This paper describes a simplified design method by which individual piles in groups can be modelled, accounting for nonlinear soil behaviour and complex pile-soil-pile interaction effects. Computer programs that can normally only model individual piles can be used in conjunction with this method to mode1 pile groups without recourse to more sophisticated foundation analysis programs. The simplified model was developed from a study of the predicted pile group behaviour which was based on the classical p-y, T-Z, Q-Z and Mindlin interaction procedures.

The model was used to represent the pile group foundation of a large Northern North Sea structure. Compatibility studies of the desired and actual behaviour of the model demonstrated its validity. Test loadcases were applied to the model and to other foundation analysis software, the which indicated good correlation.


A number of non linear computer programmes are currently available, for the analysis of offshore structures, that incorporate piled foundation behaviour. These programs usually model the structure using elastic or elastoplastic members, and the foundations as individual pile members restrained by non 1inear soil springs. Many of these programs however cannot accommodate the pile-soil-pile interaction effects required to model pile groups. A method of modelling the pile groups has been developed to provide such a facility giving structure/ foundation compatibility and permitting the individual pile head forces to be evaluated directly.

The lateral (p-y) and axial (T-Z) and (Q-Z) non linear soil springs assume Winkler soil layer modelling and are customarily developed to model the behaviour of isolated piles. Various methods for calculating these non linear springs have been described by Matlock (Reference 1), Reese Cox and Koop (Reference 2), Coyle and Reese (Reference 3) and Kraft, Ray and Kagawa (Reference 4). These soil springs are considered to be appropriate for widely spaced piles, typically pitched at more than 11 pile diameters apart as noted by Cook, Price and Tarr (Reference 5). Figure 1 Shows such a case where single piles are installed inside each of the legs of an offshore platform.

Where piles are more closely spaced, such as in the structure shown on Figure 2, methods that account for pile-soil-pile interaction, such as that of Focht and Koch (Reference 6), are usually considered to be required. The non 1inear load deflection behaviour of a pile group can be obtained by softening the behaviour of a single pile to match the predicted pile group behaviour. This manipulation can be performed in the lateral and axial directions by using y-Modifiers and Z-modifiers on p-y and T-Z (and Q-Z) soil springs respectively.

An attempt was made to obtain a simple foundation model of individual piles in a group for a Northern North Sea structure using y-Modifiers and Z-Modifiers applied to each pile. It was found that no simple relationship linking load and modifier could be developed. Subsequently a review of the predicted pile group behaviour was undertaken which 1ed to the deve1opment of an a1ternative mode11ing method.

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