The conceptual idea behind the observational method can be described as an active process where we will be able to modify and adapt our design to actual geological conditions by means of additional information coming from observation during construction. This will in many cases give a more economic design. A successful application must be based on that all parties involved in the project acknowledge the conceptual idea in which an understanding of how geological uncertainties can be reduced by observations is essential. Further formal and practical requirements must be fulfilled. The contractual prerequisites must also be established. The observational method must also have a clear role to play in the execution phase and must be integrated into the production process. In the paper the connection between the observational method and modern developments in probabilistic method
Design of underground openings includes to deal with uncertainties related to the sub-surface conditions. Safety issues and providing underground structures with an economic design taking the geological setting into account was the key considerations when the observational method was formulated by Peck [1]. The main contents of the observational method had been presented earlier by Terzaghi and Peck [2, 3]. The term ‘active design’ is used in Sweden to denote a methodology for dimensioning complex structures in rock. This approach has great similarities with the observational method. The term was introduced for emphasizing that design is an active part of the construction process. The basic elements of active design comprises to establish a preliminary design,
plan for contingency measures in case of unforeseen events,
perform relevant observations during construction and,
accomplish a final design that complies with requirements by means of active adaptation to the actual conditions.
Stille describes how active design has been used successfully in a number of complicated Swedish rock engineering projects [4]. The observational method is one of the designated design methods in the new Eurocode, EN 1997–1:2004[5]. From an engineering point of view this allows the designer to employ formal approaches towards design uncertainties as well as towards results from monitoring and observations made during construction. However, the theoretical background as well as the contractual aspects is not described in the code, only the formal requirements for applying the method is given. Quantifying uncertainties and its dependency on the degree of information is a challenge, but nevertheless a logical extension of the observational method. Connecting the observational method to modern developments in probabilistic methods will provide a firm base for its application [6, 7]. This issue is a central part of this paper. The aim for this paper is to present an overview of the observational method and its application in rock engineering. A successful application must be based on that all parties involved in the project acknowledges.
