Axial Capacity of Offshore Piles in Clay

Abstract

ABSTRACT The paper describes two main aspects related to axially loaded offshore piles in clay: methods to determine the local ultimate skin friction and effects of cyclic loading on axial pile capacity. A new 'NGI-method' for determination of ultimate skin friction is described in detail. The method is primarily based on predicting the ultimate skin friction from the state of effective stresses at onset of pile loading, coupled with untrained shear strength properties of severely remolded and reconsolidated clay as determined by direct simple shear tests. A software system for evaluating the response of an offshore pile is described. Based on a case study for piles in soft plastic clay, it is shown that the pile capacity is significantly reduced if the cyclic loads comprise a large portion of the design loads. The influence of pile length (flexibility) is also demonstrated. INTRODUCTION Significant efforts were made in the past decade to improve the methods of determining the axial capacity of offshore piles in clay. These include development of an extensive pile load test data base (Olson and Dennis1) and related statistical studies on the reliability of various common empirical design rules (Olson2, Tang3, Lacasse and Goulois 4); a number of large and small scale instrumented pile load testing programs and development of new theoretical models. As a result of these efforts, the API-RP2A 5 design guidelines underwent a pronounced change in the 17th edition, where the undrained shear strength reduction factor, ?, was related to the in situ normalized untrained shear strength ratio, rather than the absolute value of the untrained strength. Furthermore, in the commentary to the 17th edition the necessity of separately evaluating the effects of pile length or flexibility and cyclic loading on the pile capacity was pointed out. During the period 1980-84 the Norwegian Geotechnical Institute (NGI) undertook a comprehensive pile load testing program in an over consolidated clay deposit at the Haga test site, Karlsrud and Haugen 6,7 A total of 27 individual pile tests were carried out on 5 m long, heavily instrumented steel piles with a diameter of 15.4 cm. The pile loads covered a wide range of static and cyclic loading combinations, mostly in tension, but also in compression. Based on the results of these tests, various theoretical concepts and models were developed for analyzing the pile clay interaction during installation, reconsolidation and pile loading. This paper concentrates on two main aspects:Determination of the ultimate static skin friction after full set-up according to the "NGI-method", and comparison to API and some other empirical design rules.Determination of effects of cyclic loading on the axial capacity of offshore piles according to the computer model PAXCY developed by NGI.