Search

This article presents an experimental program under the constant volume condition to investigate the influence of over-consolidation on flow instability of clean and silty sand specimens prepared by a systematic increase in fines content (FC), where fines are particles smaller than 0.075 mm of non-plastic minerals. An extensive series of constant volume Direct Simple Shear (DSS) tests on normally and over-consolidated clean and silty Firoozkuh No. 161 sand specimens were carried out. The testing program covers a range of initial void ratios, effective vertical stresses, and FCs. Analysis of the laboratory data indicates that over-consolidation history leads to an increase in the slope of the instability line and a rise in normalized constant volume shear strength at the onset of flow instability. Furthermore, at the same normalized constant volume shear strength, brittleness index and maximum excess pore-water pressure ratio increase with over-consolidation. Even though an increase in FC leads to a family of non-unique critical state lines in the void ratio versus effective vertical stress plane, it is observed that the slope of the instability line and normalized constant volume shear strength vary almost uniquely with state parameter at the onset of flow instability irrespective of FC. Refereed/Peer-reviewed

Refereed/Peer-reviewed Tailings dams, which are storage structures for tailings, often are constructed using the tailings itself. A large number of tailings dam failures have been reported recently, especially due to static liquefaction, and can be attributed to a lack of knowledge of the geotechnical principles to be used and the lack of characterization of tailings as they continuously are produced and used as construction materials. Furthermore, the majority of past research work has been based on triaxial conditions even though it is believed that direct simple shear conditions may better represent the in situ condition of a tailings dam structure. This paper characterized the behavior of a tailings material from a copper tailings storage facility in Canada using the critical state soil mechanics framework using constant volume (undrained), drained, and constant shear drained shearing tests in direct simple shear space. The correlation between a modified state parameter (ψm) with instability state and the flow potential was investigated. Special attention was paid to the behavioral features of the material such as pore-water pressure generation, flow rule and dilatancy, phase transformation, characteristics state, and how they correlate with ψm. The differences in behavior from that of natural granular soils were highlighted.

Fabric anisotropy and fines content (fc) in sands modify significantly their mechanical behaviour, particularly as related to static liquefaction under undrained conditions. The fabric anisotropy aspect, expressed by means of an evolving fabric tensor F, has been addressed in the recently developed Anisotropic Critical State Theory (ACST) that enhances the two critical state conditions on stress ratio (η) and void ratio (e) of the classical Critical State Theory by an additional condition on the critical state value of F in relation to loading direction; based on this concept it introduces the dependence of dilatancy on fabric anisotropy. Various models have been successfully developed within this framework for clean sands. The fc aspect has been addressed within the Equivalent Granular State Theory (EGST) that substitutes a properly defined equivalent granular void ratio (e*) for e in any model for clean sand in order to obtain the response of sand with fines without any other change of the model structure and constants. Along these lines, a constitutive model is constructed in this work in order to address the effect of both F and fc simultaneously, by a combination of these two powerful propositions. The idea is very simple: one takes a constitutive model developed within ACST for clean sands, hence it accounts for fabric anisotropy, and substitutes the e* for e, as well as the derivative quantities of such substitution, hence it accounts for fc. The result yields a model that can simulate data on the undrained response for a range of fc, with emphasis on static liquefaction. It is shown that the inclusion of fabric anisotropy improves previous similar simulations made within the EGST but without the framework of ACST. Refereed/Peer-reviewed

Soil liquefaction or instability, one of the most catastrophic phenomena, has attracted significant research attention in recent years. The main cause of soil liquefaction or instability is the reduction in the effective stress in the soil due to the build-up of pore water pressure. Such a phenomenon has often been thought to be related to the undrained shearing of saturated or nearly saturated sandy soils. Notwithstanding, many researchers also reported soil instability under a drained condition due to the reduction in lateral stress. This condition is often referred to as the constant shear drained (CSD) condition, and it is not uncommon in nature, especially in a soil slope. Even though several catastrophic dam failures have been attributed to CSD failure, the failure mechanisms in CSD conditions are not well understood, e.g., how the volumetric strain or effective stress changes at the triggering of flow deformation. Researchers often consider the soil fabric to be one of the contributors to soil behaviour and use this parameter to explain the failure mechanism of soil. However, the soil fabric is difficult to measure in conventional laboratory tests. Due to that reason, a numerical approach capable of capturing the soil fabric, the discrete element method (DEM), is used to investigate the CSD shearing mechanism. A series of simulations on 3D assemblies of ellipsoid particles was conducted. The DEM specimens exhibited instability behaviour when the effective stress paths nearly reached the critical state line. It can be clearly observed that the axial and volumetric strains changed suddenly when the stress states were close to the critical state line. Alongside these micromechanical observations, the study also presents deeper insights into soil behaviour by relating the macro-observations to the micromechanical aspect of the soil. Refereed/Peer-reviewed

