The typical stages of geotechnical studies required for the design and construction of large civil engineering works are more or less similar around the world. Figure 1.1is a fairly common well established 'traditional' arrangement, although many variations exist. A 'fast track' approach puts all the activities on the critical path, but decision milestones would remain. The stages of the site investigation and the build-up of the geological model in a manner that embodies OM during the preliminary and the main investigations and again during construction, are critical to a successful outcome.
Our approach starts with the broad anticipation of the geology, from a desk study of that location, where the geological knowledge may be minimal, or a lot may be known already. Whichever the situation, it is necessary to:
Conventional approaches to the desk study work for the feasibility and early phases of site evaluation typically use the local maps and literature which commonly exist from many developed areas around the world. The Transport Research Laboratory Report (TRL 192), 1996, by Perry and West, on Sources of Information for Site Investigations in Britain (Revision of TRL Report LR 403) is a good example from the UK. Such works also often describe various forms of maps and remote sensing for engineering purposes.
In locations where much may be known already, e.g. geological maps do exist or there are good air photographs or other imagery, the initial local geology can be quite well anticipated by site specific models prior to any preliminary inspection (see Fookes 1997, page 348 and Figures 10, 41a to 41e, and Table 5). The site inspection, preliminary and full ground studies can then be progressed quite quickly at such a location, to give as detailed a picture of the geology as is considered sufficient or necessary. Nevertheless, this may result in some shortfall in anticipation of the total geological picture if a fundamental understanding of the regional and local geology has not been formed.
By 'anticipation of the total geological picture', the authors mean that all the geological and geomorphological characteristics of the site have been anticipated together with the range of possible variation (e.g. sizes, locations, properties) of these characteristics identified, i.e. ideally there should be no condition that comes as a surprise during construction.
Figure 1.2 shows a crude approximation of how well-designed site investigation studies, from desk stage to the project in-service stage, develop increasing geological and geotechnical knowledge. Note that it is suggested that geological knowledge rises more quickly than geotechnical in the earlier stages of a project: the gathering of geotechnical data develops faster when in situ and laboratory testing and rigorous description is introduced during the latter stages of investigations (Fookes, 1997). Rather fewer publications describe investigation activity during construction but the principle and techniques are essentially those used in pre-construction investigations. For guidance, see Eddleston, et al. (1995).