Applications of Wave -Equation Analysis to Offshore Pile Foundations
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ABSTRACTA wave-equation-formulated computer solution of the pile-driving process is shown to have considerable application to offshore foundations as a design and installation aid. Using the method, it as now possible to optimize pile drivability by proper selection of pile size, hammer, driving accessories, and field procedures. Impact stresses in both pile and hammer may be analyzed. Typical design curves are shown which are similar to those that could be developed for a handbook-type approach to preliminary design for pile drivability.INTRODUCTIONThe basic problem confronting foundation engineers with regard to high-capacity pile foundation is one of designing may choose to support large foundation loads by a smaller number of large piles. Although design philosophies in this regard may differ, the common goal as a safe but economical structure. Generally, it is thought it be more economical to install a few large piles than to install a greater number of small piles, providing the piles can be installed without great difficulty. This situation obviously leads to the problem of deciding early in the design procedure what upper limits, based on expected driving characteristics, must be set on pile diameter and depth of penetration. Where sufficient experience in the construction area or a similar area is available, this may not be too difficult a decision; in cases where experience is lacking. However, very little has been available to aid the decision making.In order to meet the need for greater driving capabilities, hammer manufacturers have developed new pile drivers with greater energy output. Unfortunately, these new hammers are correspondingly more massive and more difficult to handle in the offshore environment than their smaller predecessors. In addition, sometimes even these larger hammers may have difficulty driving pilling unassisted.Additional techniques must therefore be applied in the design and installation of pile foundation if we expect to fully meet the challenge of larger structures. These techniques should permit the following questions to be answered:Can field installation problems be anticipated in the design stage in order to provide for alternate installation techniques, or even redesign of the pile foundation?Cana pile be designed to improve its driving characteristics?Can a more efficient set of pile driver accessories be developed, or can driving accessories be tailored to the individual job?Can new pile drivers with more driving capability be developed by means other than making them more massive?E.A.L. Smith1 was successful in the early 1950's in developing a rational method of analysis foe the dynamic behavior of piling. For practical application his method required the use of high-speed digital computer and was not widely accepted initially. In 1963 Samson, Hirsch, and lowery2 programmed a more detailed version of Smith's method3 for computer solution and field-test correlations. Subsequent investigations by Hirsch and Edwards4 and Lowery5 have resulted in refinements of input data so that the simulation of pile-acceptable accuracy.Difficult conditions of large pile diameter and deep penetration requirements make wave-equation analysis especially useful for offshore applications. This approach enables the engineer to obtain a more rational solution to pile-driving problems.
