Predicting Pile Driveability: Heather As An Illustration Of The "Friction Fatigue" Theory

Abstract In the series of papers on the Heather Field pile foundation, this paper presents the backgrounds of the driveability prediction made by Heerema Engineering Service. The prediction was made according to the 'friction fatigue' theory, which is based on the assumption that skin friction in clay is gradually lost along the pile shaft as driving proceeds. A laboratory test is described which illustrates this phenomenon. A mathematical model is given for the shape of the horizontal stress distribution along the pile shaft; parameters needed in this formulation have been empirically determined, partly from laboratory tests and partly from computer post-analyses of driving experiences. This was necessary as it is as yet net possible to quantify horizontal soil stresses acting on the pile wall during driving on an analytical basis. The friction fatigue theory presented here proves to be consistent for all pile driving experience in stiff and hard clay in the North Sea investigated by the author, and it is believed, therefore, that it will in general lead to improved pile driving predictions as compared to other available methods. 1. Introduction Pile driving at Heather Field was one of the most interesting piling cases of the past few years in the North Sea. The soil consisted of very heavily overconsolidated clay at shallow depths, with undrained shear strength values up to 800 KN/m2, twice the values encountered so far in the North Sea. At depths near to design penetration of the piles, however, the clay had rather common shear strength values. It was therefore difficult to make a reliable prediction of the driveability. By conventional means of predicting (i.e., through the use of ultimate static bearing capacity calculations), it was to be expected that it was impossible to drive to the required penetration of 46 m with the hammers available. The author's research group was nevertheless optimistic about the possibility to reach design penetration. Their prediction was that it should be possible to reach design penetration with the second-largest hammer available, the Menck 8000. This proved to be correct. Although the blow count was considerably higher than expected over most of the piles' penetration depth, the expected blow count range near design penetration was correct. (fig. 1; discussion in section 7).