PSI - Issue 62

Salvatore Aiello et al. / Procedia Structural Integrity 62 (2024) 1128–1136 S. Aiello et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Following the tragic fire incident in 1999 and given the importance of the tunnel, there has been an increased focus on the management, preservation , and conservation of the Mont Blanc Tunnel’s historical and engineering heritage. This attention is particularly directed towards medium and long-term management strategies The proposed study aims to address the challenge of managing the vast amount of data generated by various inspection, management, and maintenance activities over time, and provide a solution for it. The goal is to manage this data in a way that provides a comprehensive overview, enabling informed decision-making regarding maintenance actions within the structure. To achieve this, the starting point for the development of the proposed methodology was the application of reverse engineering to the capacity of the tunnel’s concrete lining. This approach allows for a detailed understanding of the tunnel’s current state and potential areas of concern, thereby facilitating effective and efficient maintenance planning. The application of reverse engineering for assessing the resistance capacity of the concrete lining of the Mont Blanc Tunnel’s va ult is a significant aspect of tunnel maintenance and safety. This study is based on the analysis of all historical material provided by the operator, particularly the studies carried out during the repair of the vault after the 1999 fire. The Mont Blanc massif, part of the so- called “external crystalline massifs” of the Western Alpine chain, is an emerged portion of the deep European continental crust. The geological information of the Mont Blanc massif is crucial for understanding the structural integrity and stability of the Mont Blanc Tunnel. The massif is predominantly formed from a large intrusion of granite, termed a batholith, which was forced up through a basement layer of gneiss and mica schists during the Variscan mountain-forming event of the late Palaeozoic period. The summit of Mont Blanc is located at the point of contact of these two rock types. The Bieniawski parameters, specifically the uniaxial compressive strength, the Rock Quality Designation (RQD), the minimum spacing of discontinuities, the conditions of discontinuities, the inflow of water every 10 m of the tunnel, and the orientation of discontinuities, were derived from historical documents. These parameters are crucial in the Rock Mass Rating (RMR) system, a geomechanical classification system for rocks developed by Z. T. Bieniawski between 1972 and 1973 (Dal Piaz et al., 2021). The different lithologies and the structural layout of the various rock mass sectors crossed during the excavations were reconstructed through the works of Baggio and Malaroda between 1963 and 1964, and the memoirs of Gudefin and Vitel in 1971 (Baggio et al., 1963, Baggio et al., 1964, Baggio et al., 1971, Vitel et al.1965, Hassanpour et al., 2022). These works provide valuable insights into the geological and structural conditions of the tunnel. The values of Bieniawski’s 89 index, evaluated through the historical documentation of geostructural surveys, have allowed the evaluation of the corresponding value of the Geological Strength Index (GSI). The correlation adopted between 89 and GSI is that proposed by Ceballos et al. 2014 for igneous rocks and refined taking into account all the information related to the historical excavations (1). = 1.08 ∙ − 10.44 (1) The study on the Geological Strength Index (GSI) has allowed the division of the tunnel into homogeneous sections characterized by four classes: • GSI ≤25 Based on the GSI values obtained as previously described, the data relating to the covers on the tunnel section (lithostatic load), and the available values of intact rock strength ( ), the dimensions of the plastic radius were subsequently estimated for the entire length of the tunnel using the method of characteristic curves by Brown et al. 1983. This is the portion of the rock mass influenced by the detensioning following the excavation. The same methodology was used to estimate the expected deformations. • 25< GSI ≤35 • 35< GSI ≤50 • GSI ≥50