The mineralogy, geochemistry and geotechnical properties of clay samples in Ariake Bay, Japan are presented. The mineralogy and geochemistry are discussed in terms of depositional and post-depositional processes, and the correlations of the clay mineralogy and geochemistry with the geotechnical properties were examined by multiple regression analysis. The most predominant clay mineral in the deposits was found to be smectite. The vertical distribution of trace elements in the deposits was indicative of the periodical changes in the parent materials involved in the depositional processes. The chloride concentration in the pore water varied, according to a change in the sea level, from brackish to marine, and then brackish towards the surface. The higher iron oxide contents near the surface and in the deepest zones were the results of pyrite oxidation in the deposits due to weathering under subaerial conditions. Smectite content was the most predominant governing factor for the consistency limits and activity; the iron oxide content was the most predominant for the sensitivity, the Cu/p value, and the over-consolidation ratio. Overconsolidation characteristics in the near-surface zone and in the deepest zones were attributed to interparticle cementation by the iron oxides produced by pyrite oxidation. L'article présente les propriétés minéralogiques, géochimiques et géotechniques d'échantillons argileux de la baie d'Ariake, Japon. La minéralogie et la géochimie sont étudiées pendant et après le dépôt. Les propriétés géotechniques sont ensuite corrélées, à l'aide de diverses régressions, à la minéralogie et à la géochimie. La smectite est le minéral argileux le plus courant. La distribution verticale des éléments-trace dans les dépôts est représentative des variations périodiques des matériaux originels impliqués dans les processus de dépôt. Selon le niveau de la mer, la concentration en chlorures de l'eau interstitielle vane: en remontant vers la surface, elle est typique de l'eau saumâtre, puis de l'eau de mer, puis de nouveau de l'eau saum7acirc;tre. Les teneurs en oxydes de fer les plus élevées, situées à proximité de la surface et dans les zones les plus profondes, résultent de l'oxydation, par altération sous conditions subaériennes, de la pyrite contenue dam ces dépâts. Les teneurs en smectites et en oxydes de fer sont les principaux paramàtres contrôlant les limites de consistance et Pactivité pour la premiàre, la sensibilité, la valeur de culP et le degré de surconsolidation pour la seconde. Les caractéristiques de surconsolidation dans les zones proches de la surface et dans les zones les plus profondes sont sans doute dues à une cimentation de l'espace interparticulaire par des oxydes de fer issus de l'oxydation de la pyrite.

The residual strength of smectite-dominated soils from the main slip surfaces of the Kamenose landslide, Japan, was measured by ring shear test, and the relations of the residual strength to the clay fraction content and clay particle orientation were studied. The residual angles of shearing resistance, [Formula: see text], were estimated assuming that the residual cohesion is zero. The magnitude of the residual angle of shearing resistance was dependent on the <2-μm clay fraction content, CF, and on the effective normal stress, [Formula: see text], because of curved strength lines. At [Formula: see text] equivalent to the overburden pressures, the soils had [Formula: see text] of 6–8°. These residual angles of shearing resistance were fitted on the [Formula: see text] – CF line for sand–bentonite mixtures, and exhibited the sliding shear mode.The orientation of smectite particles on shear surfaces showing slickensides was assessed by X-ray diffraction technique. The orientation of smectite particles was found to reduce the residual strength considerably, and this orientation effect was revealed more clearly at low effective normal stresses. Key words: residual strength, particle orientation, landslide, smectite, shear mode, slip surface, slickenside, clay mineral, physicochemical properties, ring shear test.

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