The drained residual strength of cohesive soils has been studied extensively during the last 20 years. Various correlations between residual friction angle and index properties have been proposed and these are reviewed. Residual strength is measured with least ambiguity in the ring shear apparatus. A large number of natural soils have been tested in the ring shear apparatus developed jointly by Imperial College and the Norwegian Geotechnical Institute, and the results of these tests are summarized. The mechanisms controlling residual shearing are considered. The results of three series of tests on different soil mixtures, for which the gradings of the soils could be varied artificially, are presented. The proportions of platy particles to rotund particles present in the soil and the coefficient of interparticle friction of the platy particles are confirmed as controlling the type of residual shearing mechanism which develops. Three modes of residual shear are demonstrated; a turbulent mode in soils with a high proportion of rotund particles or with platy particles of high interparticle friction, in which preferred platy particle orientation does not occur, a sliding mode in which a low strength shear surface of strongly orientated low friction platy particles forms, and a transitional mode involving both turbulent and sliding shear. These modes are established by studies of brittleness and of the post-shear structure of the soil. The influence of these modes of residual shear on the general behaviour of cohesive soils is considered, and the transition from one to another is related to the packing and porosity of the rotund particles present. The effectiveness of correlations between residual strength and index properties is then reconsidered. La résistance résiduelle drainée de sols cohérents a fait l'objet de nombreuses études au cours des vingt dernières années. Diverses corrélations entre l'angle de friction résiduel et les propriétés caractéristiques ont été proposées et celles-ci sont passées en revue. C'est avec l'appareil de cisaillement circulaire par torsion que la mesure de la résistance résiduelle est la moins ambiguë. Un grand nombre de sols naturels ont été testés dans l'appareil de cisaillement circulaire par torsion mis au point conjointement par l'Impérial College et l'Institut Géotechnique Norvégien; les résultats de ces essais sont résumés dans le rapport. Les auteurs considèrent ensuite les mécanismes qui régissent le cisaillement résiduel. L'étude présente les résultats de trois séries d'essais sur différents melanges de sols pour lesquelles il était possible de faire varier artificiellement la granulométrie des sols. Les résultats attestent que c'est la proportion des particules plates par rapport aux particules arrondies présentes dans le sol ainsi que le coefficient de friction interparticulaire des particules plates qui régissent le type de mécanisme de cisaillement résiduel qui se produit. Trois modes de cisaillement résiduel sont démontrés; un mode turbulent dans les sols comportant une forte proportion de particules arrondies ou dont le frottement interparticulaire des particules plates est importante, dans lequel une orientation des particules plates préférentielle ne se produit pas, un mode du type glissement dans lequel se forme une surface de cisaillement à faible résistance composée de particules plates à faible friction et fortement orientées, et un mode transition impliquant à la fois un cisaillement turbulent et à glissement. Ces modes sont établis grâce à des études de la fragilité et de la structure post-cisaillement du sol. L'étude considère l'influence de ces modes de cisaillement résiduel sur le comportement général de sols cohérents et établit un rapport entre le passage de l'un à l'autre de ces modes et le tassement et la porosité des particules arrondies présentes. Elle réexamine ensuite la valeur des corrélations établies entre la résistance résiduelle et les propriétés caractéristiques.

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The influence on the mechanical properties of saturated particulate materials of the component of stress carried by the water filling the pore space is fundamental to both theoretical and experimental studies in soil mechanics. The rôle of pore pressure in controlling compressibility and shear strength is expressed in Terzaghi’s principle of effective stress to a degree of accuracy which is sufficient for most engineering purposes. However, the precise significance of the small but finite area of interparticle contact has remained uncertain in the application of this equation to shearing resistance. In the present paper the possible errors associated with the use of current expressions for intergranular stress and effective stress are examined. These errors are of significant magnitude at high values of pore pressure and low values of the yield stress of the solid forming the particles. A very accurate experimental investigation has been carried out into the sensitivity of shearing resistance to large changes in pore pressure (up to 41.4 MN/m 2 ), using particulate materials ranging in strength from Quartz sand to lead shot. The results indicate that the simple Terzaghi effective stress equation a' - o - u is consistent with all the observations, though for Quartz sand a range of pore pressure changes an order of magnitude higher is desirable for additional confirmatory evidence.

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