Staged Understanding of the Geology

As a result of their study of the Scottish road projects, Matheson and Keir suggest that the effectiveness of site investigations can be increased by efficient planning and by placing particular emphasis on the preliminary phase. A critical point: they strongly recommend that a planned preliminary site investigation should be included in all [road] projects. They go further and say that the preliminary site investigation should be of a qualitative to semi-quantitative nature, and that a high level of detail is neither necessary nor desirable: it is sufficient to indicate rather than to define potential problems. Without some prior knowledge of the ground conditions, it is difficult to plan an effective main investigation. Optimum methods and techniques cannot be chosen to suit unknown geological conditions. Information obtained at the preliminary phase is thus of paramount importance to the planning of a main site (i.e. ground) investigation. [Our bold for emphasis]

We agree with Matheson and Keir and in our paper recommend the adoption of this second wise dictum, that of a staged approach with a preliminary investigation which itself starts with a desk study, to plan the main investigation phases. However, this approach is not always seen as appropriate. For example, Ground Engineering  in an article called 'Time to Investigate', written as a result of a survey of practitioners in UK where the response 'as usual was high, reflecting the strong feelings held by firms that something had to be done ....', wrote in red for emphasis - 'An average of only 39% of investigations include a desk study, which could be considered the most important stage of the work. It appears that this is a result of ever-decreasing timescales'. Also in red, 'Worryingly, it seems that only 30% of clients appreciate the value of good geotechnics. There is a lack of awareness of the benefit of sound site investigation strategy by clients and their advisers .... some 38% of clients see site investigation as a "necessary evil"'.

It appears to us that the current trend around the world is for complex civil engineering projects to be carried out within very short time scales. Such projects often involve difficult and protracted planning consultation which leads to increased pressure on all the professionals involved to work at a very fast rate and, on occasion, at an erratic pace that is dictated without regard for good practice. In no situation is this more so than with the collection and utilisation of ground information for the design and construction phases of the project. Hence, the preliminary stage, as a precursor to a fast and efficient main site (ground) investigation, becomes all the more important and there is even greater emphasis on the engineering geologist's role in the assessment of risk arising from the uncertainty about ground conditions (Eddleston, Murfin and Walthall, 1995).

It is worth repeating that at the preliminary stage the engineering geologist probably can have the most significant influence on the project by indicating potential hazards and their consequence on the economy of design, matters of construction and expected performance of the works. Our experience shows, however, that all too often engineering geological advice is not adequately taken into account at this or following stages. Difficulties also often arise when geological conditions are relatively simple yet the significance of the conditions may not be understood or is lost sight of in a mass of other data, due to lack of understanding on the part of the decision makers.

G, as a percentage of P are, for:

a low sensitivity (S = 0.0005) building project, around 1.5%

a typical (S = 0.002) civil engineering project, about 4%

a high sensitivity (S = 0.005) tunnel project, between 6% and 8%.

Goldsworthy speculates that 'regional differences relating to typically encountered ground conditions and to contractual practices may affect the sensitivity factor distributions'.

The above estimates can be compared with current levels of expenditure on ground investigation, which are reported to be well below 1% of the total construction costs (Institution of Civil Engineers, 1991; Littlejohn, et al., 1994). The cost of claims due to unforeseen ground conditions is usually far in excess of this, let alone the extra costs of over design to cover risk of unknown ground conditions. Note, however, that we believe that the scope of investigation should be sufficient to answer the important questions, and not simply be determined as a percentage of the project costs, although they can give some guidance.