Time-dependent behaviour can have a significant influence on the compressibility characteristics of soils. However, most of the research on this topic has investigated the behaviour of soft soils. In this paper, the time-dependent behaviour of a hard clay shale (Bearpaw Shale) is investigated using both one-dimensional multi-staged loading (MSL) oedometer and constant rate of strain (CRS) oedometer consolidation tests conducted on 25.0 and 16.9 mm diameter specimens. The results show that soft clays and hard clay shales that share the same Cae/C Ã c ratio (where Cae is the secondary compression index and C Ã c is the incremental compression index) will show the same approximately 7% change in pre-consolidation pressure for an increase of one log cycle of strain rate despite the many orders of magnitude difference in pre-consolidation pressure. In the case of the Bearpaw Shale, this 7% change in pre-consolidation pressure corresponds to approximately 700 kPa. The time-dependent behaviour of the Bearpaw Shale during unloading (Cae/C Ã s , where C Ã s is the incremental swelling index) was observed to follow a similar ratio to that observed in compression (C ae /C Ã c ). While the exact nature of the compression and swelling events that have occurred over the life of the Bearpaw Formation is not clear, the influence of secondary compression cannot be ignored for interpretation of the geological history of this deposit.Résumé : Le comportement dépendant du temps peut avoir une influence significative sur les caractéristiques de compressibilité des sols. Cependant, la plupart des travaux de recherche sur ce sujet se sont penchés sur le comportement des sols mous. Dans cet article, le comportement dépendant du temps d'un schiste argileux dur (schiste de Bearpaw) est étudié à l'aide d'essais unidimensionnels de consolidation à l'odomètre avec chargement multi-étapes (CME) et avec un taux de dé-formation constant (TDC) réalisés sur des échantillons de 25,0 et 16,9 mm de diamètre. Les résultats démontrent que les schistes argileux mous et durs qui partagent le même ratio Cae/C Ã c présentent le même 7 % environ de variation dans la pression de préconsolidation pour causer une augmentation d'un log de cycle de déformation, malgré plusieurs ordres de magnitudes de différence dans la pression de préconsolidation. Dans le cas du schiste de Bearpaw, cette variation de 7 % de la pression de préconsolidation correspond à environ 700 kPa. Le comportement dépendant du temps du schiste de Bearpaw durant le déchargement (C ae /C Ã s ) suivait un ratio similaire à celui observé en compression (C ae /C Ã c ). Même si la nature exacte des événements de compression et de gonflements qui se sont produits sur la durée de vie du formation de Bearpaw n'est pas claire, l'influence de la compression secondaire ne peut pas être ignorée lors de l'interprétation de son historique géolo-gique.

There has been a significant amount of research investigating the relationship between sample disturbance and laboratory test results in soft soils. As a result of this research, the general rule adopted is that using larger specimens results in less sample disturbance. When testing these larger specimens in the laboratory, the results are more representative of in situ behavior of the materials. In contrast, there has been relatively little corresponding research performed on hard soils whose behavior typically lies on the boundary between rock and soil. Extensive unloading in hard soils, from sampling, results in large suctions (negative pore pressures) and the formation of fractures that are uncharacteristic of the material in its natural state. This poses the question of whether the use of small specimens would produce more reliable laboratory results. This paper investigates the following two hypotheses within the context of oedometric testing: (1) testing smaller diameter specimens will produce results more representative of the in situ behavior of the material, and (2) an aspect ratio of 2.5 may reduce disturbance to the specimen during preparation are tested. The results of this testing program, including the intact material properties, and a characterization of the compression behaviour of clay shale from the Bearpaw Formation is also presented herein. Results show that a reduced specimen size, when working with a hard clay shale, minimizes the effect of disturbance and produces results that are the most representative of the intact material. Decreased specimen size also aids in determining preconsolidation pressure by not only reducing the disturbance to the sample resulting from unloading, but also by enabling these high stresses to be achieved in conventional testing equipment. Two criteria from the literature were used to assess disturbance within the oedometer specimens. Overall, the methods provide a good baseline of assessing disturbance, however, there is less gradation to the quality of a specimen in a hard soil. Parameters Cc and σ′p were sensitive to sample disturbance, however, other parameters such as cv and K were less dramatically affected by disturbance. Therefore, care must be taken when assessing the compression of a hard soil as failing to achieve sufficiently higher stresses and the presence of disturbed specimens may lead to misleading Cc and σ′p values.

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