The geomechanical design of a tunnel forms the basis for setting up the construction schedule and the cost estimate. Especially for large projects where challenging ground conditions are to be encountered, geotechnical design in terms of stability of the excavated cross section is like extrapolating from small to large – taking one typical cross section and applying the results to one stretch with assumed similar geomechanical behaviour. It is left to the experience of the designer to extrapolate correctly and to take all factors for the distribution of support classes into account. Furthermore the existence of fault zones and/or periodic discontinuities like schistosity is to be taken into account. Early trials in the framework of a large tunnel project using MS Access databases were promising (it was already reported by Krenn et al., 2007) and eventually led to the concept of one database for the whole tunnel system. Alignment adoptions made in the last stage of the preliminary design could be handled very quickly. For one tunnel project in India this approach has been adopted internally. Furtheron during the internal tests the possibility to couple support classes with cycle times and excavation progress led to the estimation of construction time based on the assumed ground conditions. Any changes in the prediction of ground conditions can be reflected rather quickly and in a very transparent manner. The variations done showed the impact of any shift in ground behaviour estimate – onto the support classes but also onto the construction time.


The estimate of ground conditions and the required support measures for tunnels (especially for long ones) is crucial for the design and tendering process, in particular for the construction schedule and the costing. The Austrian guideline [1] together with [2] provides a framework for the geomechanical design of tunnels and related structures and propagates a step-wise approach. Since the available geotechnical and geological data are usually subject to refinement, and the excavation method, alignment and the projected construction logistics are subject to adaptations on site, the geomechanical design has to be worked out based on the available input data with a provision for constant revision. Since the geomechanical design results serve as the data basis for the cost and time estimate of the tunnel construction, a quick response to changes is needed. This is only possible when the approach is structured and systematic. The first experiences with databases reported by Krenn et al. [3] showed that a structured approach gives advantages in terms of response time to new findings. The main issue with a structured approach is finding a structure which is robust and yet flexible. The approach reported here is in principle based on the earlier findings, but was simplified on the one hand and extended to preliminary construction scheduling on the other hand. It is stressed that the key inputs – cycle time estimate and the support classification in terms of overburden and Rock Mass Type – have to be defined "outside" the actual data base.

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