Geotechnical and Geophysical Investigations for Offshore and Nearshore Developments.

Abstract

This review of the aspects and techniques of marine site investigation has been produced by Technical Committee 1: Offshore and nearshore geotechnical engineering, of the International Society for Soil Mechanics and Geotechnical Engineering, with the aim of providing guidance and good practice examples for the collection of site investigation data for offshore and nearshore structures. Knowledge of seabed soils and rocks is essential if offshore and nearshore structures are to be properly and safely designed and built. A large part of the commercial and operational risk involved in these works relates to uncertainties about the properties of soils and rocks at the site. It is therefore necessary to perform sufficient investigations to evaluate these risks thoroughly. Many geophysical and geotechnical techniques are available to the engineer to perform such investigations. However, offshore and nearshore site investigations involve special problems raising the question - what is sufficient? The answer largely depends on the situation. For offshore platform design, the information required on the soils is different to that, say, for a pipeline and the requirements for a dredging program are different again. Further, the geotechnical information needed for a feasibility study or a site selection is quite different from that needed for detailed design. The equipment and techniques for site investigations in deep water differ significantly from those for shallow water; those for calm water differ from those for harsher environments. It is to assist with the answer as to what is sufficient, that the authors of this booklet aspire. Many parties are involved in the planning and decision-making process of a site investigation, including the owners and clients, designers, contractors, investigators and surveyors. Rarely has any one person all the experience and expertise needed for planning the optimal programme. This document provides useful information for these construction professionals and includes information on: − Soil and rock properties and their consequences for engineering design. − Typical techniques and equipment for marine site investigation. − The relevance of different methods of data acquisition. − Guidance on the planning and scopes of work for geophysical and geotechnical investigations, and interpretation of the results. − The foundation engineering issues surrounding different sorts of structures, schemes and working methods. − Examples and case histories to illustrate special situations, difficulties and common problems. Although hydrographic surveys and other data collection of the water column are required for proper design, the requirements and techniques for such surveys and investigations are not covered in detail in this document. There are many existing international, national and industry codes and guidelines on offshore and nearshore geotechnical and geophysical investigation techniques and operations. These codes and regulations are subject to regular review, modification and improvement, and expert advice should always be sought. This booklet does not in any way purport to be a definitive or thorough dissertation on the state of industry practice, rather it is hoped that it will assist, in some measure, all those engaged in these activities by having to hand a useful practical guide to good practice.