A Simplified Silty Sand Model

Archives of Hydro-Engineering and Environmental Mechanics, 2012

The plane strain behaviour of sand is studied using, previously proposed, incremental model describing its pre-failure deformations. Original model has been formulated for the tri-axial configuration, and then generalized for 3D conditions. This 3D model was subsequently adapted to study deformations of sand in the plane strain conditions, in the x1; x3 plane. There are three unknowns in such a configuration, namely the principal strains "1; "3 and the principal stress σ2. Respective equations were derived, and then applied to study deformations of sand for chosen stress paths. The governing incremental equations were integrated numerically, and it was shown, for some loading paths, that σ2 epends linearly on the other principal stresses, so introduction of apparent Poisson’s ratio is justified, as a kind of approximation. Subsequent analysis of deformations of sand was performed using this concept, as well as using full system of governing equations.

EPJ Web of Conferences

This paper presents an experimental study of natural Camargue silty sand mechanical behavior. The analysis of test results was based on the use of the equivalent intergranular void ratio instead of the global void ratio. The calculation of equivalent intergranular void ratio requires the determination of parameter b which represents, physically, the fraction of active fines participating on the chain forces network, hence on the soil strength. A new formula for determining the parameter b using an approach based on the coordination number distribution and probability calculation is proposed. The validation of the developed relationship was done through back-analysis of published datasets in literature on the effect of fines content on silty sand behavior. It is shown that the equivalent intergranular void ratio calculated with the b value obtained by the new formula is able to provide a strong correlation, not only at the critical state, but also at the onset of instability of various silty sands, in different terms such as peak deviator stress, peak stress ratio or cyclic resistance. Therefore, it is suggested that the use of the equivalent void ratio concept and the new formula that we have put forward for b determination is highly desirable in predicting of silty sand behavior.

The use of the Structured Cam Clay model to describe unstable behaviour of sand is investigated. Based on the critical state line of sand in e-pʹ′ space, the model can be employed successfully to predict the occurrence of instability and to provide quantitative representation of the softening of soil associated with instability.

Springer Series in Geomechanics and Geoengineering, 2013

Experimental studies have revealed that plastic strains change the elastic properties of granular soils. This observation is an elastic-plastic coupling phenomenon which is considered here through the dependence of a free energy function to a proper hardening parameter. It is shown that implementation of the resulted hyperelastic model in a critical state bounding surface plasticity platform leads to more reasonable simulations.

Japanese Geotechnical Society Special Publication, 2019

The shear strength equations, based on two stress state variables, proposed by Fredlund et al. (1978) and Öberg and Sällfors (1997), respectively were compared at peak and critical states. Data of a set of drained direct shear tests for a fine sand tested under saturated and unsaturated conditions were used for the comparison. The comparison between the two equations suggests that suction contributing to the effective stress for sand used is often much smaller than predicted by the (sSr) term used in effective stress expressions of Öberg and Sällfors (1997). It is suggested that effective stress in unsaturated soils should be related to soil fabric and microstructure which caused an increase of shear strength parameters. Clusters or assemblages of particles, held by menisci, increased dilation and frictional resistance during shearing related to the different phases of water retention behaviour.

E3S Web of Conferences

The stress-strain response of compacted silty sand with over-consolidated stress history often exhibit distinct peak stress before reaching the critical stress type of response when subjected to suction-controlled triaxial shearing. Such heavily consolidated soil also tends to simultaneously manifest initial compression which transitions into dilational type volumetric response. Modelling such strain-softening type response, especially emulating the smooth transition from peak to critical state is a challenge. In this paper a previously developed generalized plasticity constitutive model, called MPZ (Modified Pastor-Zienkiewicz) is fine-tuned and calibrated using a set of suction-controlled consolidated drained triaxial tests conducted on compacted silty sand specimens. Firstly, the saturated and unsaturated silty sand characteristics and the experimental test program are briefly introduced. Secondly, the calibration of each component of the constitutive model, namely critical state...

