D _r?=?12%, 50%, and 90% before consolidation) sand–silt mixtures to the equivalent void ratio are obtained. The concept of the equivalent void ratio will then be used as a key parameter to express the dilatancy behavior of both clean and silty sand soils. Moreover, from the experimental results obtained, it is clear that the global void ratio cannot be used as a state parameter and may not represent the actual behavior of the soil as well.     

The Effects of Fines on the Behaviour of a Sand Mixture

Intergranular void ratio, e _s, can be used as an alternative indicator to assess the mechanical properties of composite matrix of coarse and fine grains. In this paper, an intensive laboratory study of saturated coarse rotund sand and fine angular sand mixtures with various mix ratios is investigated by a series of oedometer and direct shear tests. Oedometer tests performed on the mixtures show that fines percentages and stress conditions affect the compression behaviours. Tests indicated that, up to a fraction of fines, which is named as transition fines content (FC_t), compression behaviour of the mixture is mainly governed by the sand grains. As the percentage of fines exceeds FC_t finer grains govern the compression. Performed direct shear tests revealed that there is a relationship between the FC_t and shear strength, which is harmonic with the oedometer test results.     

Behavior of sand–rubber particle mixtures: experimental observations and numerical simulations

A series of laboratory experiments and numerical simulations are conducted to explore the characteristics of mixtures composed of sand and rubber particles of the same median diameter. The mixtures are prepared with different volumetric sand fractions (sf = V_sand/V_total). The experiment focuses on assessing the strain level on the characteristics of the mixture with the volume fraction of each component. Numerical simulations using the discrete element method are performed to obtain insight into the microscale behavior and internal mechanism of the mixtures. The experimental results show that the behavior of the mixtures is dependent on the relative sand and rubber particles composition with variation in the strain levels. The numerical simulation reveals the effect of the soft rubber particle inclusion in the mixture on the micromechanical parameters. In low sand fraction mixtures, a high shear stress along the contact is mobilized, and the stress state is driven to a more anisotropic condition because of the relatively high particle friction angle of the rubber. The rubber particles play different roles with the strain level in the mixture, including increasing the coordination number and controlling plasticity of the mixture in a small strain, preventing buckling of the force chain in an intermediate strain, and leading to contractive behavior in a large strain. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Undrained Cyclic and Monotonic Strength of Sand-Silt Mixtures

In an attempt to correlate the monotonic peak strength and the cyclic strength of sand-silt mixtures over a wide range of parameters and to clarify some of the existing confusing conclusions in the literature regarding the undrained strength response of sand-silt mixtures, a series of stress controlled cyclic and strain controlled monotonic triaxial tests was carried out on sand-silt mixture specimens of 50 mm diameter and 100 mm height with varying silt content. In these experiments, various measures of sample density was adopted through different approaches such as constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio approach, and constant interfine void ratio approach. Also the effect of relative density and confining pressure on these strengths was studied. It is observed that the limiting fines content and the relative density of a specimen play the key role in deciding the cyclic and monotonic resistance of sand-silt mixtures when studied through any approach. For any silt content with relative density more than 70%, cyclic and monotonic resistances are observed to be independent of silt content. When the undrained cyclic strengths of these specimens are plotted against their respective undrained monotonic peak strengths, it is observed that there exists a definite exponential relationship between the two with an excellent correlation coefficient. An expression is proposed in this regard to help engineers assess the cyclic strength of sand-silt mixtures from monotonic test results.     

Undrained monotonic response of sand–silt mixtures: effect of nonplastic fines

The effect of non-plastic fines (silt) on the undrained monotonic response of saturated and isotropically consolidated sand specimens prepared to various measures of their density was studied in detail through various approaches namely gross void ratio approach, relative density approach, sand skeleton void ratio approach, and interfine void ratio approach. Specimens of 50 mm in diameter and 100 mm in height were tested at a rate of loading of 0.6 mm/min for this purpose. The limiting silt content and the relative density of a specimen were found to influence the undrained monotonic response of sand–silt mixtures to a great extent. Undrained monotonic response was observed to be independent of silt content at very high relative densities; however the presence of fines significantly influenced this response of loose to medium dense specimens. Individual and combined analyses of undrained monotonic peak strengths which are closely related to the liquefaction related problems have been done in this paper to assess the variation patterns.     

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.     

