Cyclic and Post-cyclic Laboratory Test Results on Undisturbed Samples of Filter Pressed Mine Tailings

Rick Friedel, Len Murray

In the proceedings of: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: W1-D

ABSTRACT: Relatively little information is available regarding the cyclic behaviour of consolidated mine tailings. A major issue is that tailings often cannot be reliably classified based on conventional empirical techniques for sands or clays. This technical paper presents results from a laboratory testing program of undisturbed in situ samples of filter pressed mine tailings that were deposited in a 'dry-stack' tailings storage facility. The laboratory program included index classification tests, consolidation tests, monotonic, cyclic and post-cyclic simple shear tests. This paper will focus on the results of the cyclic and post-cyclic simple shear tests and compare them to previous tests completed on reconstituted samples of the same material. RESUME Relativement peu d'informations est disponible concernant le comportement cyclique de résidus miniers consolides. Un enjeu majeure est que souvent les résidus ne peu être classifiés de manière fiable a base de techniques classiques empiriques pour les sables ou argiles. Ce document technique présente les résultats d'un programme d'essais de laboratoire d'échantillons intactes in situ de résidus miniers presse au filtre qui on été déposes dans une « pile sèche » d'une installation de stockage de résidus. Le programme de laboratoire inclue des analyses de classifications index, des analyses de consolidations, et des analyses de cisaillement simple mono toniques, cycliques et post-cycliques. Ce document va accentuer les résultats des analyses cycliques de cisaillement simple et tentera de caractériser le comportement comme celui du sable ou de l'argile. 1 INTRODUCTION Relatively little information is available regarding the behaviour of consolidated mine tailings under cyclic loading. Experience and available information indicate that this unique material cannot be reliably classified based on conventional empirical techniques for sands or clays. This paper presents the results of a laboratory testing program of undisturbed samples collected in situ from filter pressed mine tailings that were deposited in a 'dry-stack' tailings disposal facility (TDF). These 'dry' filter pressed tailings were hauled to the storage facility in standard haul trucks, placed in lifts and lightly compacted. The tailings sampled and tested had been in situ for up to 15 years. The climate at the site is generally poor for fill placement. For the majority of the year the site is under persistent rainfall; and snow falls during winter months. On average the site receives approximately 1.5 m of rainfall and 2.9 m of snowfall annually. Conventional Standard Penetration Test (SPT) or Cone Penetration Test (CPT) investigation programs provided inconsistent and contradicting information regarding cyclic resistance and liquefaction susceptibility of the tailings. A laboratory testing program was undertaken to help resolve the conflicting information from in situ testing. Mill tailings at the site can be classified into two categories: 'Old' tailings which were produced prior to a prolonged mill shutdown during which the milling process was changed; and 'New' tailings which were produced using the new process. The laboratory program included index classification tests, consolidation tests, monotonic, cyclic and post-cyclic simple shear tests on undisturbed samples. Cyclic simple shear tests are compared to previous tests completed on reconstituted samples of the same material. 2 CYCLIC RESPONSE AND EMPIRICAL CLASSIFICATION OF SOILS TYPES For this project the objective of the investigation was to assess the potential for slope failure or large deformations during cyclic loading. A key step of predicting material behaviour during cyclic loading is proper classification. Boulanger and Idriss (2004) proposed classifying cyclic behaviour of materials into three groups: sand-like, clay-like and transitional. Development of elevated porewater pressures and large shear strains during undrained cyclic loading is referred to as 'liquefaction' in sand-like soils and as 'cyclic failure' in clay-like soils (Boulanger and Idriss, 2004). During liquefaction of sand-like loose granular materials can undergo significant sudden strength loss. Strength loss during cyclic failure of clay-like soils is less severe and generally builds up progressively over several load cycles. Both liquefaction and cyclic failure are capable of producing large scale deformation. Boulanger and Idriss (2004), note that the potential consequences of 168

RÉSUMÉ: and Post-cyclic Laboratory Test Results on Undisturbed Samples of Filter

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