Undrained shear strength from piezocone tests

With the advent of piezocones -- penetrometers measuring both the mechanical resistance and induced pore pressures near the tip during penetration into the soil -- a new interpretation of penetrometer test data is possible. This paper presents a review of available interpretation methods for obtaining the undrained shear strength of soft soils, and introduces a new interpretation taking into account measured pore pressures. The undrained shear strength is considered to be solely related to the ultimate cavity expansion pressure, which is one of the components of the tip resistance. The other component is calculated assuming that effective friction is developed at the cone -- soil interface. Parametric studies on the parameters required for strength determination based on the proposed method are also presented. Special self-boring pressuremeter tests to obtain relevant values of soil rigidity index, which is a key parameter for cavity expansion modelling, are described. Tests were conducted at three sites having the characteristics of soft sensitive clay, stiff sensitive clay, and clayey silt. The operational undrained strength mobilized during the cone insertion derived from the proposed model is equal to or lower than the CKsub0U triaxial strength, which depends on soil brittleness. The proposed approach yields results consistent with known soil behaviour at all three sites.L'utilisation du pi\ue9zoc\uf4ne permet une nouvelle interpr\ue9tation des r\ue9sultats du test de p\ue9n\ue9tration gr\ue2ce aux mesures combin\ue9es de la r\ue9sistance en pointe et de la pression interstitielle g\ue9n\ue9r\ue9e pr\ue8s de la pointe. Cet article revoie les principales m\ue9thodes d'interpr\ue9tation en vue d'obtenir la r\ue9sistance au cisaillement non drain\ue9 et pr\ue9sente une nouvelle m\ue9thode qui tient compte des pressions interstitielles. La r\ue9sistance au cisaillement non drain\ue9 est calcul\ue9e \ue0 partir de la pression ultime d'expansion de cavit\ue9 qui est une des deux composantes de la r\ue9sistance \ue0 la p\ue9n\ue9tration. Le frottement d\ue9velop\ue9 le long de l'interface c\uf4ne -- sol, qui est d\ue9termin\ue9 en fonction des contraintes effectives, est la deuxi\ue8me composante. L'article pr\ue9sente \ue9galement le resultats d'\ue9tudes param\ue9triques et d\ue9crit des essais sp\ue9ciaux de pressiom\ue8tre autoforeur permettant la d\ue9termination de l'indice de rigidit\ue9 du sol correspondant aux conditions d'expansion de cavit\ue9 non drain\ue9es. Des essais ont \ue9t\ue9 effectu\ue9 \ue0 trois sites pr\ue9sentant les caract\ue9ristiques d'argile molle et sensible, d'argile raide et sensible et de limon argileux. La r\ue9sistance au cisaillement globale mobilis\ue9e pendant la p\ue9n\ue9tration calcul\ue9e en utilisant les \ue9quations de la m\ue9thode propos\ue9e est \ue9gale ou inf\ue9rieure \ue0 la r\ue9sistance d'essais triaxiaux CKsubU, qui est d\ue9pendant de la fragilit\ue9 du sol. La m\ue9thode propos\ue9e produit des r\ue9sultats en accord avec le comportement observ\ue9 dans d'autres \ue9tudes pour les sols des trois sites.Peer reviewed: YesNRC publication: Ye