Three-dimensional DEM modeling of the stress–strain behavior for the gap-graded soils subjected to internal erosion

In this work, 3D discrete element method modeling of drained shearing tests with gap-graded soils after internal erosion is carried out based on published experimental results. The erosion in the model is achieved by randomly deleting fine particles, mimicking the salt dissolving process in the experiments. The present model successfully simulates the stress–strain behavior of the physical test by employing the roll resistance and lateral membrane. The case without erosion shows a strain-softening and dilative response, while strain-hardening and contractive response starts to occur as the degree of erosion increases. The dilative to contractive transition is mainly caused by the increase in void ratio due to the loss of fine particles. The change from dilative behavior to contractive behavior is more abrupt for the specimen with larger fine particle percentage because the soil skeleton is mainly controlled by the fine particles instead of by the coarse soil particles. The transition from “fines in sand” to “sand in fines” might be associated with the rapid increasing in the contacts associated with fine particles in the specimen as the percentage of fine content increases. The erosion scenario based on the hydraulic gradient is also modeled by deleting the fine particles based on the ranking of the contact force. Compared with the scenario based on random deletion, the remaining fine particles for the erosion scenario based on the ranking of contact force are more dispersedly distributed, which might benefit the small strain stiffness but result in a smaller strength. This work provides some insights for better understanding the mechanism behind the internal erosion and the associated stress–strain behavior of soil. The gradient of the critical state line increases with more loss of fine particles denoting that the fine particles are helpful for holding the structure of the soils from larger deformation.

     

饱和粉砂不稳定性的试验研究

通过对净砂和级配良好粉砂(含10 %粉土)进行一系列三轴固结不排水试验（CU）,研究了粉土、孔隙比和围压对饱和粉砂不稳定性的影响。试验结果表明,净砂与粉砂在不排水剪切条件下均会出现应变软化现象（即不稳定性）。同一围压下脆性指数(IB)随孔隙比增加,但不稳定线的应力比随孔隙比增加而减小。引用等效粒间孔隙比(ege)后,净砂和粉砂在ege-ln p?平面上拥有基本相同的临界状态线。在临界状态理论及等效粒间孔隙比的基础上,提出在同一修正状态参数(?ge)下净砂和级配良好粉砂有相似的不稳定性。     

Stress–dilatancy based modelling of granular materials and extensions to soils with crushable grains

Stress–dilatancy relations have played a crucial role in the understanding of the mechanical behaviour of soils and in the development of realistic constitutive models for their response. Recent investigations on the mechanical behaviour of materials with crushable grains have called into question the validity of classical relations such as those used in critical state soil mechanics. In this paper, a method to construct thermodynamically consistent (isotropic, three‐invariant) elasto‐plastic models based on a given stress–dilatancy relation is discussed. Extensions to cover the case of granular materials with crushable grains are also presented, based on the interpretation of some classical model parameters (e.g. the stress ratio at critical state) as internal variables that evolve according to suitable hardening laws. Copyright © 2004 John Wiley & Sons, Ltd.     

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...     

Comprehensive laboratory study on stress–strain of granular soils at constant global void ratio: combined effects of fabrics and silt content

The published literature has revealed conflicting results regarding the effect of low plastic fines fraction (I_p?≤?5.0%) on the mechanical behavior of sandy soils. For this reason, the use of different sample initial structures as (initial relative density approach, global void ratio index approach, etc.) could explain these different mechanical responses of granular materials. Thus, it is necessary to evaluate the quantitative aspect of the low plastic fines effects on the undrained monotonic response of sand-silt mixtures using the global void ratio approach. To achieve this goal, an experimental testing program through controlled monotonic triaxial tests was carried out on reconstituted saturated Chlef sand containing from 0 to 50% silt with an interval of 10% at three global void ratios (e?=?0.64, 0.66 and 0.68) and subjected to constant confining pressure (σ'₃?=?100 kPa). The different samples were reconstituted using two different preparation techniques: DFP and MT. The obtained results show that the low plastic fines content appears as a very relevant parameter in the characterization of the mechanical response of sand-silt mixture samples reconstituted at constant global void ratios, where the steady state shear strength and instability shear strength decreased with the increase in low plastic fines content up to the limiting fines contents (F_c?=?40% and F_c?=?10%) considering both studied initial structures (Dry funnel pluviation and Moist tamping), respectively. Beyond these thresholds fines contents, a reverse trend was observed for all parameters under study. Moreover, the test results indicate that the brittleness index, flow potential (V_f), friction index, equivalent void ratio (e*) and equivalent relative density (Dr*) could be considered as reliable parameters in the prediction of the mechanical behavior of the silty sand soils under study.

