Hostun RF Research Articles

Liquefaction of unsaturated soils- volume change and residual shear strength

This study presents a series of triaxial tests to highlight the effect of saturation degree ranging from 79.8% to 100% on the behavior of sand subjected to cyclic loading, the development of pore water pressure increment, the volumetric strain at liquefaction state, and the behavior of sand subjected to monotonic loading after liquefaction. The tested sand was Hostun RF sand, fine clear sand that has been widely used in laboratory testing in France. The results show that there is a difference between the behavior of samples having saturation degrees higher and lower than 95%. Besides, the development of pore water pressure increment is affected significantly by the value of cyclic stress ratio (CSR) and saturation degree. The Saturation degree also affects the appearance of criteria of liquefaction as Excess Pore-water Pressure (EPP) and Double Amplitude of axial strain (DA). The analysis of the effect of saturation degree on sample volumetric strain exhibits nonlinear liquefaction criteria for a given maximum value of CSR. The monotonic loading tests after liquefaction showed a small difference between the monotonic compressed behavior of the samples before and after liquefaction.

A multiaxial constitutive model for fibre-reinforced sand

Fibre orientation in fibre-reinforced sand (FRS) is highly anisotropic due to compaction during sample preparation or field construction. This makes the mechanical behaviour of FRS, such as strength and dilatancy, highly dependent on the strain increment direction. While constitutive models that are able to capture such anisotropic behaviour of FRS have been proposed for conventional triaxial compression and extension conditions only, this paper proposes for the first time a full anisotropic model for FRS formulated in the general multiaxial stress space. The new model is developed based on the assumption that the strain of FRS is dependent on the deformation of the sand skeleton. In turn, the fibre presence affects the void ratio and effective stress of the soil skeleton, which governs the elastic properties, dilatancy and plastic hardening of the FRS. The effect of anisotropic fibre orientation on the FRS behaviour is considered through an anisotropic variable which characterises the relative orientation between the loading direction tensor and fibre orientation tensor. The model does not require direct measurement of the stress–strain relationship of individual fibres. Although the model is for FRS under multiaxial loading conditions, the parameters associated with the fibre inclusion can be determined based on triaxial test results, provided that the orientation of fibres is known. The model has been used to predict the stress–strain relationship of fibre-reinforced Hostun RF (S28) sand under multiaxial loading conditions. Satisfactory agreement between the experimental data and model predictions is observed.

Parameter Evaluation of Exponential-Form Critical State Line of a State-Dependent Sand Constitutive Model

In sand constitutive models, it is of cardinal importance to consider a state parameter to distinguish the real dilatancy for cohesionless soils (sand), which is different from cohesive soils (clay). Thus, one of the key issues in simulating the sand behaviour is the better representation and parameter calibration of critical state line (CSL) for estimating contraction in loose state and dilatancy in dense state, respectively. For this purpose, a new exponential form for CSL with two model constants a and b has been presented in the literature. This paper provides a valuable insight into the two model constants, controlling the shape of the critical state line by simulating a uniform quartz reference sand (Hostun RF) in loose and dense states under undrained triaxial conditions. It can be concluded that the liquefaction behaviour in loose state is fundamentally affected by even a minor variation in model constant a , but insensitive to model constant b . Moreover, the linear fitting calibration of CSL recommended in the literature is complicated in consideration of the non-unified critical state line. Thus, the maximum void ratio in the natural state could be considered as a comparison basis on which to evaluate the liquefaction potential as an alternative. The numerical results showed good agreement with real experimental data. However, in dense state, the dilatant behaviour of sand was found to be mainly controlled by model parameter b . In addition, the influence of a non-unified critical state under various confining pressures on the determination of b should not be neglected. With the correction of b , the numerical results were found to be consistent with the experimental data concerning Hostun RF sand.

Stress–dilatancy relationship for fiber-reinforced sand and its modeling

The stress–dilatancy relationship could be employed as the foundation to develop a constitutive model for polypropylene fiber-reinforced (PFR) soils. In this study, a number of triaxial compression tests were carried out to investigate the effect of uniform distributed fiber reinforcements on the stress dilatancy relationship of Nanjing sand. A new parameter representing the increase in the effective confining stress $$\sigma _{\rm FR}$$ was introduced to describe the stress–dilatancy of PFR sand, and a new stress–dilatancy relationship was proposed for PFR sand based on Rowe’s stress–dilatancy equation for granular materials. Moreover, the stress–dilatancy relationships in the conventional triaxial compression, extension and plane stain conditions were discussed in this paper. The stress–dilatancy relationship is validated against a series of triaxial tests on Nanjing sand and Hostun RF sand mixed with discrete polypropylene fibers. It is shown that the predicted results are in a good agreement with the experimental results.

