Modeling the behavior of sand-fines mixtures using a critical state model (original) (raw)
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On equivalent granular void ratio and steady state behaviour of loose sand with fines
Canadian Geotechnical Journal, 2008
Void ratio has traditionally been used as a state variable for predicting the liquefaction behaviour of soils under the critical state soil mechanics framework. Recent publications show that void ratio may not be a good state variable for characterizing sand with fines. An alternative state variable referred to as the equivalent granular void ratio has been proposed to resolve this problem. To calculate this alternative state variable, a b parameter is needed. This b parameter represents the fraction of fines that actively participate in the force structure of the solid skeleton. However, predicting the value of b is problematic. Most, if not all, of the b values reported were determined by case-specific back-analysis, that is, the b value was selected so that a single correlation between equivalent granular void ratio and the measured steady state strength (or cyclic resistance) could be achieved. This paper examines the factors affecting the b value based on published work on bina...
Acta Geotechnica, 2019
The influence of non-plastic fines on undrained monotonic behaviour of sand was investigated over a wide range of fines content (FC = 0-40%), global void ratio and initial mean effective stress (p 0 0 = 100 kPa to 500 kPa), by performing triaxial compression tests with isotropic and anisotropic consolidation. For the sand-silt mixtures, steady-state line in the elog(p 0) space was found to be dependent on fines content with the existence of a limiting fines content, which defines the transition from a ''fines in sand'' to a ''sand in fines'' soil fabric. It is demonstrated that by means of the concept of equivalent granular void ratio, e*, all steady-state/critical state data points can be well described by a unique relationship in the e*-log(p 0) space called the equivalent steady-state line (EG-SSL), regardless of fines content. The equivalent granular state parameter (w*), defined in terms of e*, and the EG-SSL can be effectively adopted for predicting the undrained monotonic behaviour of sand-silt mixtures and the onset of static instability, irrespective of fines content and initial state of the sand-silt mixtures.
A unified critical state model has been developed for both clean sand and silty sand using the modified Cam-clay model (MCC). The main feature of the proposed model is a new critical state line equation in the e-ln(p) plane that is capable of handling both straight and curved test results. With this feature, the error in calculating plastic volumetric strain is eliminated in theory. Another crucial feature of the model is the transformed stress tensor based on the SMP (spatially mobilized plane) criterion, which takes into account the proper shear yield and failure of soil under three-dimensional stresses. Additionally, the proposed model applies the intergranular void ratio with the fines influence factor for silty sand. Only eight soil parameters are required for clean sand, and a total number of twelve soil parameters are needed for silty sand.
Exploring Static Instability Behavior of Sand-Fines Mixtures Based on Different State Variables
Lecture Notes in Civil Engineering, 2019
The paper presents a study aimed at evaluating the effect of nonplastic fines on undrained static liquefaction and instability behavior of silty sands considering two different state variables: (global) void ratio (e o) and equivalent granular void ratio (e*). The last parameter was introduced as an alternative index for mixed soils with a "sand-dominated" fabric. Undrained monotonic triaxial tests were carried out on Ticino sand-silt mixtures (FC = 0-30%) for a wide range of initial void ratios and mean effective confining stresses. The study provides evidence that, when the (global) void ratio is used as the comparison basis, the effect of fines is to cause an increase in the degree of contractiveness and higher susceptibility to collapse. Nevertheless, when the comparison is made at constant equivalent granular void ratio, undrained shear behavior and collapsibility of the silty sands are closely related to e*, which appears an effective index for unifying the response of mixed soils with varying fines content.
A comparison of critical state models for sand under conditions of axial symmetry
Géotechnique, 2010
Three constitutive models constructed within the spirit of critical state soil mechanics are summarised and compared under conditions of axial symmetry. The models have been calibrated using drained and undrained triaxial compression tests on Hostun sand. The quality of the individual simulations is strongly influenced by the shape chosen for the critical state line at high stresses. Stress response envelopes have been produced in order to contrast the incremental stiffnesses that the models predict for stress and strain paths other than those used for simulation. Plots of second-order work have also been presented for each model. Negative second-order work for certain strain paths indicates the potential for instability and bifurcation of response. Non-associated flow is a basic feature of these models and potentially unstable states were found to occur at mobilised friction angles lower than the peak of the stress–strain response. The hardening moduli of the models are similar, ev...
On intergranular void ratio of loose sand with small amount of fines
2007
Traditionally, void ratio has been used as an index to predict stress-strain behaviour of soil under steady state framework. Recent publications show that void ratio is not a proper index for gap graded sand with fines. Therefore, intergranular void ratio is proposed to resolve this problem. It is found that intergranular void ratio can predict stress-strain response of sand with low fines content (say 10%) only. To extend the applicability of intergranular void ratio to higher fines content, an additional parameter "b" has been introduced by some researchers in the calculation of intergranular void ratio. The parameter "b" represents the fraction of fines that actively take part on the parent sand force structure. However, there is a lack of theoretical basis for determining a b-value. Different "b" values have been selected by different researchers to enable the fitting of a single trend line, but this paper examines the physical significances of "b" by considering particles packing. This leads to a semi-empirical equation for predicting the value of "b" based on fines size and fines content. Published data from different researchers appears to be in support the proposed equation.
Modeling sand behavior using a critical state model implemented in FLAC
2009
A critical state constitutive model previously developed at the University of Alberta for the prediction of the behavior of sands was implemented in the computer program FLAC using its specific FISH programming code. Accuracy of the predictions obtained by the program was verified in the single-element scale by comparing predicted and observed results of triaxial tests on Toyoura and Syncrude sands, and in the full-scale level by modeling the field event of the CANLEX project, and comparing predictions with readings of field instrumentations. The current study showed that in order to obtain correct results from the analysis, it is important to use an appropriate soil constitutive model, to account for the simultaneous pore pressure generation and dissipation during loading, and to consider soil anisotropy. RÉSUMÉ Un état critique le modèle que constitutif s'est développé à l'université d'Alberta pour la prévision du comportement des sables a été ajouté au programme infor...
Peak and Critical State Conditions for Unsaturated Sand
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.