Seven series of tests have been performed on specimens from five natural soft clay sites to determine the variation of the vertical and horizontal permeabilities when the soil is unidimensionally compressed. The results show that the permeability anisotropy does not increase significantly with compression up to strains of 25% for marine clays. In varved clays, mostly because the clay laminae are more compressible than the silty laminae, the anisotropy increases with vertical compression. Key words: permeability, clay, anisotropy, laboratory, compression.

Isotropic and anisotropic compression tests, and drained triaxial shear tests were performed on a collapsible unsaturated silty soil under various suctions. The results show that the concepts of yielding and critical state originally developed for saturated clays can be applied to this unsaturated material when the matrix suction is considered as an independent variable. Pre-yield and post-yield behaviour is clearly defined, the yield curve increases in size with suction, and the effect of suction on the critical state strength decreases with increasing net mean stress. Des essais de compression isotrope et anisotrope et des essais triaxiaux de cisaillement drainé ont été réalisés, pour différentes valeurs de succion, sur des cols silteux non saturés effondrables. II apparaît que les concepts d'état limite et d'état critique, initialement développés pour les argiles saturées, sont applicables à ce type de matériau non saturé si la succion matricielle est considérée comme variable indépendante. Les comportements pré et post état limite apparaissent nettement, la courbe d'état limite s'accroît lorsque la succion augmente et l'influence de la succion sur la résistance à l'état critique diminue lorsque la contrainte moyenne augmente.

The behaviour of a clay element during the sampling operation is first analysed in the light of the limit state theory. The stress variation and the resulting distortion of the clay element during sampling is equivalent to a destructuration of the clay, and its mechanical properties are greatly affected. The causes of sampling disturbance are analysed and the principles which should govern the design of a good quality sampler are reviewed in the light of previous studies. It is shown how the reshaping of the sampling tubes can improve the sample quality. The new 200 mm sampler designed at Université Laval uses the overcoring technique to eliminate suction and to insure the recovery of large samples without a piston. The quality of these large samples has been proven to be equivalent to block samples. The advantages and limitations of the Laval sampler are discussed.

Radforth peat is a term introduced here to describe peats with a negligible content of mineral matter. The Sphagnum peat discussed in this paper consists mostly of fibrous particles such as stems, leaves, fiber hairs, etc. Outstanding characteristics of Radforth peats are their extremely high compressibility and high rate of creep. The variability of peat is often almost unpredictable, even within an apparently uniform deposit. Predictions of magnitude and rate of settlement are therefore difficult. Frequent disturbed sampling and classification combined with empirical relationships may give better results than infrequent undisturbed sampling, consolidation testing, and theoretical relationships. Peat fibers affect the geotechnical behavior of peat by providing an internal lateral resistance to shear deformations in the triaxial mode of shear. Since pore pressures reduce this resistance, loading under drained conditions will provide better stability through higher lateral resistance. The shear strength of peat in ring shear would be virtually unaffected by fiber reinforcement. The behavior of the Escuminac peat in ring shear was surprisingly consistent and gave parameters of c = 2.4 kPa and φ = 27 to 32 deg within a load range of 3 to 50 kPa. These values would be directly applicable in the case of retaining structures on peat, for which triaxial φ-values of 40 to 50 deg, as consistently reported in the literature, could in fact be misleading.

Triaxial, stress controlled, drained tests on four undisturbed, overconsolidated clays from eastern Canada show that the strain energy can be used with advantage to define the limit state surface of such clays since a clear discontinuity in the energy–stress relationship exists for all possible stress paths. Along a K0 stress path the threshold energy, at the limit state, is a linear function of the preconsolidation pressure, i.e. of the vertical effective stress at the limit state. For other stress paths, at a given void ratio, the threshold energy is a function of the stress state at the limit state. Drained creep triaxial tests show that the strain energy is also a good indicator of the creep behaviour of overconsolidated clays. In particular the rate of dissipation of the strain energy is shown to depend essentially on the relative position of the applied effective creep stresses to the limit state surface of the clay. Des essais triaxiaux drainés à contraintes contrôlées sur quatre argiles surconsolidées intactes de l'est du Canada montrent que l'énergie de déformation constitue un bon paramètre de définition de la surface d'état limite de ces argiles. En effet une discontinuité nette de la relation énergie-contrainte se produit sur tous les chemins de contrainte possibles. Le long d'un chemin de contrainte K0, l'énergie limite, à l'état limite, est une fonction linéaire de la pression de préconsolidation, c'est-à-dire de la contrainte effective verticale à l'état limite. Le long d'autre chemins, à un indice des vides donné, l'énergie limite est fonction de l'état de contraintes à l'état limite. Des essais tri-axiaux de fluage drainé montrent que l'énergie de déformation est aussi un bon indicateur du comportement en fluage des argiles surconsolidées. En particulier la vitesse de dissipation de l'énergie de déformation dépend essentiellement de la position relative des contraintes effectives appliquées pendant le fluage et de la surface d'état limite de l'argile.