Influence of relative density on the cyclic shear strength of sands

This paper presents a laboratory study of the influence of relative density on the liquefaction potential of a soil. The study is based on undrained triaxial tests that were performed on samples with relative density Id = 0.15, 0.5 and 0.65. The article is composed of three parts. First, we present the materials and characteristics of the studied sands. the second part deals with the procedure and the device used. The third part studies the influence of the relative density on the liquefaction potential of the three sands (Hostun Rf, Chlef and Rass). This study also makes it possible to explore the influence of granulometry on the liquefaction potential. The results of the tests show that concordant results have been obtained which clearly show that the increase of the relative density leads to a significant improvement in the resistance to liquefaction of the sands. This effect is very significant when the initial relative density Id = 0.50 to Id = 0.65.

Influence of relative density on the cyclic shear strength of sands

This paper presents a laboratory study of the influence of relative density on the liquefaction potential of a soil. The study is based on undrained triaxial tests that were performed on samples with relative density Id = 0.15, 0.5 and 0.65. The article is composed of three parts. First, we present the materials and characteristics of the studied sands. the second part deals with the procedure and the device used. The third part studies the influence of the relative density on the liquefaction potential of the three sands (Hostun Rf, Chlef and Rass). This study also makes it possible to explore the influence of granulometry on the liquefaction potential. The results of the tests show that concordant results have been obtained which clearly show that the increase of the relative density leads to a significant improvement in the resistance to liquefaction of the sands. This effect is very significant when the initial relative density Id = 0.50 to Id = 0.65.

Analytical definition of collapse surface in multiaxial space as a criterion for flow liquefaction occurrence

Prediction of flow liquefaction occurrence in multiaxial stress space is presented in this paper. The closed-form relation analytically attained by means of Dafalias-Manzari constitutive equations gives the flow stress ratio as a function of model parameters, state parameter, direction of undrained loading and especially initial condition of consolidation. These proposed flow condition can predict the occurrence of flow liquefaction initiated by any loadings whether the soil is consolidated isotropically or anisotropically. Suggested predictions are compared with the results of triaxial tests performed on Hostun RF sand. The results show that increasing consolidation stress ratio leads to flow stress ratio increase.

Characteristics of Cyclic Shear Behavior of Sandy Soils: A Laboratory Study

This paper presents a laboratory study of the influence of relative density on the liquefaction potential of a sandy soil using the triaxial apparatus. The study is based on undrained triaxial tests performed on samples at an initial relative density RD = 15, 50 and 65 % under a confining pressure of 100 kPa using a dry deposition method. Samples were subjected to quasi-static undrained cyclic tests. The paper is composed of three parts. In the first part the used materials and their characteristics are presented. The second part is devoted to the experimental procedures and the device used. The third part investigates the influence of relative density on the liquefaction potential of the three sands (Hostun Rf, Chlef and Rass). This study also explores the influence of particle size on the liquefaction potential. The test results indicate that consistent results were obtained and show clearly that increasing the relative density leads to an important improvement in the liquefaction resistance of sand. This effect is very pronounced when the initial relative density increases from 50 to 65 %.

Saturation Effect on Behaviour of Sandy Soil Under Monotonic and Cyclic Loading: A Laboratory Investigation

This paper presents a laboratory study of the influence of saturation degree on the liquefaction potential of sand. The study is based on drained, undrained monotonic and cyclic triaxial tests. Tests were conducted on a medium dense Hostun RF sand (France) with an initial relative density Dr = 4, 50, 64 and 78 % and the coefficient of Skempton ranging between 0.95 (saturated) and 0.25. For all Skempton’s coefficient B values, the tests were carried out for different loading levels. The monotonic tests show that an increase in the Skempton’s coefficient B leads to a reduction of liquefaction resistance. The decrement of the coefficient of saturation B does not affect the instability and steady state lines. The test results indicate that the liquefaction resistance increases with the decrease of Skempton’s coefficient B. For the high level of loading (RCC = 0.5), the number required to cause liquefaction varied little with the Skempton’s coefficient B (Nc = 9, 10 and 11 for B = 0.67, 0.36 and 0.25 respectively). For the low level (RCC < 0.40), the number of cycles needed for liquefaction increases susceptibly with the reduction of the coefficient of Skempton (B). A simple mathematical relation has been suggested to correlate the liquefaction resistance with the coefficient of Skempton B.

