Tunnelling is foremostIy an empirical technology. Nevertheless, the engineer needs theoretical models for the structural behaviour of the ground and the tunnelstructure in order to find the best tunnelling method and the most economical design. At the time of tendering the engineer must rely - besides experiences - on a model from which he may derive criteria whether a design is suitable, safe enough and economical too.
In different countries different methods are in use for designing the tunnel. They are different in the assumed loads acting on the lining, in the statical model, with which the inner stresses and the deformations are derived, and in the criteria on which safety is defined.
Therefore, the International Tunneling Association (ITA) set up a working group in 1978 for gathering in-formations on these models via a questionnaire. Engineers of the member nations were asked to respond to questions with regard to the design procedure for four characteristical examples of tunnels:
the driving of a tunnel by using a shield and by placing tubbings,
tunnelling by supporting the ground with steel arches and concrete,
deep tunnel in moderately hard rock,
tunnel, built by cut and cover method.
The answers will be published in the form of a comparative synopsis /7/. The report given here covers only the main topics on the two kinds of tunnelling a) and b) in soft soil. Readers are referred for more details to the publication /7/.
(Figure in full paper)
The graph in Fig. shows the consecutive phases of the design procedure for tunnels. If the design procedure, we are running through for example designing a bridge, is analogously applied to tunnels, the engineer's work may be devided into the following main phases (Fig. 1):
The application of geological and hydrological site investigations determine whether a tunnel is feasible at all.
Experience, practical knowledge and preliminary design calculations determine the excavation method and the choice of the structural elements of the tunnel.
If we want to predict by rational criteria whether this tunnelIing procedure wiII succeed, we need a model for the analysis and calculation. The geometry of ground and tunnel, the primary stresses, the phases of the excavation and the consecutive placing of the supporting structural elements, the stress-strain-time dependent behaviour of rock and structural materials, all have to be transformed and translated into a mathematical-mechanical model. It should yield characteristic quantities as design criteria(e.g. stresses or displacements or ultimate states).
These design criteria are drawn from a safety concept, different hypotheses for failure possibilities. This safety concept should enable the engineer to decide whether the proposed tunnelIing method is safe enough or not and whether work on driving the tunnel may start. A change of the design may also be necessary if the chosen excavation procedure proves to be too expensive, or if the structural elements assumed earlier in the design procedure are too conservative in regard to their functional tasks.