ABSTRACT

An analytical study was undertaken to develop a set of closed form equations to make preliminary estimates of the pile loads, stresses, dynamic resistance and other important parameters in the installation of offshore piling. Solutions were developed for cushioned and cushionless hammers alike, along with illustrations of sample results and comparisons with field-correlated wave equation results.

INTRODUCTION

Since the pioneering work done by Smith (Reference 1), the development of discrete wave equation analysis of pile dynamics during driving has proceeded to the point where there are well developed and accepted algorithms of this method of pile drivability and capacity analysis (References 2,3). These methods have increased our understanding of the workings of stress and strain distribution during driving and allow us to accurately estimate the ultimate pile capacity (or, more significantly offshore with tensile loading, resistance) of a driven pile. As a consequence, the use of the old dynamic formulae, derived as they are from Newtonian impact theory (Reference 4), are obsolete, so much so that they are unknown in the design of pile foundations for offshore structures.

A consequence of the use of wave equation programs, as they are presently used, is that they are by nature "job specific", that is to say the results they produce are unique to a given hammer, pile, and soil system and thus to one specific situation. This is an advantage in that the foundation designer is able to predict an outcome based on the actual pile system; it is a disadvantage when one needs to look at matters from a more general standpoint, such as in preliminary analyses, equipment design, surveys, general hammer capability studies, and general platform specifications (such as API RP 2A (Reference 5)). In such situations the answer might be to use the have equation many times; this can be expensive and one still must choose such parameters as soil type, pile length, etc., which will necessarily limit the scope of the results.

One possibility would be to simplify the wave equation program for preliminary survey use; this has been done by Stockard (References 6, 7) and Lowery et. al. (Reference 8). Another possibility would be to develop closed form equations based on wave mechanics, which then could be applied with ease to a large number of situations. This has been done in Japan (Reference 9); however, this formula involves the use of pile rebound per blow (as opposed to the customary set) and knowledge of SPT results that make it more job specific than desirable for general use.

Another, simpler, formula, is that of Kummel (Reference 10); it is given as (Mathematical equation available in full paper)

This formula has been successfully tested with diesel hammers (Reference 11). It is very simple and fully derived from wave mechanics; however, one must be supplied with, amongst other things, hammer forces, which can vary with cushion, pile hammer efficiency, etc ..

This report contains a description of a method using a simple pile capacity formula derived from wave generation and propagation theory (as with the Kummel Formula), to estimate for preliminary purposes the hammer forces, pile stresses, and pile capacities and resistances to driving.

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