Quantifying Morphology of Sands Using 3D Imaging

AbstractParticle morphology plays a significant role in influencing engineering behavior of granular materials. Surface texture, roundness, and sphericity represent distinct multiscale measures needed to fully describe particle morphology. Most studies reported in the literature rely on two-dimensional (2D) projected images of particles with a few three-dimensional (3D) images that mostly focused on relatively large-size aggregate samples. In this paper, 3D synchrotron microcomputed tomography (SMT) was used to acquire high-resolution images of glass beads, F-35 Ottawa sand, #1 dry glass sand, GS#40 Columbia sand, Toyoura sand, and Hostun RF sand. New roundness and sphericity indexes are proposed and calculated for the samples based on 3D measurements of surface area, volume, and three orthogonal diameters of particles. In addition, the surface texture of particles were measured using optical interferometry technique. The measurements reported in this paper can serve as a good source for other researchers...

On the undrained strain-induced anisotropy of loose sand

An experimental study was carried out to investigate the effects of previous deviatoric strain histories on the undrained behaviour of loose and saturated Toyoura sand and compared with known results of Hostun RF sand. From an initial isotropic stress state, recent deviatoric strain histories in the compression side of the triaxial plane were generated by a standard drained presheared cycle up to a specified mobilized stress ratio. Mainly, the fully liquefied, contractive, unstable and softening behaviour of loose sand was progressively transformed into the non-liquefied, dilative, fully stable and hardening behaviour of dense-like sand, while remaining within a narrow range of loose density. The paper validates and extends the current understanding of strain-induced anisotropy of loose sand. New experimental data support the directional dependency of the instability cone on the stress increment direction, suggest the bifurcation characteristics of loose sand and evidence the important role of past deviatoric strain histories.

Probing into the strain induced anisotropy of Hostun RF loose sand

Several recent linear drained preloading histories with fixed direction were especially designed to study the effects of strain induced anisotropy of loose Hostun RF sand in the compression side of the classical triaxial plane. Nearly identical void ratio and a same initial isotropic stress state prior to the final undrained shearing in compression are the requirements of the experimental program to take into account only the deviatoric strain histories. The effects of previous deviatoric strain histories on the undrained response of loose Hostun RF sand are identified: mainly the progressive transformation of a contractive and unstable behaviour of very loose sand into a dilative and stable behaviour of dense-like sand by previous linear drained history, while remaining in the same state of loose density. Experimental data evidence the directional dependency of the initial gradient of the effective stress paths, independent of the length of the approaching linear stress paths; the large common non linear effective stress response up to the deviatoric stress peak; the progressive appearance of the dilatancy domain and the unexpected evolution of the undrained behaviour of loose and presheared sand. The paper provides new insights into the mechanisms of strain induced anisotropy of loose sand created by simple linear triaxial stress paths from an isotropic stress state.

Hypoplastic model for sands with loading surface

Although the hypoplastic models for sands have exhibited good predictive capability in monotonic loading, they are not able to reproduce memory effects and predict excessive plastic accumulation under cyclic loading. To overcome these issues, a loading surface has been incorporated into a hypoplastic model. This surface is capped and has two hardening variables. Notions from the bounding surface plasticity were borrowed in order to formulate the hardening functions. With this novel model, some salient features can be described: the model can account for the accumulation of plastic deformation, a memory effect is provided by the new surface, and stress-induced anisotropy effects observed in sands are successfully simulated. A short calibration guide of the parameters is given, and some simulations for Hostun RF loose sand and Toyoura sand are presented.

Effects of Previous Deviatoric Strain Histories on the Undrained Behaviour of Hostun RF Loose Sand

The effects of previous deviatoric strain histories on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, strain histories are generated by isotropic consolidation followed by standard drained triaxial preshear or presheared cycle, either in compression or in extension, up to a desired value of axial strain or mobilized stress ratio. Deviatoric strain histories are achieved by having nearly the same void ratio at the beginning of the undrained shearing for all tested samples. Subsequent undrained behaviour in triaxial compression and extension is analyzed in detail. Previous deviatoric strain histories can progressively transform the compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand, while being loose. Experiments show a common response induced by recent strain histories in terms of effective stress paths, independently of the axial strain attained during the drained presheared cycle, a unique initial gradient of the effective stress paths, a progressive appearance of dilatancy, an evolution the undrained behaviour and a systematic partial static liquefaction associated with softer behaviour when sheared in the opposite direction of the initial presheared direction. This paper offers a comprehensive understanding of the mechanisms of a specialized induced anisotropy created by simple linear stress paths in the classical triaxial plane.

