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Introduction The authors present an interesting and detailed examination of the failure mechanism of the Mount Polley Tailings Storage Facility (TSF). Indeed, such rigorous modelling seems likely to contribute to [...]

We thank the discussers for their interest in the work and their comments. First, the authors in general agree with the discussers in their view that further work is necessary, [...]

The most widely used in situ testing instrument for tailings storage facility (TSF) is the cone penetration test (CPT), which uses calibration chamber (CC) test data as the preferred method to correlate the soil's state with the CPT acquired data. Many, if not most, mine tailings are different from the soils historically used in CC studies (mainly sands). Moreover, these tests were conducted at denser states than typically found in TSFs. Additionally, CC tests cannot practically be performed for every new soil-specific project. For these reasons, practitioners usually perform numerical simulations to approximate these correlations. One of the most popular techniques for this is the spherical cavity expansion approach. This study compares novel small CC testing of two different silty mine tailings, and their respective spherical cavity expansion simulations. Since these simulations use the NorSand constitutive model, this study also presents the calibration process that includes the triaxial laboratory testing to calibrate the NorSand properties, and the corresponding iterative process to infer the additional plastic and elastic parameters of the model. Finally, an adjustment equation is introduced for the contractive state of these silty materials. Key words: CPT, silt, critical state, laboratory tests, numerical simulations L'instrument d'essai in situ le plus largement utilise pour les installations de stockage de residus (TSF) est l'essai de penetration au cone (CPT), qui utilise les donnees de l'essai en chambre d'etalonnage (CC) comme methode privilegiee pour correler l'etat du sol avec les donnees acquises par le CPT. Beaucoup, sinon la plupart, des residus miniers sont differents des sols historiquement utilises dans les etudes de CC (principalement des sables). De plus, ces essais ont ete realises dans des etats plus denses que ceux que l'on trouve generalement dans les TSF. De plus, les tests CC ne peuvent pratiquement pas etre realises pour chaque nouveau projet specifique au sol. Pour ces raisons, les praticiens effectuent generalement des simulations numeriques pour approximer ces correlations. L'une des techniques les plus populaires a cet egard est l'approche de l'expansion de la cavite spherique. Cette etude compare les nouveaux essais de petits CC de deux residus miniers limoneux differents, et leurs simulations respectives d'expansion de cavites spheriques. Comme ces simulations utilisent le modele constitutif NorSand, cette etude presente egalement le processus de calibrage qui comprend les essais triaxiaux en laboratoire pour calibrer les proprietes de NorSand, et le processus iteratif correspondant pour deduire les parametres plastiques et elastiques supplementaires du modele. Enfin, une equation d'ajustement est introduite pour l'etat de contraction de ces materiaux limoneux. [Traduit par la Redaction] Mots-cles : CPT, limon, etat critique, tests de laboratoire, simulations numeriques, Introduction In the course of the last decade, the failure susceptibility of tailings storage facilities (TSFs) has generated concern around the world (Morgenstern et al. 2015, 2016; Jefferies et al. [...]

Past studies suggested various methods to determine the onset of instability of soil under constant shear drained (CSD) stress path using triaxial equipment. These methods were based on the characteristic features observed on the CSD stress path, axial and volumetric strains. However, the characteristic features are not similar for every soil leading to inconsistencies in predicting the onset of instability under the CSD stress path. This paper presents a strain ratio ([[epsilon].sub.v]/[[epsilon].sub.a]) - mean effective stress (p') based method for determining the onset of instability of sand in the CSD stress path. The proposed method identifies the onset of instability at the drop of the [[epsilon].sub.v]/ [[epsilon].sub.a]-p' curve and features a distinctive value to mark the onset of instability. A series of CSD tests have been carried out on sand samples to verify the applicability of the proposed method. The proposed method captured the onset of instability similar to the other methods available in the literature. Key words: sand, constant shear drained, instability, triaxial test. Des etudes anterieures ont propose diverses methodes pour determiner le debut de l'instabilite d'un sol soumis a une contrainte de cisaillement constant draine (CSD) en utilisant un equipement triaxial. Ces methodes etaient basees sur les caracteristiques observees sur la trajectoire des contraintes CSD, les deformations axiales et volumetriques. Cependant, les caracteristiques ne sont pas les memes pour tous les sols, ce qui entraine des incoherences dans la prediction du debut de l'instabilite sous la contrainte CSD. Cet article presente une methode basee sur le rapport de deformation ([[epsilon].sub.v]/[[epsilon].sub.a]) - la contrainte effective moyenne (p') pour determiner le debut de l'instabilite du sable dans le chemin de contrainte CSD. La methode proposee identifie le debut de l'instabilite a la chute de la courbe [[epsilon].sub.v]/ [[epsilon].sub.a]-p' et presente une valeur distinctive pour marquer le debut de l'instabilite. Une serie de tests CSD a ete realisee sur des echantillons de sable pour verifier l'applicabilite de la methode proposee. La methode proposee a capture le debut de l'instabilite de maniere similaire aux autres methodes disponibles dans la litterature. [Traduit par la Redaction] Mots-cles : sable, cisaillement constant draine, instabilite, essai triaxial., Introduction The failures of many natural slopes and embankments including tailing dams founded on granular soils are due to the instability caused by the infiltration of water or rise in [...]

