drained residual strength of cohesive soils

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 résidu