Computers and Geotechnics, 2010

A simple incremental model describing the pre-failure behaviour of granular soils is presented. The model describes both the dry/fully drained and undrained response. It takes into account an initial anisotropy of soil and an initial state defined as either contractive or dilative. A physically sound definition of loading/unloading is assumed, which differs from elasto-plastic approaches. The model is based on extensive empirical data and gives predictions conformable with experimental results. It also describes prefailure instabilities of granular soils, both dry/fully drained and undrained. The Hill's criterion was used to examine stability. It was shown that this condition can be formulated either in terms of the effective stresses or by the total stresses. In the extreme cases of either dry/fully drained or undrained conditions, these alternative formulations are equivalent. This is not so in the case of partial drainage of pore water and associated volumetric deformations as well as pore pressure changes. The model describes the prefailure instabilities well, and additionally allows for analytical derivation of the instability line. It was shown that the second order work, appearing in the Hill's condition, is equivalent to the entropy source.

Science in China Series E: Technological Sciences, 2008

A new double hardening elasto-plastic model is proposed in this paper based on the existing unified hardening model (UH model). By assuming that there is part coupling effect between the plastic volumetric strain and plastic shear strain, hardening parameters consisting of a coupled and an uncoupled components are adopted in this model. A unique feature of this model is that it can describe not only the conventional drained and undrained behaviors of soil, but also the stress-strain relationships of soil under partially drained conditions which can be volumetric compression or dilation. Adopting the asymptotic state concept, simple equations for estimating the limiting stress ratio under undrained or earth pressure at rest (i.e. K 0 ) conditions are derived. The new model is relatively simple to be adopted in practice for two reasons. First, the same soil parameters as in Cam-clay model are used except the addition of one extra parameter, the stress ratio at the characteristic state. Second, all the parameters can be determined using conventional triaxial compression tests.

International Journal for Numerical and Analytical Methods in Geomechanics, 2002

This paper evaluates the performance of a generalized effective stress soil model for predicting the rate independent behaviour of freshly deposited sands, while a companion paper describes model capabilities for clays and silts. Most material parameters can be obtained from standard laboratory data, including hydrostatic or one-dimensional compression, drained and undrained triaxial shear testing. A compilation of data on compression behaviour allows for estimation of compression parameters when this type of data is not available. Extensive comparisons of model predictions with measured data from undrained triaxial shear tests shows that the model gives excellent predictions of the transition from dilative to contractive shear response as the confining pressure and/or the initial formation void ratio increases. A parametric study of drained response shows that the model describes realistically the variation of peak friction angle and dilation rate as a function of confining pressure and density when compared with an empirical correlation valid for many sands. The proposed formulation predicts a unique critical state locus for both drained and undrained triaxial testing which is non-linear over the entire range of stresses and is in excellent agreement with recent experimental data. Overall, the model provides excellent predictions of the stress-strain-strength relationships over a wide range of confining pressures and formation densities.

Soil and Tillage Research, 2002

A validation of an extended critical state model was conducted using a heavy agricultural clay soil from Quebec Province, eastern Canada. The numerical model used for this purpose simulated the behaviour of Sainte-Rosalie clay under triaxial compression and one-dimensional compression loading paths. Under triaxial compression, simulations were done for three soil suctions: 0, 300, and 500 kPa. Under one-dimensional compression, simulations were done for suctions 0 and 300 kPa. The model used five parameters while simulating soil mechanical behaviour under saturated condition. Three additional parameters were required for simulating soil mechanical behaviour under unsaturated condition. Results of simulations were then compared to those of laboratory tests. Experimental data compared well with simulation results. It was found that the model reasonably predicted soil behaviour under both saturated and unsaturated conditions. However, some discrepancies arose in both soil conditions: the model predicted an earlier rupture in each case of triaxial compression and it failed to reach the smallest void ratio obtained during one-dimensional compression. These small divergences were attributed to the fact that numerical development of the tested model was based on densified sand behaviour.