Reinterpretation of the West Sole platform «WC» pile load tests

There is a considerable lack of field testing of prototype piles on offshore structures, the state of the art at present being such that most design methods for piles currently used have been extrapolated from lightly loaded land case histories. Since owing to scale effects the behaviour of piles offshore differs qualitatively from those onshore, field load tests are required particularly in stiff normally and overconsolidated clays. The basis of soil mechanics is field testing at full scale, which, unfortunately, is not practical in many cases offshore. The West Sole pile load tests, being the only major offshore pile load tests carried out in the North Sea, form an important part of the data base used in the development of the American Petroleum Institute's procedure (API RP2A) for the determination of axial pile capacity for single piles driven in cohesive material. The original interpretation of the tests used site investigation data obtained in 1968 from a location 200m from the platform position. A subsequent investigation in 1978, using more sophisticated drilling and sampling techniques at the correct location, disclosed that soil strengths were underestimated in the original investigation. The results of this more recent site investigation and subsequent reinterpretation demonstrate the need for site investigations of a high quality, preferably at the correct location. The reinterpretation of the data revealed that the adhesion or α factor (the ratio of soil–pile adhesion to undrained shear strength) used at present in the total stress method of design of piles in overconsolidated clays in the North Sea may be unconservative and may lead to a lower factor of safety against failure than that designed for. The study revealed that the back-calculated α values relevant at the time of the West Sole pile tests were slightly less than the 0·5 value recommended by the American Petroleum Institute for such soils. I1 y a un manque considérable d'essais de chantier effectués sur des pieux prototypes en mer; il en résulte que les règles de l'art relatives à la construction actuelles des pieux ont été extrapolées à partir d'essais faiblement Iégèrement chargés g terre. A cause des effets d'échelle le comportement des pieux en mer diffère de façon qualitative de leur comportement à terre, et par conséquent on a besoin d'essais de chargement en place, particulièrement dans les argiles raides normalement consolidées et surconsolidées. La mécanique des sols est ba