Analysis of pathologies and long term behaviour of the Swiss national road tunnels

This project, supported by the Swiss Federal Roads Authority (OFROU), focuses on the long term behaviour of road tunnels and on their main pathologies. As a tunnel is composed by lining structure and rock mass, it has been necessary to consider the effects of degradation on both of them. Degradation processes have been divided into three main classes depending on process type and effects: ageing, weathering, and other mechanical and physical processes. Initial conditions (i.e. tunnel depth, construction features as waterproofing system, and age of the structure, together with geological and hydrogeological conditions), operation conditions (e.g. traffic pollution and de-icing salts), and inspection results (disorders) have been taken into account for determining the long term effects of the degradation processes. After a detailed literature review and based on advices from tunnel inspectors, a data base (Tunnel Data Base) and a technical form for data collection have been created. Several information sources have been compiled. Apart from general information, the data base stores technical information, data about construction and tunnel inspections results, collected directly by consulting cantonal archives and responsibles for the Swiss National Roads. Also geological and hydrogeological information, together with traffic data, have been integrated using G.I.S. tools. A comprehensive analysis of the collected data has been performed (168 tunnels; 122 tunnels with disorders data from principal inspections). Typical disorders affecting Swiss National Roads tunnels have been identified. Moreover, using Correspondence Analysis (multivariate statistics method) it has been possible to select, within all the tunnel features, the main factors involved in the development of disorders. Both the degradation potential, due to the tunnel initial conditions, and the degradation rate, due to operation conditions, have been investigated. This analysis better pointed out which factors (or combination of factors) mainly contribute to disorders development and change the long term behaviour of the tunnel. Based on the initial conditions and the degradation rate of both rock mass and lining, it is possible to assess the evolution with time of the tunnel stability conditions. Some examples illustrating the application of this methodology have been proposed in the framework of the convergence-confinement method. Within the basic assumptions of the method, mathematical models were used for describing the time dependent degradation processes (e.g. viscoelastic and viscoplastic behaviour, lining thickness and strength reduction). The results (i.e. changing equilibrium conditions) have been interpreted in terms of Safety Factor of the tunnel structure. Due to the restrictive assumptions on which the convergence-confinement method is based, the aim is not to replace the detailed stability analysis, which remains necessary to determine the long term stability conditions of a tunnel, but to identify the main factors involved in changing equilibrium conditions due to degradation processes. The results show clearly the importance of a detailed characterisation of the tunnel, since its construction and during its service life (by means of inspection results), in order to perform a complete diagnostic of the degradation process and of the main factors involved. Moreover, this study shows the importance of taking into account the delayed pathologies that affect both the rock mass and the concrete lining for a correct assessment of the tunnel future behaviour. Actually, a better knowledge of the tunnel and of its "health" is the basis for an effective tunnel maintenance.


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