The critical state (CS), the anchor concept in the critical state soil mechanics (CSSM) framework, has been comprehensively evaluated in triaxial stress conditions, where all the principal stresses are known but the pure shear stresses in the shear directions are absent. Therefore, the evolution of first and second invariants of the stress tensor (σ′) i.e. the mean effective stress (p′) and deviatoric stress (q) along with volumetric strain ( e v ) is often used to identify the CS. However, the same does not apply for a direct simple shear (DSS) test, which is more representative of the real ground condition, as in most cases the minor principal stresses are not measured. This leads to the challenge of determining the CS line in the e - l o g ( p ' ) space and thus the concept of state parameter ( ψ ) cannot be used to characterise soil behaviour. Using DEM, this study evaluates CS for DSS, using evolution σ′ and e v , in the e - σ ' N space, compare its location with triaxial CS lines in the e - l o g ( p ' ) space. The CS line for DSS in e - σ ' N space was then used to define a modified state parameter, ψ , ( ψ m ). It was found that both ψ and ψ m showed good correlations with characteristic behaviour e.g. onset of liquefaction, phase transformation state, fabric anisotropy etc. Therefore, ψ m maybe a good alternative of ψ for DSS test where the minor principal effective stresses are not measured to define a CSL in e - l o g ( p ' ) space.

Loose sandy soil subjected to undrained shearing manifests deviatoric strain softening and such a behaviour has been referred to as static liquefaction. The triggering of static liquefaction corresponds to effective stress ratio, referred to as instability ratio, ɳɪS. The ɳɪS for isotopically consolidated specimen exhibits a unique relation with the state parameter, Ψ which has significant potential in estimating liquefaction risk both static and cyclic liquefaction (instability type). However, most natural soils are anisotropically, K ≠ 1 consolidated (most like Kₒ) and ɳɪS-Ψ relation for K = 1 consolidation is not directly transferable. This study presents discrete element method (DEM) simulations for undrained tri-axial condition for K = 1 and Kₒ consolidated specimen which exhibited distinctly different ɳɪS-Ψ relation. The fabric anisotropy (and invariant of deviatoric stress and fabric tensors) was found to be the key factor that changes those relations. The fabric anisotropy has to be considered in constitutive relation for modelling such behaviour.

An experimental study was carried out to investigate the static liquefaction behaviour of sand with a small amount of plastic and nonplastic fines. Five series of tests were conducted in drained and undrained conditions. The drained test results indicate not only that the failure line coincides with the critical state, but also that the development of volumetric strain during shearing was not sensitive to the initial confining pressure. In both isotropically and anisotropically consolidated undrained tests, a so-called “reverse behaviour” was consistently observed. The results were also interpreted in the critical state framework. The critical and steady state (CS/SS) data were found to trace along the same curve in e–log( p′) space, irrespective of the stress history and effective stress paths. A comparison between the isotropic consolidation line (ICL) and critical state (CS) curve showed that a small amount of fines can significantly change the shape and position of the ICL relative to the CS curve. Furthermore, the soil behaviour manifested in both drained and undrained shearing led to the development of a modified state parameter.

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 binary packing. This leads to a simple semi-empirical equation for predicting the value of b based on fines size and fines content. Published data appear to be in support of the proposed equation. A series of experiments were conducted on a specially designed sand–fines type to provide additional validation of the proposed equation and to reinforce the use of equivalent granular void ratio in a more generalized context.

The steady state line, SSL for sandy soil moves downward with increasing fines content, fc up to a limiting fc, then it moves upward with increasing fc. Thus, one cannot have a single framework within critical state soil mechanics, CSSM. However, equivalent granular void ratio, e* instead of e was proposed by many researchers to achieve a single framework based on the studies on a host sand with a single fines type. But, the effects of fines type have not been studied in details. This paper discusses the effects of fines type using triaxial tests on a host sand with two types of fines. The experimental results are then synthesized in terms of e*. The prediction approach is used to obtain e* instead of back analysis as reported in many publications. A single trend for SS data points is achieved in e*-log(p′) space irrespective of fines type. The effective stress path and stress-strain behaviour for clean sand and sand with two types of fines are consistent within a CSSM framework. Refereed/Peer-reviewed

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. Refereed/Peer-reviewed