Current state of practice for the assessment of liquefied strengths of tailings relies primarily on empirical or semi-empirical correlations based on penetration testing. That is, despite liquefied strength being arguably the most important strength parameter for the design of brittle tailings storage facilities, there is much less success or acceptance of the use of laboratory element tests to support strength selection compared to other forms of strengths inferred in geotechnical engineering. This is particularly the case for tailings at a state near or slightly dense of the critical state line (CSL) for which there is ample evidence of field-scale flow liquefaction but where laboratory element tests often behave in a manner inconsistent with such field-scale response – at least at large strains. The current paper examines the quasi steady state (QSS) strength of sands and tailings for which the CSL has been measured, linking the observed strengths to inferred in situ behaviour through the state parameter. Particular focus is placed on QSS strengths obtained from simple shear tests carried out within a hollow cylinder torsional shear system where the stress state in the test is a better representation of in situ below-slope conditions that the triaxial compression test. In particular, the marked effect of intermediate principal stress on the QSS in sands is highlighted. Alternatively, the negligible anisotropy seen in a sandy silt gold tailings, and the potential implications in the context of QSS strengths and field-scale behaviour, are examined and emphasised.

Results of torsional shear hollow cylinder (TSHC) tests carried out on a reconstituted sand maintaining a constant direction of major principal stress relative to the vertical axis (α) and intermediate principal stress ratio (b) during shearing are presented. Tests were undertaken following the constant shear drained (CSD) stress path, which simulates stress conditions under a rising phreatic surface. The test programme was complemented by undrained TSHC tests to provide further insight regarding the behaviour of the sand when sheared under a range of α values. Various reconstitution methods (i.e. moist tamped (MT), dry pluviated (DP) and wet pluviated (WP)) were included to examine the effects of fabric on CSD triggering. Only MT and DP specimens could be prepared loose enough to exhibit liquefaction behaviour; hence the focus of this study was on these preparation methods. The programme indicated that the instability stress ratio (ηIL) under the CSD stress paths decreases as α increases, suggesting that cross-anisotropy strongly influences the shearing behaviour of sands under this trigger mechanism. In addition, the DP sand showed a greater decrease of ηIL with increasing α compared to the MT sand, consistent with other studies examining the effect of fabric and preparation methods on inherent anisotropy. The results of this study suggest that ignoring the effect of cross-anisotropy on the CSD trigger mechanism will lead to unconservative slope stability assessments. [ABSTRACT FROM AUTHOR]

Assessing the risk of slope failures of earth structures comprising loose, saturated soils has seen increased focus recently owing to a series of prominent tailings and water dam failures. The potential for the triggering of slope instability by a rising phreatic surface—commonly referred to as the constant shear drained (CSD) stress path—has been well documented over the past few decades for sands and sandy soils under a wide range of loading modes. Alternatively, there has been limited CSD element testing of more clayey soils, with the available data rather showing a dilative tendency through the CSD stress path and mobilised strengths greater than the critical friction ratio, even for specimens commencing CSD shearing from an initially loose state. The current study expands on the CSD data of clayey soils by testing an iron ore tailings and kaolin clay in both triaxial and direct simple shear (DSS) devices, with an emphasis on the DSS. Owing to the tendency for the CSD stress path to result in an overconsolidated condition (i.e., unloading), the tests in the current study were interpreted in the SHANSEP framework given its frequent use to analyse clay behaviour. The results indicated that the SHANSEP framework can reasonably characterise some aspects of CSD behaviour of clays in DSS loading, with some caveats. [ABSTRACT FROM AUTHOR]

Tailings storage facilities (TSFs) have undergone an unacceptably high failure rate for decades, leading to an increased focus on improving the safety of these structures. One aspect that has not received sufficient attention is examining how reliably and consistently engineers analyse the stability of TSFs. An understanding of the current state of practice is needed as there are a range of analytical methods available to engineers, while the high failure rate of these structures strongly points to some deficiencies in practice. To examine some of these issues, a tailings-focussed slope stability comparative design exercise was organised to compare the methods and results used by tailings engineers to analyse the same slope within which the phreatic surface was to rise under a specified pattern. Twenty-eight practitioners participated in the exercise to predict at what phreatic surface level the slope would fail. A wide range of predictions were made, ranging from assuming the slope would liquefy and fail instantly (i.e. before any rise in phreatic surface) to a range of techniques based on (i) yield strength ratios, (ii) stress path methods, and (iii) numerical analyses, each predicting various failure levels, and finally some predictors who applied drained strengths, resulting in predictions that the slope would not fail. [ABSTRACT FROM AUTHOR]

