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We performed a field investigation into the effective stress, the shaft friction at the pile-soil interface, and the pile-end base resistance of a 29.5 m long prestressed concrete pipe pile in soft clay under static and cyclic loadings (50 000 cycles). Our results indicate that under low-level cyclic loading (CLR [less than or equal to] 0.2, where CLR is the ratio of cyclic load amplitude to the ultimate capacity), the pore water pressure at the pile-soil interface initially accumulates with the number of cycles, followed by gradual dissipation after reaching its maximum value. The effective stress also initially decreases and then increases. The shaft friction increases after cyclic loading. Owing to axial load redistribution, the base resistance decreases with the number of cycles. When the pile is subjected to high-level cyclic loading (CLR [greater than or equal to] 0.5), the pore water pressure at the pile-soil interface keeps accumulating while the effective stress continues to decrease, leading to the degradation of shaft friction. The base resistance increases as a result of axial load redistribution. In particular, we analyzed the effects of the static load level and the cyclic load level on the changes shaft friction. Key words: pore water pressure, effective stress, shaft friction, base resistance, cyclic loading, soft clay. Cette recherche presente une etude de terrain sur l'evolution de la contrainte effective, du frottement de l'arbre a l'interface pieu-sol et de la resistance de la base de l'extremite du pieu d'un pieu tubulaire en beton precontraint de 29,5 m de long dans de l'argile molle sous des charges statiques et cycliques (50 000 cycles). Les resultats revelent que sous une charge cyclique de faible niveau (CLR [less than or equal to] 0,2; CLR designe le rapport entre l'amplitude de la charge cyclique et la capacite ultime), la pression de l'eau interstitielle a l'interface pieu-sol s'accumule initialement avec le nombre de cycles, puis se dissipe progressivement apres avoir atteint sa valeur maximale. La contrainte effective diminue d'abord, puis augmente a nouveau. On observe que le frottement de l'arbre augmente apres un chargement cyclique. La resistance de base diminue avec le nombre de cycles en raison de la redistribution de la charge axiale. Lorsque le pieu est soumis a une charge cyclique elevee (CLR [greater than or equal to] 0,5), la pression d'eau interstitielle a l'interface pieu-sol continue de s'accumuler tandis que la contrainte effective continue de diminuer en consequence, entrainant la degradation du frottement de l'arbre. La resistance de base augmente en raison de la redistribution de la charge axiale. En particulier, les effets du niveau de charge statique et du niveau de charge cyclique sur les variations du frottement de l'arbre sont analyses. [Traduit par la Redaction] Mots-cles : pression d'eau interstitielle, contrainte effective, frottement de l'arbre, resistance de la base, chargement cyclique, argile molle., Introduction Piled embankments have been widely used in high-speed railways, especially in the coastal areas of southeast China where extensive deposits of soft clay are constantly encountered (Chen et al. [...]

Excavation-induced soil arching is a key factor influencing the responses of the ground and underlying structures. This soil arching below an excavation base is defined as downward soil arching in comparison with that above a trapdoor or tunnel, featured with the downward developing direction. To recognize and assess the downward soil arching, two centrifuge tests in the plane-strain condition were conducted in the dense and loose sand. Variations of Earth pressure, basal heave, and shear wave velocity below the excavation base were observed, and the influence of the relative density was also investigated. The test results indicated the existence of differential deformation and stress transfer below the excavation base, and the downward soil arching is confirmed. The contributions of the downward soil arching to the restriction of the excavation-induced stress release and the ground response were found. Further, the boundary of the loosened zone and the width of the arch foot were determined in the dense and loose sand, respectively. In addition, the arched-type basal heave and stress-related shear wave velocity were observed. The significant ground heave and reduction of the shear wave velocity were found within the loosened zone. [ABSTRACT FROM AUTHOR]

The tetrapod jacket foundation is one of the most common types of foundations used for offshore structures. Although the static bearing capacity of the jacket foundation is important, a safe design must be taken into consideration for accumulative deformation and stiffness degradation during cycling. To address this, a set of centrifuge model tests of a tetrapod jacket foundation subjected to cyclic and monotonic loading was conducted in kaolin. The test results mainly reveal (i) the influence of loading history on the lateral bearing capacity of the jacket foundation; (ii) the evolution of accumulative deformation of the jacket foundation during cycling; (iii) the stiffness degradation of soil-pile interaction under cyclic loading. Comparing typical 'lateral soil resistance'-'pile lateral displacement' (p-y) curve approaches with test results, a hyperbolic tangent p-y curve method is adopted to calculate the soil-pile interaction of the jacket foundation. The values of the p-multipliers of leading-row and trailing-row piles in soft soils are clearly lower than that in sands. Finally, a cyclic p-y curve method related to cycles and embedded depths is developed for the jacket foundation in the hope of providing guidance for the design of the jacket foundation in soft soil. Key words: offshore foundation, centrifuge modelling, soft soil, lateral loading. La fondation a gaine tetrapode est l'un des types de fondations les plus courants utilises pour les structures en mer. Bien que la capacite de charge statique de la fondation a gaine soit importante, une conception sure doit etre prise en consideration pour la deformation accumulee et la degradation de la rigidite pendant le cyclage. Pour y remedier, une serie d'essais sur modele de centrifugeuse d'une fondation a gaine tetrapode soumise a une charge cyclique et monotone a ete realisee dans du kaolin. Les resultats des essais revelent principalement : (i) l'influence de l'historique de la charge sur la capacite de charge laterale de la fondation de la gaine; (ii) l'evolution de la deformation cumulative de la fondation de la gaine au cours de cyclage; (iii) la degradation de la rigidite de l'interaction sol-pile sous une charge cyclique. En comparant les approches typiques de la courbe << resistance laterale du sol >>-<< deplacement lateral du pieu >> (p-y) avec les resultats des essais, une methode de courbe p-y tangente hyperbolique est adoptee pour calculer l'interaction sol-pile de la fondation de la gaine. Les valeurs des p-multiplicateurs des pieux des rangees de tete et de queue dans les sols mous sont nettement inferieures a celles des sables. Enfin, une methode de courbe p-y cyclique liee aux cycles et aux profondeurs d'enfouissement est mise au point pour la fondation de la gaine dans l'espoir de fournir des orientations pour la conception de la fondation de la gaine dans les sols mous. [Traduit par la Redaction] Mots-cles : fondation en mer, modelisation par centrifuge, sol mou, charge laterale., 1. Introduction Offshore wind power offers a potential source of renewable energy and as a result, Markets for offshore wind farms have expanded rapidly within the past decade. Most current [...]