Observational Method and Insight in the Most Common Design Approaches from that Point of View Panciera A.1 1Lombardi Engineering Limited, Giubiasco, Switzerland Online Published on 28 September, 2023. Abstract Observational Method has been defined as the generalised approach for studying projects in com-plex geological conditions according to the Eurocode 7 [13], especially for tunnel projects. Various reasons stay behind the suitability of this approach. It is valid for all kind of underground work, in every geological and operational condition. The task of the engineer is to ascertain the geological features for defining the boundary condition for the project execution under safe conditions. The characteristics of the project and its finalization are part of the equation. The set of all determinant situations has to be identified in detail as per available information. For doing this, no classification is sufficient per se. Geological survey and investigations are to be collected in the geological-geotechnical model. The conditions along the tunnel have to be consequently expressed in terms of lithology, geotechnical characteristics, associated aspects such as cover, water presence, water sensitivity, time dependency, operational loading conditions, etc. These conditions affect the project, requesting a differentiated solution pattern and, finally, determining the cost and time for the project execution and commissioning. The approach allowing to evaluate the hazards, the way how to consider these in the design process, hence in the construction and relevant organization, is the so-called hazard scenario method. The pattern of solutions (conceived, analysed and verified) for all identified hazards is the base of the design. This needs to be calibrated to the possible variation of the conditions, which cannot be known in advance in all details. The actual response of the support measures to the excavation and progress has to be monitored and checked under various points of view. The adjustment of the measures to be applied for making the construction and the operation safe is a continuous process, based on observation. This approach clarifies the origin of the definition of the Observational Method. The process is based on 4 typical stages: Plan, as studying the situations and hazards that define the boundary of the de-sign environment; Do, as actualising the design through the construction and all requested measures; Check (or Study), as evaluating the behaviour that the support measures and the construction are showing; Act, as reacting to the behaviour by adopting or re-evaluating the suitable countermeasures in the given conditions (PDCA or PDSA in short, from Deming in the 50’s [4] and 1982 [5] and Mizuno 1959). Other design approaches are rather known and currently applied with satisfactory results in our professional sector: NATM (New Austrian Tunnelling Method) [7] [12] [14] [15] [16], ADECO-RS (Analysis of Controlled De-formation in Rocks and Soils) [17] [19], NMT (Norwegian Method of Tunnelling) [6] [10] [11] [18] [20], RMR approach (Rock Mass Rating) [9]. These approaches are, if ever, only partially valid, not systematically applicable and representative of the tunnelling process in limited circumstances. The aim of the author is to present lead these approaches within the point of view of the Observational Method and highlight their characteristics and limits, in order to lead to their careful application or disregard towards the most general approach of the hazard scenario method by applying the PDSA cycle. Top |