This document is a discussion of a research paper titled "A Critical Assessment of the Effect of Initial Fabric on Key Small-Strain Design Parameters of Slurry-Deposited Silts and Sands." The authors of the discussion raise several queries and observations related to the proposed slurry deposition naming convention, measurement of the critical state line, end restraint, triaxial specimen uniformity, and the accessible in situ states and full-scale behavior of silts. They question the authors' claims and suggest alternative perspectives and evidence. The discussion provides valuable insights and raises important considerations for further research in the field of geotechnical engineering. [Extracted from the article]

For a soil that is contractive and strain softening at fully saturated conditions the question remains on which degree of saturation can trigger a strain softening response. A common engineering practice is to adopt a saturation threshold (e.g., 85%) and then assume a fully saturated response when saturation is above that value. This study aims to understand how the strain softening response change when tailings are not fully saturated and to propose a systematic method for determining the saturation threshold. A restraint when preparing unsaturated loose samples is that it is likely to have collapse during flashing, thus changing void ratio and making comparisons between different degrees of saturation problematic. A simple procedure to obtain loose samples at different degrees of saturation but at similar post-flushing void ratios is explained and preliminary results of undrained shear strength obtained for gold tailings in triaxial device are presented and discussed.

Laboratory-controlled cone penetration test results of a silty gold tailings in a variety of saturated and unsaturated states, obtained using two calibration chambers, are presented then interpreted using a state parameter-based approach. The research was conducted as part of the TAILLIQ project (tailliq.com), funded by the Australian Research Council and six mining companies, and involving four Australian universities.Cone penetration resistances, which increase due to the presence of suction when the tailings are unsaturated, can be normalised using the initial mean effective stress to establish a relationship with the initial state parameter. The relationship is applicable to saturated and unsaturated conditions, as long as the presence of suction hardening as well as the influence of suction on the mean effective stress are accounted for, and as long as the cone penetrations occur under pseudo drained conditions. The relationships enable state parameters and void ratios to be back-calculated from normalised cone penetration resistances.

A series of samples obtained from adjacent to piezocone penetration test (CPTu) locations within a gold tailings storage facility with hypersaline pore fluid was tested to compare the inferred state parameter vfrom CPTu results to the in situ density and critical state line (CSL) from reconstituted samples, and to assess the effects of salt concentration in the sample pore fluid on mechanical behaviour of the material. Preparation of samples with similar salt concentrations to those measured in situ gave CSLs that compared favourably to estimates of vfrom the CPTu, in light of measured in situ density. Tests prepared without salt did not tend towards the same CSL as the salt samples, generally being at higher densities both following consolidation and during subsequent shearing. These results are consistent with the observed effects of salt on mechanical behaviour in previous testing on clays. The implications and limitations of the testing program are outlined and the need for further investigation of these issues is emphasized owing to the importance of perimeter embankment stability for a tailings storage facility. Key words: piezocone penetration test (CPTu), critical state, salinity, triaxial. Une serie d'echantillons preleves a des endroits adjacents a des emplacements d'essai de penetration de piezocone (CPTu) dans une installation de stockage de residus auriferes avec un fluide interstitiel hypersalin a ete testee pour comparer le parametre d'etat Vinfere des resultats du CPTu a la densite in situ et a la ligne d'etat critique (CSL) des echantillons reconstitues, ainsi que pour evaluer les effets des concentrations en sel du fluide des pores sur le comportement mecanique du materiau. La preparation d'echantillons ayant des concentrations en sel similaires a celles mesurees in situ a donne des CSL dont leur comparaison aux estimations de V du CPTu, a la lumiere de la densite mesuree in situ a ete favorable. Les essais prepares sans sel ne tendaient pas vers la meme CSL que les echantillons de sel, affichant generalement des densites plus elevees a la fois apres consolidation et apres cisaillement. Ces resultats sont coherents avec les effets observes du sel sur le comportement mecanique lors d'essais anterieurs sur les argiles. Les implications et les limites du programme d'essais sont soulignees, ainsi que la necessite d'approfondir ces questions est soulignee en raison de l'importance de la stabilite du remblai du perimetre de l'installation de stockage des residus. [Traduit par la Redaction] Mots-cles : essai de penetration de piezocone (CPTu), etat critique, salinite, triaxial., Introduction General Tailings characterization involves a number of challenges: (i) as an intermediate soil, the consolidation characteristics are often such that partial drainage occurs during a piezocone penetration test (CPTu); [...]