It is suggested that the structure of clays may be distinguished simply as either ‘sedimentation’ or ‘post-sedimentation’, depending on whether gross yield in one-dimensional compression in void ratio-vertical effective stress plane occurs on the original sedimentation compression curve or to the right of this curve. A behavioural framework is proposed for clays having either of these structures, the magnitude of the strength sensitivity St when consolidated to the gross yield state (in compression) quantifying the effects of structure. The behaviour of natural clay consolidated to stress states pre-gross yield is related to the undisturbed clay sensitivity St; the gross yield curve is generally of arch shape, its size relative to that of the reconstituted clay depending on this sensitivity. Clays having St > 1 consolidated to stresses beyond gross yield exhibit post-gross yield behaviour in which values of sensitivity reduce with increasing post-yield strain. It is found that following normalization for composition (using the stress ratio at critical state M), volume (using p*e) and structure (using the sensitivity at gross yield St ), many different clays have the same gross yield curve. A basically frictional linear flow rule is found to operate from small or medium to large strains for both natural and reconstituted clays.

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.

The paper discusses the geological history, intrinsic properties, structural features and mechanical behaviour of three differently fissured clays outcropping within the Apennine chain in southern Italy. Based on a large experimental database, the mechanical behaviour of the clays is investigated in the light of their different fissuring features, which have been distinguished and characterised by means of a new chart. The study assumed the soil to be a continuum, despite the different fissuring features of the clay fabric. Therefore laboratory tests were carried out on both natural and reconstituted clay samples, and the results were compared with those recognised in the literature to be typical of unfissured sensitive clays. Based on these comparisons, a behavioural framework is proposed for clays possessing certain fissure structures. The results of the analysis show that the mini- to mesostructure of clays of fissuring intensity I5–I6 can be modelled as part of the structure variable controlling the clay behaviour. Where the structure variable refers solely to the micro scale for unfissured homogeneous clays, for fissured clays I5–I6 it spans from the micro to the meso scale. As for the microstructure of unfissured clays, this micro- to mesostructure influences the soil response as an internal state variable in addition to specific volume in controlling the mechanical response. In particular, it appears that for clays of fissuring intensity I5–I6, structure is detrimental to strength, so that the material is even weaker than the reconstituted clay.

In slopes formed by tectonized clayey turbidites, the soil fissuring recurrently influences the hydro-mechanical soil properties, determining an impoverishment in strength and an increase in permeability of the slope that make them predisposing factors of landsliding. This paper presents three case histories of slopes within tectonized clayey turbidites that are representative of several others in the Southern Apennines and, more widely, in the southern Mediterranean. The paper reports a novel attempt to connect tightly the slope geomorphological and hydromechanical features to the slope geological history, through an introductory presentation of the geological setting and history of the chain where the slopes occur. The slopes, location of very slow landslides, have been reconstructed based upon field surveys and investigations, multi-aerial photo-interpretation, laboratory testing, monitoring and numerical modelling. Furthermore, novel is the attempt to present, all together, the behaviour of the soils involved in the three landslide case studies, in the light of the mechanical modelling approach to fissured clays recently presented in the literature.

The paper discusses the results of research into the causes of weathering of stiff Pleistocene clays located in the Montemesola Basin (TA, Italy), and on the effects of weathering on the clay's mechanical behaviour. The weathered clays are yellow-brown and overlie the original grey clays in the whole basin. The liquidity indexes of both the yellow and the grey clays, their fabric, as observed by means of scanning electron microscopy, as well as the yellow clay oxidation features suggest that drying has been the main weathering process in the basin. This is confirmed by the results of drying–wetting cycle tests carried out in the laboratory on undisturbed clay samples, which show the drying–wetting cycles to produce changes in the grey clay similar to those present in situ due to weathering. Results of oedometer, stress path and triaxial tests show that weathering has caused a degradation of the clay bonding and an associated reduction in the size of the clay state boundary surface. Both one-dimensional and iso-tropic compression data and the shear data indicate that the effects of weathering on the mechanics of the clay may be established according to the decrease in the clay stress sensitivity. Weathering also causes a decrease in the normalised shear stiffness, as observed by means of bender element tests, which is also related to a decrease in the stress sensitivity.