The reaction of clays to sampling stress relief

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Abstract
The Paper describes investigations into the influence of stress relief caused by sampling on the undrained stress-strain behaviour of normally consolidated clays. The study was conducted on two clays, kaolin and illite, manufactured in the laboratory from consolidated slurries. In situ behaviour was measured from clay specimens consolidated to in situ stresses in the triaxial cell and then tested undrained from these in situ stresses without unloading. Samples were prepared from consolidated blocks of material which were unloaded and stored for various times before being tested in undrained compression. Unloadings equivalent to appreciable sampling depths were simulated. The tests on both clays showed that, compared with the in situ soil, samples suffered considerable loss in strength, increase in failure strain and produced appreciably different effective stress paths to failure. This behaviour became more pronounced with increasing sample age and was accompanied by dissipation of residual negative pore pressure in the samples. The strength losses were more acute in the kaolin compared with the less permeable illite. The results indicate that the unconsolidated undrained test is misleading for predicting in situ undrained behaviour where stress relief is appreciable. It was found that in situ behaviour of these young and relatively insensitive clays could be reproduced with reasonable accuracy from samples in the consolidated undrained test provided that the samples are reconsolidated to the in situ stresses. These conclusions apply to relatively deep samples of young, intact clays of relatively low sensitivity. Cet article décrit des études de l'influence de la réduction de contrainte provoquée par le prélèvement d'échantillons sur le comportement contrainte-déformation non-drainé des argiles consolidées normalement. Les études on été exécutées sur deux argiles (le kaolin et l'illite) produites en laboratoire à partir de coulis consolidés. Le comportement in situ a été mesuré sur des échantillons d'argile consolidée à des contraintes in situ dans la cellule triaxiale et puis testée non-drainée à partir de ces contraintes in situ sans déchargement. On a préparé des échantillons à partir de blocs consolidés de matière qui ont été déchargés et emmagasinés pendant des périodes différentes avant d'être testés sous compression non-drainée. Des déchargements ont été simulés qui équivalaient à des profondeurs notables de prélèvement d'échantillons. Les tests faits sur les deux argiles ont montré qu'en comparaison avec le sol in situ les échantillons subissaient une perte considérable de résistance et une augmentation de rupture par contrainte et produisaient des chemins de contrainte menant à la rupture qui différaient de façon appréciable entre eux. Ce comportement s'est accentué à mesure que l'âge des échantillons s'augmentait et a été accompagné d'une dissipation de la pression interstitielle négative résiduelle dans les échantillons. Les pertes de résistance étaient plus prononcées dans le cas du kaolin, que dans le cas de l'illite plus perméable. Les résultats indiquent que le test non-consolidé et non-drainé est trompeur en ce qui concerne la prédiction du comportement in situ non-drainé là où il y a une réduction de contrainte notable. On a découvert que le comportement in situ de ces argiles jeunes et relativement insensibles pouvaient se reproduire avec un degré de précision raisonnable à partir des échantillons dans le test consolidé drainé, à condition que les échantillons soient reconsolidés aux contraintes in situ. Ces conclusions s'appliquent à des échantillons relativement profonds de jeunes argiles intactes de sensibilité relativement basse. The Paper describes investigations into the influence of stress relief caused by sampling on the undrained stress-strain behaviour of normally consolidated clays. The study was conducted on two clays, kaolin and illite, manufactured in the laboratory from consolidated slurries. In situ behaviour was measured from clay specimens consolidated to in situ stresses in the triaxial cell and then tested undrained from these in situ stresses without unloading. Samples were prepared from consolidated blocks of material which were unloaded and stored for various times before being tested in undrained compression. Unloadings equivalent to appreciable sampling depths were simulated. The tests on both clays showed that, compared with the in situ soil, samples suffered considerable loss in strength, increase in failure strain and produced appreciably different effective stress paths to failure. This behaviour became more pronounced with increasing sample age and was accompanied by dissipation of residual negative pore pressure in the samples. The strength losses were more acute in the kaolin compared with the less permeable illite. The results indicate that the unconsolidated undrained test is misleading for predicting in situ undrained behaviour where stress relief is appreciable. It was found that in situ behaviour of these young and relatively insensitive clays could be reproduced with reasonable accuracy from samples in the consolidated undrained test provided that the samples are reconsolidated to the in situ stresses. These conclusions apply to relatively deep samples of young, intact clays of relatively low sensitivity. Cet article décrit des études de l'influence de la réduction de contrainte provoquée par le prélèvement d'échantillons sur...

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