EN-1997 Geotechnical Design-referred to here as EC7 - is a limit states design (LSD) standard that allows application of the Observational method (OM). However, to date, it has not been shown how the OM satisfies the requirements of LSD. In this paper, we investigate this issue. We assume the OM in rock engineering can be regarded as a series of measurements of structural responses when the structure and loading system are uncertain. We rationalize this by examining the behaviour of a beam whose bending stiffness and support conditions are only partially known, as a simple analogue to the OM applied to roof stability of an underground excavation. Our results show that unambiguous definition of the structural mechanism is essential, and hence the current presentation of the OM in EC7 is generally insufficient to allow verification of limit states. Moreover, we discuss strength and time dependent aspects of design in the context of application of the OM within the LSD paradigm. We make suggestions for critical areas which require further research and development in order to allow the OM to be used in rock engineering design and construction such that it permits limit state verification.


Since 2010, EN-1997 Geotechnical Design (CEN, 2004) - referred to here as EC7 - has been the reference design code for publicly funded construction in the EU and, importantly, applies to rock engineering design. EC7 is part of the structural Eurocode suite (CEN, 2002), and as this suite implements limit state design, EC7 requires all relevant limit states to be verified during design, construction and the service life of a structure.

Uniquely in the Eurocode suite, EC7 allows the application of the procedure known as the "Observational Method" (OM) (Peck 1969) for cases where "prediction of geotechnical behaviour is difficult". However, it seems that there has never been a formal investigation of how the OM satisfies the LSD paradigm. As application of the OM is commonplace in rock engineering, we believe it is critical that this is done. Thus, this paper re-examines the OM in terms of its compatibility with EC7 requirements.

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