Consolidation Characteristics and Volume Change Behaviour of Fine-grained Thickened Tailings in Consolidated Drained Triaxial Testing

R. Moghaddam

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: ENERGY RESOURCES - I Mining and Tailings

ABSTRACT: The consolidation behaviour of tailings in the form of cemented paste backfill (CPB) and uncemented thickened tailings isof interest in the emerging trend in integrated mine waste management. The behaviour of the tailings is conventionallydescribed using soil mechanics theories developed for sandy soils and extended to silty soils. This paper presents theconsolidation characteristics and volume change behaviour of non-segregating fine-grained tailings in the consolidateddrained triaxial testand the standard one-dimensional oedometer test.In the standard one-dimensional oedometer tests,it was shown that the primary consolidation of thickened tailings and CPB specimens is well represented by Terzaghi•smodel.However, the initial compression cannot be described by that model. In addition,theconsolidation curveschangeascement hydrates. It was also shown that the silt-sized tailings in the form of thickenedtailingsor paste have highercompressibility compared to sand. The consolidated drained triaxial test results showed thatthat the failure line obtainedin the consolidated drained tests lies above the constant stress ratio line obtained from the consolidated undrained triaxialtest. Although this is consistent with the behaviour of medium dense sand, the tailings do not fully recover their volumebefore reaching peak stress (i.e., failure point). In this case, the peak stress corresponds with the point of maximum dilation(i.e., maximum rate of volumetricstain/axial strain). The high compressibility of the tailings relative to the sands may beresponsible for the difference in thedrained behaviour of the materials.R†SUM†Le comportement de consolidation des r‡sidus sous forme de remblai de p…teciment‡e et des r‡sidus ‡paissis non-ciment‡ est important dans domaine ‡mergent de la gestion int‡gr‡e des d‡chets miniers. Le comportement des r‡sidusest conventionnellement d‡crite en utilisant th‡ories de la m‡canique des sols d‡velopp‡es pour les sols sableux et‡tendues aux sols limoneux. Ce document pr‡sente les caract‡ristiques de consolidation et de changement de volumepour les r‡sidus — grains fins non s‡gr‡g‡s durant des essais triaxiaux en conditionsdrain‡es et durant essaisoedom–triques unidimensionnelles. Dans les essais oedom‡triques unidimensionnelles, c•‡tait d‡montr‡ que laconsolidation primaire des r‡sidus ‡paissis et des ‡chantillons (CPB) est bien repr‡sent‡e par le mod–le de Terzaghi.Cependant, la compression initiale ne peut pasƒtre d‡crit par ce mod–le. De plus, les courbes de consolidation changentavec l'hydratation du ciment. Il a ‡t‡ ‡galement d‡montr‡ que les r‡sidus ‡paissis ou de p…te de taille limoneux ont unecompressibilit‡ plus ‡lev‡ par rapport au sable. Les r‡sultats ont montr‡ que la ligne de rupture obtenue dans les essaistriaxiaux en conditionsdrain‡s se situe au-dessus de la ligne du ratio de stress constant obtenu — partir de l'essai triaxialconsolid‡ et non drain‡. Bien que cela conforme avec le comportementde sable moyennement dense, les r‡sidus ner‡cup–rentcompl–tementleur volume avant d'atteindre leur tension maximale (le point de rupture). Dans ce cas, le tensionmaximale correspond au point de dilatation maximale (le taux maximal de d‡formation volum‡trique /d‡formation axiale).La compressibilit‡ ‡lev‡e des r‡sidus relative aux sables peut ƒtre responsable pour la diff‡rence dans le comportementdrain‡ des mat‡riaux.1INTRODUCTIONThe consolidation behaviour ofnon-segregating, fine-grainedtailings in the form of cemented paste backfill(CPB) and uncemented thickened tailings is of interest inthe emerging trend in integrated mine waste managementthat includes two main components: underground minebackfilling and surface disposal.Generally, the mechanical behaviour of tailings isconventionally described using soil mechanics theoriesdeveloped for sandy soils and extended to silty soils.Thispaper presents the consolidation characteristics andvolume change behaviour of thickened tailings and CPBusingthestandard one-dimensional oedometerandconsolidated drained triaxial testsand in comparison withthe conventional soil mechanics theoriesand experiments.2BACKGROUNDThe volume change of uncemented thickened tailings withtime is controlled by self-weight consolidation and dryingduring surface deposition. The volume change due toconsolidation continues over a prolonged period of time asthe degree of consolidation approaches 100%. On theother hand, the volume change due to drying diminishesquickly when the soil matric suction overcomes the air entryvalue of the tailings (Li et al, 2012). In contrast, the volumechange of CPB with time in an underground CPB system iscontrolled by self-weight consolidation as well as hydration

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