Keywords: pile, driving, capacity, monitoring, re-driving, hard soils.

The paper presents and describes the design process and the results obtained from a successful pile installation in the very hard and difficult soils of the offshore area in the Gulf of Guinea. In particular, the initial study of the possibility of pile installation by driving and, then, the elements that were inferred by the engineering interpretation of the data collected by the pile dynamic monitoring that has been requested, are discussed in the paper.

The scope of the publication is the engineering process and the measured data and is to report the lessons learned, that confirms what has been observed and documented by several authors working on the very hard clays of this part of the world.

Data are published courtesy of ENI, with the understandable request to keep confidential the position and the name of the locations of interest. For a reference in the text, they will be named with capital letters, A, B, C, and so on.

The design of pile installations by means of driving had to cope with the high strength of the soils and with the lack of geotechnical information at the new location that was to be considered.

The design process was organized in three successive steps. A feasibility study was initially done to select the best geometry of the pile, that had to be short enough so as to minimise the risk of encountering soft rock layers that would be hard enough to stop pile driving for refusal, but also to safely guarantee that the necessary axial bearing capacity was available. Then, the dynamic monitoring of the pile was organized with the capability of giving the elements and data to measure the energy transmitted to the pile, thus allowing the direct check of the proper functioning of the driving system. Finally, both at the end of initial driving and at the subsequent re-driving after a halt of a few hours, measurements were interpreted by back-analysis to estimate the axial capacity.

Large, steel pipe piles, of 84 inches in diameter, were to be installed, to about a 50m depth. Hammers of the hydraulic type MHU 1000 and MHU 1700 (Menck) were adopted for the pile driving.

The condition of the presence of hard soil, potentially able to let the driving system experience refusal, associated with the (exceptional) lack of a dedicated site investigation at the location of interest, had to be faced by means of an engineering analysis of the most representative soil condition, to which some contingencies for the presence of hard rock layers, randomly distributed with depth but with quantifiable strength, has been added.

At the end, all the phases of pile installation occurred as predicted, with no surprises. Despite the (predicted) low resistance to penetration that was measured during the continuous driving, the pile ultimate capacity, that was inferred by the interpretation of the dynamic measurements and by the analysis of the global driving, was greater than calculated, so that to allow for a confident acceptance of the pile.

It has to be underlined, as concluding comment, that the data that allowed for the confident pile acceptance, were available as a result of the instruments installed for the dynamic monitoring. As well, the process demonstrated the paramount importance of measurements and monitoring for geotechnical engineering of large foundations of offshore steel platforms.

Monitoring and data collection regarding the behaviour of real structures that involve soils is rare, but still today it is of primary importance not only to understand the basics of their behaviour, but to help in the engineering design of new structures in the future by reducing the probability of failures and improving the reliability of the engineering design.

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