     

Undrained Cyclic Pore Pressure Response of Sand–Silt Mixtures: Effect of Nonplastic Fines and Other Parameters

Literature regarding the pore pressure generation characteristics and in turn the cyclic resistance behaviour of silty sand deposits is confusing. In an attempt to clarify the effect of nonplastic fines on undrained cyclic pore pressure response of sand–silt mixtures, an experimental programme utilising around 289 stress-controlled cyclic triaxial tests on specimens of size 50 mm diameter and 100 mm height was carried out at a frequency of 0.1 Hz. Specimens were prepared to various measures of density through constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio approach, and constant interfine void ratio approach to study the effect of nonplastic fines on pore pressure response of sand–silt mixtures. The effect of relative density, confining pressure as well as the frequency and magnitude of cyclic loading was also studied. It was observed that the pore pressure response is greatly influenced by the limiting silt content and the relative density of a specimen corresponding to any approach. The influence of other parameters such as relative density, confining pressure and magnitude of cyclic loading was as usual but an increase in frequency of cyclic loading was seen to generate excess pore pressure at a higher rate indicating an impact load type of behaviour at higher frequency. Utilising the entire test results over a wide range of parameters a new pore pressure band for sand–silt mixtures in line with Lee and Albaisa (1974) has been proposed. Similarly another pore pressure band corresponding to 10th cycle of loading as suggested by Dobry (1985) and up to a shear strain of around 25% has been proposed. These two bands can readily be used by researchers and field engineers to readily assess the pore pressure response of sand–silt mixtures.     

Experimental Study on Mechanical Behavior of Unsaturated Silty Sand in Constant Equivalent Granular Void Ratio

In this paper effect of fine mineralogy on mechanical behavior of unsaturated silty sand in different fine contents and divers confining pressure has been studied. All samples were molded with constant equivalent granular void ratio well-known parameter already proposed for characterizing silty sand behavior in saturated state. This manner of study allow to investigate also the validity of equivalent granular void ratio concept in unsaturated state. For this purpose, a series of triaxial tests were performed on the sand specimens with different percentages of silt in the undrained saturated (CU) and unsaturated (CW) conditions. The results showed that the material types and aggregate distribution of the fines have enormous effects on the silty sand behavior. In addition, the shear strength in the unsaturated specimens changed as a function of the initial applied matric suction. A fewer performance of equivalent intergranular void ratio in the case of unsaturated state in comparison of saturated states was observed.

     

A comparative study of suction stress between sand and silt under unsaturated conditions

The purpose of this study is to estimate and compare suction stress between sand and silt sampled from the coast of Korea. The water content and matric suction of sand (Joomunjin) and silt (Saemangeum) were first examined using an automated soil–water characteristic curve (SWCC) apparatus based on the axis translation technique. SWCCs were then estimated from the test results using the van Genuchten (1980) model. At equal matric suction, the corresponding water content of silt was higher than that of sand. Moreover, the saturated water content and air-entry value (AEV) of silt were larger than those of sand. Using the fitting SWCC parameters, suction stress characteristic curves (SSCCs) were estimated according to the method proposed by Lu and Likos (2006). The SSCC behavior for sand and silt was different and significantly depended on the material properties, particularly pore size and pore size distribution. For sand, the suction stress exhibited rapid variation with changes in matric suction, but for silt, the suction stress approached a constant value as the matric suction increased. In addition, when the matric suction was smaller than the AEV of soil, the suction stress was equal to the magnitude of the matric suction. In contrast, when the matric suction exceeded the AEV of soil, suction stress had a nonlinear shape with respect to the matric suction.     

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。     

Equivalent granular state parameter and undrained behaviour of sand�Cfines mixtures

State parameter defined using void ratio, e, and the steady-state line has been shown to be effective in predicting the undrained behaviour of sand. However, steady-state line for sand with fines is dependent on fines content. To overcome this problem, the concept of equivalent granular void ratio, e*, has been well investigated. However, the conversion from e to e* has been essentially a back-analysis process. A methodology for converting e to e* without the need of a back-analysis process was first presented. The concept of equivalent granular state parameter, ψ*, defined in terms of e*, and equivalent granular steady-state line was then developed. An extensive experimental study was conducted to investigate whether ψ* can capture the effects of fines content, and thus can be used to correlate undrained behaviour of sand–fines mixtures without the need of separately considering the effects of fines content. This study suggested that the effective stress path and deviatoric stress–strain responses in undrained shearing can be correlated with the ψ* value at the start of undrained shearing irrespective of fines content.