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.
The engineering properties of naturally occurring sedimentary and residual deposits which are usually treated in geotechnical engineering as ‘soils’ are reviewed, and it is shown that usually they have characteristics due to bonded structure which are similar to those of porous weak rock. While this structure can arise from many causes, its effects follow a simple general pattern that involves stiff behaviour followed by yield. This yield can be described in a similar way to that occurring due to overconsolidation, although it is a separate phenomenon. The effects of structure are as important in determining engineering behaviour as are the effects of initial porosity and stress-history, which are the basic concepts of soil mechanics. As it can be described in a general way, it is concluded that structure and its effects should be treated as a further basic concept of equal importance. L'article passe en revue les propriétés des dépôts sédimentaires et résiduels naturels qui sont nor-malement traités comme des sols en géotechnique et démontre que généralement ils possèdent des caractéristiques dues à la structure liée qui ressemblent à celles du rocher tendre poreux. Bien que cette structure puisse provenir de beaucoup de facteurs, ses effets suivent un cours général très simple comprenant un comportement raide suivi par l'ecoulement. Cet écoulement peut être comparé a celui résultant de la surconsolidation, bien qu'il représente un phénomène different. Dans la détermination du comportement dans la construction les effets de la structure ont une importance égale a celle des effets de la porosité initiate et de l'historique des contraintes, qui représented les conceptions classiques de la mécanique des sols. On tire la conclusion que la structure et ses effets devraient être traités comme une autre conception de base d'importance égale, puisqu'il est possible de les décrire d'une façon génerate.
The paper presents results from a laboratory investigation into the in¯uence of fast rates of displacement on the residual strength of soil. This in¯uence was studied in the ring shear apparatus. Shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement. Results from a wide range of natural soils are presented. All soils show initially an increase in strength when resheared at fast rates of displacement. The strength then tends to decrease with fast displacement to a minimum valueÐthe fast residual strength. Three types of rate effects on the residual strength are identi®ed: a positive rate effect in soils showing a fast residual strength higher than the slow residual; a neutral rate effect in soils showing a constant fast residual strength, equal to the slow residual, irrespective of rate of displacement; and a negative rate effect in soils showing a signi®cant drop in fast residual strength below the slow residual when sheared at rates higher than a critical value. The relation between the types of displacement rate effect and the basic shearing modes of slow residual strength (turbulent, transitional and sliding) is considered and the causes of the negative rate effect and the in¯uence of the apparatus on it have been investigated. The practical implications of the results in the context of slope stability and displacement piles are also discussed.
Delayed failure of railway cutting slopes in stiff clays excavated in the nineteenth and early twentieth centuries has been studied for some forty years. Known failures have generally been deep-seated. More recently failures have been a problem in the slopes of motorway cuttings and embankments, although generally these have been shallow. The average operational strength at failure in these slides has been significantly less than the peak strength, and progressive failure has been postulated as the probable cause of this. Progressive failure can now be analysed using advanced numerical techniques. A series of coupled finite element analyses have been conducted assuming strain-softening soil with properties based on the Brown London Clay, and the results are reported. They show that progressive failure is considerable, and fully explain the observed field behaviour. The delays experienced in the field are also recovered by the analyses. Progressive failure is generated primarily by the high lateral stresses in the soil prior to excavation. The rupture surface generated spreads horizontally from the toe as the soil swells, and differs significantly from the critical surface predicted by limit equilibrium analysis. The average strength on it at collapse is significantly lower than that obtained by back analysis by limit equilibrium methods. It becomes lower as lateral stress increases, but the effect is compensated by the increasing depth of rupture. The roll of the hydraulic surface boundary condition, which is controlled by climate, is important in controlling collapse. The implications of the findings for monitoring and for remedial works in existing slopes is discussed.
Progressive failure occurs when brittle soils are loaded non-uniformly. The average shear stress at collapse is less than peak strength. The effect cannot be examined by conventional limit equilibrium stability analysis. The Carsington embankment slipped in 1984, just before it reached full height. The slide was 30 m deep and 500 m long, and instrumentation had been installed and read up to the moment of collapse. The strengths of the soils involved were carefully established, as was the exact geometry of the slip. Limit equilibrium analysis showed that, at collapse, the average shear stress mobilized was less than peak strength, with a safety factor based on peak strength of 1.2. Since the soils involved were brittle, progressive failure was a probable cause of the discrepancy. Finite element analyses were performed in which strainsoftening soil properties were assumed. Some of these analyses are reported here. The pre-failure displacements, the collapse height and collapse mechanism were recovered by the analysis, and the role of progressive failure was confirmed. The sensitivity of the results obtained to the assump tions made was examined by parameteric studies. It was quite low, indicating that finite element analysis can make predictions of the stability of strain-softening soils of an accuracy adequate for engineering purposes.La rupture progressive a lieu lorsque des sols fragiles sont soumis $ un chargement non-uniforme. La contrainte de cisaillement moyenne lors de la rupture est infirieure P la rbistance de pit. Cet effet ne peut pas s'btudier par I'analyse classique de la stabilite d%quilibre limite. Le remblai de Carsiugton glissa en 1984 peu de temps avant d'atteindre sa hauteur compl&e. Le glissement avait une profondeur de 30 m et une longueur de 500 m. Les instruments de mesure install& avaient donni! leurs indications jusqu'au moment de la rupture. Les r&stances des sols en cause ont & soigneusement &ablies et la gbmetrie exacte du glissement fut difinie. L'analyse d'bquilibre limite indiqua qu'au moment de la rupture la contrainte de cisaillement moyenne etait infirieure a la r&s-tance maximale dans un rapport de 1.2 puis que les sols prbents Ctaient fragiles, la diffbrence fut probablement cau&e par la rupture progressive. Des analyses par &ments finis furent effect&s dans lesquelles on admettait que le sol avaient des prop rietb radoucissantes. L'article pr6sente quelquesunes de ces analyses.L'analyse r&v&la les dbplacements pr&&dant la rupture aussi bien que la hauteur et le m&car&me de la rupture, et le r8le joui? par la rupture progressive fut confirme. Des btudes parambtriques dCmontr&ent que les rbultats obtenus n'btaient que trb peu influencb par les suppositions adopt&es, indiquant de la sorte que I'analyse par Cli?ments finis peut donner des prbdictions de la stabilitk des sols radoucissants avec une prbision sufflsante pour des buts de construction.
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