Driven piles in clay—the effects of installation and subsequent consolidation

Abstract

This paper describes the results of numerical analysis of the effects of installing a driven pile. The geometry of the problem has been simplified by the assumption of plane strain conditions in addition to axial symmetry. Pile installation has been modelled as the undrained expansion of a cylindrical cavity. The excess pore pressures generated in this process have subsequently been assumed to dissipate by means of outward radial flow of pore water. The consolidation of the soil has been studied using a work-hardening elasto–plastic soil model which has the unique feature of allowing the strength of the soil to change as the water content changes. Thus it is possible to calculate the new intrinsic soil strength at any stage during consolidation. In particular the long-term shaft capacity of a driven pile may be estimated from the final effective stress state and intrinsic strength of the soil adjacent to the pile. A parametric study has been made of the effect of the past consolidation history of the soil on the stress changes due to installation of the pile. The results indicate that for anyinitial value of overconsolidation ratio, the final stress state adjacent to the pile is similar to that in a normally one-dimensionally consolidated soil except that the radial stress is the major principal stress. A method is described whereby the model of pile installation and subsequent consolidation may be extended to clays which are sensitive. The method is used to predict changes in the strength and water content of soil adjacent to a driven pile which compare well with measurementsfrom two field tests on driven piles. It is also shown that the rate of increase of bearing capacity of a driven pile may be estimated with reasonable accuracy from the rate of increase in shear strength of the soil predicted from the analysis. Cet article fait part des résultats d'analyses numériques des effets occasionnés par la mise en place d'un pieu battu. L'hypothése de conditions de déformation plane en plus dúne symétrie axiale a été adoptée pour simplifier la géométrie du problème. Le battage du pieua été représenté par l'expansion non drainée d'une cavité cylindrique. On a supposé par la suite que les surpressions interstitielles engendrées au cours de ce processus se dissipaient en raison de l'écoulement radial vers l'extérieur de l'eau interstitielle. L'étude de la consolidation du sol a fait appel à un modèle de sol élasto-plastique rigide, qui a pour propriété unique de permettre des modifications de la résistance du sol en fonction des variations dela teneur en eau. Grâce à cela, il est possible de calculer la nouvelle résistance intrinsèque du sol à une étape quelconque de la consolidation. Notamment, il est possible d'estimer le frottement latiére à long terme d'un pieu battu à partir de l'état de contrainte efficace finale et de la résistance intrinséque du sol adjacent au pieu. L'effet de l'historique de consolidation antérieure du sol sur les variations de contraintes dues a l'installation du pieu a fait l'objet d'une étude paramétrique. Les résultats montrent que pour toute valeur initiale du degré de surconsolidation, l'état de contrainte final au voisinage du pieu est identique à celui d'un sol à consolidation unidimensionnelle normale sauf que lacontrainte radiale constitue la contrainte principale majeure. L'article decrit une méthode suivant laquelle le modèle d'installation de pieux et de consolidation subséquente peut être appliqué aux argiles qui sont sensibles au remaniement. La méthode sert à prévoir les variations de la résistance et de la teneur en eau du sol adjacent à un pieu battu, lesquelles sont tres voisines des mesures provenant de deux essais in situ de pieux battus. Il appalaît également que le taux d'accroissement de la capacité portante d'un pieu battu peut etre estimé dans des limites de préision raisonnables à partir du taux d'accroissement de la résistance à la rupture au cisaillement du sol prévu par l'analyse.

The shaft resistance of axially loaded piles in clay

“…Changes in pore-pressures and radial total stresses close to the pile were recorded during the installation and the subsequent consolidation, and the variation of the local undrained shear strength was determined by means of vane tests. The undrained shear strength of the clay adjacent to the pile increased by 50% as a result of the consolidation following pile driving; this figure accords well with -20-predictions based on cavity expansion theory (see Randolph, Carter and Wroth, 1979). After consolidation, the radial effective stress acting at a distance of one pile radius from the pile shaft was found to average 1.6 times the current local undrained shear strength (or 2.5 times the initial undrained shear strength).…”

Interpretation of Cone Penetration tests in Cohesive Soils

“…After Vesic, significant progress was made in developing cavity expansion solutions by adapting improved soil stress-strain models and yield criteria in both clay and sand (Cater et al 1986, Yu and Houlsby 1991, Salgado et al 1997, Salgado and Randolph 2001. More specifically, many researchers have related limit pressure solutions to practical values, such as pile end bearing or cone resistances (Randolph et al 1979, Salgado 1993, Yasufuku and Hyde 1995, Salgado and Randolph 2001. All cone factors N k derived from cavity expansion solutions depend on the rigidity index I r of soil.…”

Numerical analyses of stone column installation in Bothkennar clay

“…Three different areas may be distinguished: (1) an elastic one (beyond 11.5-13.5 column radii depending on the soil model) where soil behaviour is always elastic and hoop stresses decrease, (2) an area that is plastic during undrained expansion of the cavity but is not after consolidation, where vertical stresses change only slightly and (3) points that are on the yield surface also after consolidation (closer than 4.5-6 column radii), where densification and the increase in mean effective stresses are important. Randolph et al (1979) presented similar results for normally consolidated Boston Blue clay using the MCC model and a similar numerical model for driven piles. The results from this study with the MCC model are compared with those results ( Figure 3).…”