Residual shear strength of fine-grained soils and soil-solid interfaces at low effective normal stresses

A laboratory research program was undertaken to study the large-strain shear strength characteristics of fine-grained soils under low effective normal stresses (~3-7 kPa). Soils that cover a wide range of plasticity and composition were utilized in the program. The interface shear strength of these soils against a number of solid surfaces having different roughness was also investigated at similar low effective normal stress levels. The findings contribute to advancing the knowledge of the parameters needed for the design of pipelines placed on sea beds and the stability analysis of shallow soil slopes. A Bromhead-type torsional ring-shear apparatus was modified to suit measuring soil-soil and soil-solid interface residual shear strengths at the low effective normal stresses. In consideration of increasing the accuracy of assessment and depicting the full-scale field behavior, the interface residual shear strengths were also measured using a macroscale interface direct shear device with a plan interface shear area of ~3.0 [m.sup.2]. Correlations are developed to estimate the soil-soil and soil-solid interface residual shear strengths at low effective normal stresses. The correlations are compared with soil-soil and soil-solid interface drained residual shear strengths and correlations presented in the literature. Key words: pipelines, offshore engineering, residual strength, large-strain shear strength, interface strength, shear strength, roughness, coating, slope stability, normal stress, soils. Un programme de recherche en laboratoire a ete monte pour etudier les caracteristiques de resistance au cisaillement a grand deplacement de sols fins soumis a des contraintes normales effectives faibles (~3 a 7 kPa). Pour ce programme, des sols couvrant une grande variete de plasticite et de composition ont ete utilises. La resistance au cisaillement a l'interface entre ces sols et plusieurs surfaces solides ayant differentes rugosites a ete investiguee pour des niveaux de contrainte normale effective faibles et similaires. Les resultats contribuent a ameliorer les connaissances sur les parametres necessaries pour la conception de pipelines places sur les fonds marins, et sur les analyses de stabilite de pentes de sol peu profond. Un appareil de cisaillement en anneau de torsion de type Bromhead a ete modifie pour permettre les mesures des resistances au cisaillement residuelles aux interfaces sol-sol et sol-solide pour des contraintes normales effectives faibles. Afin d'augmenter la precision de revaluation et pour representer le comportement a l'echelle du terrain, les resistances au cisaillement residuelles a l'interface ont aussi ete mesurees a l'aide d'un appareil a l'echelle macro de cisaillement direct a l'interface, avec une surface plane de cisaillement a l'interface d'environ 3,0 [m.sup.2]. Des correlations sont developpees pour estimer les resistances au cisaillement residuelles a l'interface sol-sol et sol-solide pour des contraintes normales effectives faibles. Ces correlations sont comparees aux resistances au cisaillement residuelles (trainees a l'interface sol-sol et sol-solide de meme qu'aux correlations presentees dans la litterature. [Traduit par la Redaction] Mots-cles : pipelines, ingenierie en mer, resistance residuelle, resistance au cisaillement a grande deformation, resistance a l'interface, resistance au cisaillement, rugosite, revetement, stabilite de pente, contrainte normale, sols.
Introduction It has been recognized that the drained residual shear strength envelope of fine-grained soils is nonlinear. This stress dependency is more pronounced for plastic soils especially at low effective [...]