The yielding of Bothkennar clay

Abstract

The Paper describes the yielding characteristics of Bothkennar clay, as shown by stress-path triaxial tests. Computer-controlled equipment and high resolution instrumentation were used to obtain a detailed picture of the behaviour of the undisturbed clay at very small and large strains. The results confirm an earlier hypothesis that the soil's behaviour at small strains can be described by two kinematic sub-yield surfaces existing within an initial bounding surface. Consolidation beyond the initial bounding surface is shown to bring about a progressive breakdown of the clay's initial sedimentary structure. Once this has occurred the soil's stress—strain and undrained strength characteristics become highly dependent on the stress conditions under which consolidation takes place. Cet article décrit les caractéristiques de fluage de l'argile de Bothkennar, comme le montrent des essais triaxiaux avec chemin des contraintes. Du matériel contrôlé par ordinateur et des instruments de mesure à grande définition utilisés pour obtenir une vision détaillée du comportement de l'argile non remaniée pour de très petites et de grandes déformations. Les résultats confirment une hypothèse précédente: le comportement du sol pour de petites déformations peut être décrit par deux surfaces cinématiques de sous-fluage existant à l'intérieur d'une surface limite initiale. On montre que la consolidation au delà de la surface initiale limite amène une rupture progressive de la structure sédimentaire initiale de l'argile. Passé ce point les caractéristiques contrainte déformation et résistance au cisaillement deviennent principalement dépendantes des conditions de contraintes sous lesquelles s'effectue la consolidation.

Behaviour of Unsaturated Aggregated Soil in Oedometric Condition

“…They showed that the void ratio for both residual and sedimentary structured soils is greater than can be sustained by the same soil from which structure has been removed by remolding. Similar evidences have been reported by Burland (1990) and Smith et al (1992). Interpreting their results, Leroueil and Vaughan (1990) stated that while bonded structure in these materials might arise from diŠerent causes, its consequences on the material behaviour is similar and corresponds to a stiŠ behaviour followed by yield.…”

“…The experimental evidence of the soil structure eŠects on the stress-strain response of soils have been reported for a wide variety of natural structured soils, including soft rocks (Tavenas and Leroueil, 1990;Smith et al, 1992), stiŠ clays and clay shales (Calabresi and Scarpelli, 1985;Anagnostopoulos et al, 1991;Burland et al, 1996), granular soils (Mitchell and Solymar, 1984;Coop and Atkinson, 1993) and residual soils (Vaughan et al, 1988). The extra features arising from soil structure are often identiˆed in comparison with reconstituted soil of the same mineralogy as a reference state with intrinsic properties (Burland, 1990).…”

“…Accordingly, they described this yield in a similar way to that occurring due to overconsolidation, although it is a diŠerent phenomenon. As for yielding behaviour, natural soils often exhibit a brittle behaviour with the post-yield compression index being much greater than that of reconstituted soils (Burland, 1990;Smith et al, 1992). It has been commonly reported that the compression curve of structured clay tends to converge to that of reconstituted clay, as the applied stress increases (Burland, 1990;Callisto and Calabresi, 1998).…”

Discussion: Soil stiffness in stress paths relevant to diaphragm walls in clay

“…In addition, the lengths of the ®rst two outgoing stress paths were approximately 9 kPa, which resulted in an axial strain of more than 0´06%. This is well in excess of the strain level of 0´02% required to induce plastic strains in Bothkennar clay (Smith et al, 1992). The stiffness for each outgoing stress path is shown in Fig.…”

Stiffness of Frozen Soils Subjected to K0 Consolidation Before Freezing

“…Clayton conducted the triaxial experiment to observe the behaviour at very small strains of three wildly diŠerent natural materials: Bothkennar clay, London clay and a high-porosity chalk (Clayton and Heymann, 2001). At the same time, corresponding developments in mathematical modeling allow a number of non-linear and inelastic soil characteristics to be included in numerical analyses (Smith, 1992 Whereas the factors controlling the small strain response are well understood for unfrozen soils, there have been few researches dealing with the small strain stiŠness of frozen soils, especially for artiˆcially frozen soils in deep alluvium. The main reason may be attributed to that the study of the deep artiˆcially frozen soils in the laboratory demands special specimen preparation and testing procedures.…”