Estudio en laboratorio sobre licuefacción de arena parcialmente saturada

This experimental study was designed to assess the effects of soil water saturation on the liquefaction of Hostun RF sand. Cyclic undrained triaxial tests were conducted at different soil saturation levels, as given by Skempton’s coefficient, and liquefaction potential curves constructed for each value of this coefficient. Our findings indicate that a lower soil saturation level results in the increased resistance of the sand to liquefaction, in agreement with the tendency observed in other sands. In addition, the variation in sand resistance to liquefaction produced with Skempton’s coefficient was found to be consistent with the semi-empirical relation proposed by Yang et al. (2004).

Performance of mixed and enhanced finite elements for strain localization in hypoplasticity

SUMMARYDisplacement and mixed finite element formulations of shear localization in materials are presented. The formulations are based on hypoplastic constitutive laws for soils and the mixed enhanced treatment involving displacement, strain and stress rates as independently varied fields. Included in these formulations are the standard displacement method, the three‐field mixed formulation, the enhanced assumed strain method and the mixed enhanced strain method. Several numerical examples demonstrating the capability and performance of the different finite element formulations are presented. The numerical results are compared with available experimental data for Hostun RF sand and numerical results for Karlsruhe sand on biaxial tests. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Exploring the undrained induced anisotropy of Hostun RF loose sand

The effects of recent history on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, recent histories are generated by isotropic consolidation followed by standard drained triaxial preshear in compression, up to a desired value of axial strain or mobilized stress ratio, and unloading to an initial stress ratio. Subsequent undrained behaviour in triaxial compression is analysed in detail. This paper contributes to the traits explaining the progressive transformation of a compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand by previous history, while remaining in the same state of loose density. Experiments show a large pseudo-elastic domain induced by recent history in terms of effective stress paths, function of the initially mobilized stress ratio level, a unique initial gradient of the effective stress paths depending on the stress ratio at the beginning of the undrained shearing, a progressive appearance of dilatancy and a surprising evolution the undrained behaviour of loose sand. Experimental results evidence the important role of the recent deviatoric strain history, from any initial isotropic or anisotropic stress state. This paper also offers a comprehensive understanding of the history mechanisms created by simple linear stress paths with fixed direction in the classical triaxial plane.

Time-Dependent Behaviour and Static Liquefaction Phenomenon of Sand

Recent laboratory and field experiments have confirmed that sand does indeed exhibit time-dependent behaviour. Based on these findings, it was considered necessary to revisit some of the published experimental results on the static liquefaction phenomenon of loose anisotropic Hostun RF sand. Time-dependency might have had a significant influence on the observed undrained response of anisotropic consolidated sand specimens. Specific triaxial tests have been performed and a qualitative analysis is presented in this paper. It is shown that, despite the differences on the anisotropic consolidation path employed, different specimens show qualitatively identical undrained responses if creep periods are performed at identical test stages or if the anisotropic consolidation takes place very slowly. With time, the undrained stiffness and strength are considerably improved and this may explain why the static liquefaction phenomenon is not as common in practice as could be predicted based on an instability line concept. Whereas the original instability line concept was developed independently of time-dependency, in field situations, the liquefaction resistance of the sand can increase with time.

Finite‐element formulations of shear localization in granular materials

AbstractDisplacement and mixed finite element formulations of shear localization in granular materials are presented. The formulations are based on hypoplastic constitutive laws for soils and the mixed‐enhanced treatment involving displacement, strain and stress rates as independently varied fields. Included in these formulations are the standard displacement method, the three‐field mixed formulation, the method of incompatible modes, the enhanced assumed strain method and the mixed enhanced strain method. Several numerical examples demonstrating the capability and performance of the different finite element formulations are presented. The numerical results are compared with available experimental data of Hostun RF sand and numerical results of Karlsruhe sand on biaxial tests. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Failure and Dilatancy Properties of Sand at Relatively Low Stresses

Analysis of geotechnical problems concerned by low confinement such as design of shallow foundations and analysis of slope stability and soil liquefaction requires modeling of the soil behavior at low stresses. This note includes a laboratory study of the behavior of Hostun RF sand at low cell pressure (20–50 kPa) . Isotropic and triaxial compression drained tests were performed. Drained tests show that both failure and dilatancy angles at low stresses are stress dependent. The contractive/dilative phase transition is observed for loose sand, which may result from the overconsolidated nature of this sand for low values of cell pressure.