The Applicability of Geoelectrical Methods in Pre-Investigation for Construction in Rock

Abstract: Construction in rock is associated with risks because knowledge of the geology and ground conditions is limited. Unforeseen rock conditions involve a large risk to the project and can in the end entail delays and extra costs. To minimize the risks, an optimized pre-investigation program has to be conducted where essential information is gathered in order to make the best decisions throughout the construction project. In this Ph.D. thesis the main focus has been on the applicability of geoelectrical methods as a tool for predicting geological and rock mass conditions. The application of the geoelectrical methods at different scales has been proved to provide useful information at different stages of rock tunnel construction. In the geological setting at the Hallandsås Horst the method can indicate fractured, water bearing rock, weathered rock and to some extent lithology changes in crystalline bedrock. Large scale geoelectrical imaging is useful in the design/production planning stage and in the construction stage. Geoelectrical measurements are performed at a more detailed scale between two horizontal boreholes mainly in the construction stage. At even more detailed scale, geoelectrical methods may be combined with other geophysical methods in borehole logging and be applied late in the design/production planning stage. Additionally, borehole geophysics is important for in situ correlation/verification of the large-scale geoelectrical data. In an attempt to demonstrate the applicability of geoelectrical imaging in pre-investigations for rock tunnel construction, a framework for Value of Information Analysis (VOIA) has been developed. The VOIA is used for choosing the pre-investigation program best suited for a specific geological environment. VOIA is based on Bayesian statistics and cost-benefit analysis and is suitable for problems where different alternatives are evaluated and compared. In VOIA the cost for new information is compared with the reduced risk for taking an economically unfavourable decision. New information is only interesting when it can change the outcome of the decision and thus is of value for the decision-maker. The cost of an investigation or measurement should be less than what is expected to be saved; otherwise the investigation should not be made. The VOIA of geophysical methods used in pre-investigation showed indisputably that the value of performing geoelectrical imaging and ground based magnetic measurements prior to drillings has a higher value than only drilling. This result is only valid for this particular geological setting and is site specific. Nevertheless the framework can help designing the best measurement program for a specific geological setting if the VOIA is used to decide between different geophysical methods, e.g. geoelectrical imaging, seismic, magnetic or a combination. The framework developed has the potential to become an integral part of any pre-investigation. With an optimized pre-investigation with well integrated results, the uncertainty in the engineering geological prognosis is reduced and the risk that something unexpected happens is reduced. Geoelectrical imaging and borehole geophysics contributes to reduce the uncertainties and should therefore be considered as a prospective part of all pre-investigations as well as the production stage.