Increasing the Ability to Drive Long Off-Shore Piles
Authors: Lowery Lee L. , Edwards Thomas C. , Finley James R. 
Published: October 2008

Because offshore pile foundations are being required to support ever-increasing axial loads, two of the most important questions the Design Engineer must answer is whether or not it will be possible to drive these piles to the design penetration, or whether jetting or drilling will be required, and also, what static resistance to penetration will the soil afford at final penetration. Not only are other methods more expensive than driving alone, they also introduce major uncertainties in the load capacity of the pile.

This paper discusses how pile make-up, size and type of hammer, and driving accessories affect the ability to drive long offshore piles, and how the resistance to penetration may be determined. The one dimensional wave equation was used to determine the significance of changes in parameters such as the cross-sectional area of the pile, cushion stiffness, ram weight and velocity, and others.

Although it might be expected that increasing the ram weight or ram velocity would be the major influences on increasing the ability to drive a given pile, it was found in this study that in many cases, increasing the cross-sectional area of the pile was of far greater importance. Also, the cross-sectional area was always found to be of importance in utilizing any increase of hammer energy.

It was further determined that for a given hammer, an increase in the cushion stiffness or coefficient of restitution could often be used to greatly increase the ability of a given pile to be driven.

Probably the most significant conclusion shown by this study is that by use of the wave equation the Design Engineer can for the first time determine quantitatively, rather than only qualitatively, the effects of any of the numerous variables involved, i.e., how much any variable affects the ability of a pile to be driven.

Introduction

During the past several years, the use of the wave equation to investigate the dynamic behavior of piling during driving has become more and more popular. Widespread interest in the method was begun in 1960 by E.A.L. Smith (1), who proposed a numerical solution to be handled by highspeed digital computers, which permitted the investigation of such factors as ram weight, ram velocity, cushion and pile properties, and dynamic behavior of soils during driving. Since this time, a vast amount of experimental data has been taken to determine accurate input values for this method and a multitude of full-scale pile tests have been correlated which now permits an accurate analysis of the dynamic behavior of piles during driving.

At the present time, a number of the major oil companies and construction firms are using the wave equation to answer such questions as:Can a given hammer drive a pile to the required depth of penetration?What driving stresses will be induced in the pile and hammer?What rate of penetration will the proposed hammer provide, and consequently, how long will the installation require?What changes in the hammer, driving accessories, pile configuration, or installation procedure can be made to improve the situation?