Yielding and plastic flow of sensitive cemented clay

Abstract

The stress-strain relations of a cemented sensitive clay are studied experimentally by loading soil samples along different effective stress paths under axisymmetric and plane strain conditions. The soil behaviour is considered to be quasi-elastic workhardening plastic. In the quasi-elastic range the soil may be considered istrotopic but the relevant parameters are stress state dependent. Initial yielding is associated with the breaking down of cementation bonds and a unique yield envelope, independent of stress path, is defined. Based on an experi mentally defined non-associated flow rule, a plasti city theory is developed to describe the post-yield stress-strain behaviour of cemented sensitive soils. Reasonable agreement is obtained between theoreti cal predictions and experimental results. Aniso tropic yielding is also considered. Les relations contrainte-déformation d'une argile sensible cimentée sont étudiées expérimentalement en chargeant des échantillons de sol selon différentes trajectoires de contraintes effectives dans des conditions de déformation axisymétrique et plane. Le comportement du sol est considéré comme étant quasi-élastique et plastique. Dans le domaine quasi-élastique, le sol peut étre considéré isotrope mais les paramttres caractéristiques dépendent de 1‘état de contrainte. L'écoulement initial est associé à I'effondrement des hens de cementation et une enveloppe de rupture unique, indépendante de la trajectoire des contraintes, est définie. Basée sur une régle d'ecoulement non-associée et définie expérimentalement, une théorie de plasticité est developpée afin de décrire le comportement aprés rupture contrainte-déformation des sols sensibles ci mentés. Un accord raisonnable est obtenu entre les predictions théoriques et les résuhats expérimentaux. La rupture anisotrope est aussi considérée. The Champlain Sea deposits of Eastern Canada generally have liquidity indices greater than unity and sensitivities in excess of 30. These characteristic properties have been attributed to the development of interparticle cementation immediately following deposition (Crawford, 1963; Penner, 1965). The geological history of these post-glacial clays has been well docu mented in geotechnical literature, e.g. Karrow (1961) and Gadd (1962) and possible physico chemical reasons for cementation have been suggested by Townsend (1965), Kenney (1964) and Sangrey (1972a). The compression and shear behaviour of these clays have been the subject of a large number of geotechnical publications many of which are referenced in Sangrey (1972b). The general behaviour may be stated as follows: when the soil structure is subjected to normal foundation design loads (factor of safety 2 2) the soil skeleton deforms in a nearly elastic man ner similar to a space frame where individual members are rigidly joined together. When the interparticle bonds rupture under larger applied loads the soil will exhibit a catastrophic shear failure or will undergo excessive distortion and volume change leading eventually to a state of frictional equilibrium. Earthflows, bearing capacity failures and excessive settlements of foundations are common geotechnical problems in these soils. The results of drained triaxial and plane strain tests on a typical Ottawa area Champlain Sea clay are reported in this Paper and a theoretical approach to analysing the stress-strain be haviour of vertically oriented triaxial samples is proposed.