Ultimate limit state design for linings of bored tunnels

Tübbingbemessung im Grenzzustand der Tragfähigkeit beim Schildvortrieb

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According to modern design codes, such as the Eurocode 7, also Soil Retaining structures should be designed according to Ultimate Limit State analysis, see [1]. From an economic point of view, the design of the lining, i.e. the thickness and reinforcement of it, would be optimal if the loading during construction is less critical than the loading during service life, i.e. to the overburden loading. If necessary one could take measures to prevent that any loading during construction becomes 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 our present situation with respect to lining design, some observations from engineering practice are iscussed: the first, from the construction of the 2nd Heinenoord tunnel, where the damage to the lining during construction was above average; the second from the construction of the Green Hart tunnel where measurements show, that flexibility of the tube and the influence of interaction between structure and underground can also lead to critical loading conditions for the lining. Finally the analyses and observations are generalized and some conclusions with respect to lining design are drawn.