Geotechnical Characterisation of Soils and Rocks: a Geological Perspective

摘要

The integration of site geology with engineering requirements is the basis of engineering geology. Despite the record of case histories and the development of field and laboratory investigation techniques, there continues to be a need to describe site geology in terms appropriate for the analyses of deformation and stability of ground in situ. Two developments hold the potential to improve such descriptions: the characterisation of soil and rock, and the wider use of numerical modelling. Methods of characterisation can now be tested with the aid of numerical analyses, and the suitability of the predictions they lead to can be compared with site data from instrumentation. This paper reviews the areas of site description that remain in need of research; material properties, particularly those of their boundary layers, the geological history of sites and the soils and rocks they contain, and the content of ground investigations designed to provide data for numerical analyses. It then illustrates a characterisation appropriate for soils that reveals important elements of the stress history and strength of sediment, using the concept of the Sensitivity Framework. This links to intrinsic properties of the sediment via the Intrinsic Compression Line of Burland (1990), and leads to a similar relationship in terms of strength: the Intrinsic Strength Line. Such characterisation holds the potential of describing sediments in ways other than their material composition, and is an example of input that could prove to be of value in modelling strength and deformation of the ground. Similar characterisations have developed in engineering rock mechanics, particularly the Geological Strength Index proposed by Hoek et al. Here the geological character of rock material, together with a visual assessment of the mass it forms, are used as a direct input to the selection of parameters relevant for the prediction of rock-mass strength. This approach enables a rock mass to be considered as a mechanical continuum without loosing the influence that its geology has on its mechanical properties. It also provides a field method for characterising difficult-to-describe rock masses. Such characterisations can be used with borehole logs and in-situ tests.