Coupling Effects of Uplift Loading and Lateral Loading on Capacity of Piles
Authors: Darr* K.A. , Reese L.C. , Wang S.T. 
Published: October 2008

ABSTRACT
New concepts for platform design have been developed recently to allow for the production of oil and gas in deep water, The foundation for each leg of the platform is usually a group of piles that are subjected to both uplift and lateral loads. A conventional approach to the design of the piles is to neglect the interaction between these two types of loading, but evidence indicates that the tensile capacity and the lateral capacity can both be affected because of the combined loading. The paper describes an approach to a rational solution of the interaction problem. Both analytical and experimental studies were conducted and the influence of cyclic loading is considered.

INTRODUCTION
Piles that are used for foundations of offshore platforms in deep water are frequently subjected to both uplift and lateral loading. The conventional approach in design is to assume no interaction between the two types of loading. Analysis shows that soils near the mudline are most important with regard to lateral loading, and in most instances are relatively unimportant with respect to axial loads. While this assumption has apparently been satisfactory for lateral and compressive loading, some special consideration is warranted for cases where the axial loading is tensile. There are two important differences in behavior of piles in compression and in tension that might decrease the capacity of the latter type:piles loaded in compression are more apt to exhibit an increased resistance with increase in settlement, partly due to end bearing; andthe loads in skin friction along the upper portion of a pile act against a free surface in the case of uplift, but against supporting layers in the case of compression.
These two handicaps for a pile in tension may be of minor importance, but could be critical in cases where the load transfer in side resistance is decreased due to deflection-softening. A consideration of decreased load transfer with relative movement between the pile and the soil indicates the need to consider the possibility of progressive failure in uplift, especially for large offshore structures. This paper considers briefly the concept that coupling of uplift and tensile loading results in the need to consider reduced load transfer in lateral resistance and in side resistance. Models for the uncoupled method are given, results from small-scale studies are presented, a model for the coupled method is shown, and design recommendations are suggested.

MODELS FOR THE UNCOUPLED PROBLEM
According to methods presented by the American Petroleum Institute in RP 2A (1987), the computation of axial and lateral load employs the following equations. For axial loading:(mathematical equation)(available in full paper) where:Qd = total load,Qf = skin friction resistance,Qp = total end bearing, zero for uplift resistance,f = unit skin friction capacity,As = side surface area of pile,q = unit and end bearing capacity, zero for uplift resistance, andAp = gross end area of pile.
With regard to the computation of the axial movement of a pile, RP 2A implies the use of the t-z method (Coyle and Reese, 1966), where t is the load transfer and z is the relative movement of the pile.