Ultimate limit state design for linings of bored tunnels / Tübbingbemessung im Grenzzustand der Tragfähigkeit beim Schildvortrieb

According to modern design codes such as the Eurocode 7, soilretaining structures should also be designed according to Ultimate Limit State (ULS) analysis. From an economic point of view, the design of the lining, its thickness and reinforcement, would be optimal if the loading during construction is less critical than the loading during service life, i.e. under overburden loading. If necessary, measures could be taken to prevent any loading during construction becoming more critical than the overburden loading. If this can be achieved, the structural design would be limited to establishing the overburden pressures and calculating equilibrium between lining strength and overburden. For optimisation, one could use probabilistic theory and risk analytic techniques to establish a sufficient distance between actual loading and design parameters to get the most economic lining thickness and reinforcement.
In order to evaluate the present situation with respect to lining design, some observations from engineering practice are discussed: firstly with the example of the construction of the 2nd Heinenoord tunnel, where damage to the lining during construction was above average; and secondly from the construction of the Green Hart tunnel, where measurements show that flexibility of the tube and the influence of interaction between structure and ground can also lead to critical loading conditions for the lining. Finally, the analyses and observations are generalised and some conclusions with are drawn respect to lining design.