Offshore Pile Design For Sites With Complex Soil Conditions

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ABSTRACT The assessment of axial pile behaviour is often difficult due to the presence of complex soil stratigraphy at offshore platform locations. There are few recommendations in the literature, or in codes of practice, for the design of offshore piles in such conditions. This paper provides guidance for the evaluation of both skin friction and end bearing in multi-layered soils. INTRODUCTION Over the past two decades significant research has been undertaken to better understand the factors controlling pile capacity, with particular emphasis on skin friction in cohesive soils. In 1987 the American Petroleum Institute's "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms" (API RP 2A)1 changed the procedure given in the main text for assessing skin friction in clay, and introduced a "Commentary on Axial Pile Capacity in Clay" giving five alternative methods. There is, however, little guidance for pile design in layered soils in the code. Foundation economics can be governed by stratigraphy. To understand the factors controlling skin friction is undoubtedly important, and worthy of continued research. However, in actual foundation design, possible reduction in end bearing due to the presence of weak layers may have a far greater influence on pile length or structure reliability than a detailed understanding of unit skin friction. This paper sets out to quantify the likely effect on end bearing of weak layers near the pile tip. Results from numerical analyses, published data and pile driving performance data are used to evaluate bearing capacity in layered soils. The findings are presented as the relationship between foundation geometry and ratio of layer bearing capacities, and provide a rational method of design, rather than the commonly used "3-D in, 3-D out" approach to end bearing in layered soils. Recommendations are also made for the calculation of skin friction in multi-layered soils. CURRENT DESIGN GUIDANCE The most widely used code for offshore pile design is the American Petroleum Institute's API RP 2A (1991)2. Detailed recommendations are given for assessing unit skin friction and end bearing in clays and sands. However, stratigraphic considerations are only briefly covered in Sections 6.4.2 (cohesive layers) and 6.4.3 (cohesionless layers), with the general recommendations that:piles should penetrate two to three diameters into the stratum to develop full end bearing, andthe pile tip should be approximately three diameters above the base of an end bearing stratum to preclude punch. through. The possible effect of soil layering on skin friction is not discussed in API RP 2A. STRATIGRAPHY In this paper it has been assumed that adequate site investigation has been undertaken to confidently assess stratigraphy and soil characteristics. If a relatively thin end bearing stratum were present at a site it would be imprudent to design platform foundations using the stratigraphy from a single boring. Where such a stratum exists, with potentially significant benefits for pile penetration, detailed information comprising both geotechnical and geophysical data will be required to provide confidence in foundation design.