Geological Characterisation and the Observational Method. Application of Value of Information Analysis

Abstract: The implementation of the new European standards for geotechnical design calls for new practice in the underground construction industry in Sweden. The standard proposes the Observational Method as an alternative method when the geotechnical behaviour is difficult to predict. The method involves iterative updating of models and design, which demands that the procedure for rock mass characterisation facilitates the updating. Geological investigations are often associated with high costs and uncertainties. This thesis presents how a decision-maker can prepare for a continuous updating process at the exploration phase of a project. The primary objectives of the thesis were: to describe how Bayesian statistical methods can be useful in rock mass characterisation according to the Observational Method; and to show how Value of Information Analysis (VOIA) can be used as a tool when working according to the Observational Method. VOIA is based on Bayesian statistics and cost-benefit analysis and is suitable for problems when different alternatives are evaluated and compared. The method also facilitates iterative updating in the course of the project. VOIA in tunnelling and rock mass characterisation for grouting in hard rock were tested in two hypothetical examples. The suggested method for VOIA in rock mass characterisation can contribute to good structure in geological surveys when the geology is difficult to predict and when repeated updating is necessary. It leads to well-motivated investigation programmes where geotechnical value is weighed up against execution costs. A prerequisite for the method is efficient communication and knowledge transfer between everyone involved in the project. A purpose-driven rock mass characterisation will, using VOIA, contribute to a transparent decision-making procedure and to an investigation programme that is well adapted to the statutes of the Observational Method.