Friction Degradation And Set-Up Effects In Hard Clays Offshore Congo And Angola

Abstract

ABSTRACT Piles driven into stiff to hard clays encountered offshore Congo and Angola clearly show both clay-type effects which are i) friction degradation, with very low driving resistances during continuous driving, and II) set-up after driving interruptions. Both phenomena were studied by back-analysis of driving records, including results of pile driving monitoring. It allowed one to: deduce the friction distribution along the piles during driving. show how the shaft friction at any depth reduces as the pile is driven further into the ground. show that the shaft friction after driving delays can be close to the estimated static friction capacity of the piles. The SRD calculation method proposed by the authors takes the friction degradation effect into account, leading to improved pile drivability predictions. Four case histories are presented to illustrate the results obtained. INTRODUCTION Within the pile drivability problems encountered in the soils of the Southern Gulf of Guinea, those related to driving in hard clays found offshore Congo and Angola include i) friction degradation, with very low driving resistances during continuous driving, and ii) strong and rapid set-up effects after driving interruptions which may lead to premature refusal of piles. In a continuous effort to improve our understanding of the behaviour of difficult soils of the Southern Gulf of Guinea (1), the present study, conducted jointly by ELF Aquitaine Production and GEODIA, aimed at taking into account these effects in the drivability predictions. This may have contractual and operational consequences when considering the selection of hammers for pile installation. The so-called "friction fatigue" theory, for the description of skin friction loss during driving, was first introduced by Heerema (2) for the drivability predictions related to the installation of the Heather pile foundations in hard over consolidated clays in the North Sea. The gradual decrease of skin friction along the pile shaft as the pile is driven further was attributed to both clay remolding and horizontal stress decrease in the surrounding soil. Based on simple cyclic laboratory tests and wave equation post analyses of driving experiences, an exponential distribution of the horizontal stress profile acting along the pile shaft was proposed to fit into the drivability predictions. The same model was applied in the under to normally consolidated clays of the Gulf of Mexico (3) and it was shown that i) the process of friction degradation (fatigue) and restoration (set-up) was fully reversible in the Gulf of Mexico clays, and ii) soft and firm clays exhibit a smaller friction fatigue effect than do stiff clays. More recently, and in good agreement with the previous studies, back-analysis of stress-wave data from two 30" diameter test piles driven into both normally consolidated silty clay and very stiff over consolidated clay, showed that the shaft friction at any given level appears to reduce considerably as the pile is driven further into the ground (4). It was also concluded that the residual shaft friction was determined by the radial effective stress acting around the pile and by the remoulded shear strength of the soil.