Special Offshore Piledriving Test

Abstract

ABSTRACT Predictions of the bearing capacity of foundation piles generally are based upon soils investigations on site and in the laboratory. In addition, load tests on completed piles have been performed. These provide the checks on prediction methods and experimental validation of established design methods. Actual load tests on large offshore piles however, are very few and tests on piles with bearing capacities of 20 to 40 MN (4.500 to 9.000 kips) would lead to prohibitive costs. Hence an increasing importance is attached to the significance of the Soil Resistance during Driving (SRD) as a yardstick for - or at least a check on - the ultimate static bearing capacity. The SRD may be determined by dynamic measurement of strains and accelerations in the pile and also by post-analysis of blow-counts. On a test site in the Netherlands a 84" O.D. batter pile (6:1) was driven by a Hydroblok type HBM 4000 hammer to 70 m penetration. The pile was instrumented to determine SRD. Accelerations were determined by a new strain difference technique as an alternative to conventional accelerometers. An extensive soils investigation on site and in the laboratory was included in the test program. Post-analysis of blow-count lead to values close to the static bearing capacity at successive levels predicted according to the Cone Penetration Test (CPT)-method. The SRD obtained by measurements proved to be a lower bound for the predicted static bearing capacity (80%). Offshore field evidence from the driving of anchor piles in the Eastern Scheldt show the same correlation between SRD Obtained by post-analysis of blow counts and predicted static bearing capacity on the basis of Cone Penetration Tests, except in cases where the CPT's indicated extremely high values for cone resistance and local skin friction. INTRODUCTION The delivery procedure for two Hydroblok HEM 4000 hammers included an endurance driving test. The opportunity to gather information of a more general nature on the behaviour of the interaction between soil, pile and hammer generated sufficient international support of the industry, certifying authorities and scientific organisations to include soils investigations and instrumentation of the pile in the test program. In this paper some of the results and additional field evidence are presented. The methods used to obtain SRD values and static bearing capacity are summarized and discussed. Data on the test programme and additional field evidence are introduced, followed by a summary of the results and conclusions from comparisons of SRD values and predicted bearing capacities. The objective of the paper is to contribute to the development of methods to determine axial bearing capacity of piles during the actual driving process. Controlled pile-driving presents the opportunity to collect fairly accurate information on the soils during driving. The problem needing the most attention in the future is the interpretation of this information with respect to the in service behaviour of the piles during the lifetime of the construction.