The influence of structure on the pre-failure behaviour of a natural clay

Abstract

Results are presented of an extensive laboratory and field investigation of a Pleistocene stiff clay in its natural and reconstituted states, including constant rate of strain oedometer and triaxial tests over a wide stress range, scanning electron microscopy and chemical micro-analysis. The relationships between the structure of the clay, its geological history and its mechanical response are demonstrated. The structure of the clay is strongly influenced by bonding, apparently largely due to a film of amorphous calcite which developed in situ at the end of normal consolidation. The effects of structure on the stiffness of the clay are identified, with sensitivity as the parameter quantifying the mechanical effects of structure. A frictional flow rule is found generally to be applicable over a wide strain range, including yield, even when strong bonding is present, although its form is influenced by structure. A new normalization shows that the gross yield surface is isotropically hardening in terms of both volume and structural change. L'article présente les résultats de recherches approfondies, en laboratoire et sur le terrain, sur une argile raide du Pléistocène à l'état naturel et reconstituée, y compris des essais oedométriques et triaxiaux à taux de contrainte constant, sur un large éventail de contraintes, ainsi que les résultats de travaux de microscopie électronique à balayage et de micro-analyse chimique. Les auteurs montrent les rapports entre la structure de l'argile, son histoire géologique et sa réaction mécanique. La structure de l'argile est fortement influencée par l'adhérence, qui semble en grande partie due à une pellicule de calcite amorphe qui s'est formée sur place à la fin de la consolidation normale. Les auteurs identifient les effets de la structure sur la rigidité de l'argile, la sensibilité étant le paramètre qui quantifie les effets mécaniques de la structure. On constate qu'un écoulement de frottement s'applique généralement à un large éventail de contraintes, y compris à la limite d'élasticité, même quand l'adhérence est forte, bien que sa forme soit influencée par la structure. Une nouvelle normalisation montre que la surface brute d'écoulement subit un durcissement isotrope tant en volume qu'en changement structural.

A Multilaminate Model with Destructuration Considering Anisotropic Strength and Anisotropic Bonding

“…At the limit state, bonds are still present but after large deformations (critical state) most of the bonds are broken. The critical state of structured soils is inside the limit state curve while it coincides with limit state curves for destructured soils (Saihi et al, 2002;Burland et al, 1996;Cotecchia and Chandler, 1997).…”

“…The term structure is deˆned by Lambe and Whitman (1969), and Cotecchia and Chandler (1997) as the combination of fabric, the arrangement of the soil component particles, and bonding, those interparticle forces which are not of a purely frictional nature. The initial state of the structure of a soil is called intact state.…”

“…To verify the proposed model, some numerical triaxial tests in undrained conditions and some oedometer tests on two diŠerent stiŠ clays have been carried out. The aforementioned natural clays are Pietraˆtta clay (Burland et al, 1996) and Pappadai clay (Cotecchia and Chandler, 1997). Although there is no information about the anisotropic behaviour of Pietraˆtta clay available, some numerical tests have been performed by assuming inherent anisotropy in bonding to show the eŠect of anisotropy on …”

“…There was subsequent erosion of some 120 m at the sampling location, resulting in the clay being overconsolidated. The mechanical behaviour of reconstituted and natural Pappadai clay has been studied by Cotecchia (1996) and published in Cotecchia and Chandler (1997). The experimental parameters have been taken from the experimental graphs which were provided in Cotecchia and Chandler (1997).…”

“…The mechanical behaviour of reconstituted and natural Pappadai clay has been studied by Cotecchia (1996) and published in Cotecchia and Chandler (1997). The experimental parameters have been taken from the experimental graphs which were provided in Cotecchia and Chandler (1997). Table 3 summarises the values of experimental compressibility and strength parameters of reconstituted and natural Pappadai clay.…”