ABSTRACT

The challenges to build a long and deep tunnel are manifold. They can be grouped into effects either due to the length or the depth of a tunnel. The main challenges that are related to the depth of a tunnel arise from the limited possibility of investigation, the geological and hydrogeological aspects and their hazard scenarios. A great tunnel depth renders the acquisition of a reliable picture of the geological and hydrogeological situation, through which the tunnel has to be driven, extremely difficult and expensive. Zones with poor rock quality and difficult geomechanical behavior are of special relevance in this context. Nevertheless, due to the complex design of modern underground infrastructures or underground power schemes, zones with favorable and particularly favorable geological conditions are also of importance, as for the determination of geologically acceptable locations for complex structural parts. For deep tunnels, geological investigation can quickly reach an economical limit. The investment in further investigation in relation to the results which might be obtained is then no longer justified. Deep tunnels are therefore to be built with a higher degree of uncertainty in the geological forecast and higher remaining risks. These risks are to be covered by a corresponding design and adequate technical measures, cost estimates and budgets and, last but not least, by fair contracts.

1. INTRODUCTION

The challenges to build a long and deep tunnel are manifold, but they may mostly be grouped into effects either due to the length or the depth of a tunnel. A special problem typically related to the length of a tunnel is logistics for example, as heading 15 to 20 km off the portal makes logistics a key element of success or failure. The main challenges that are related to the depth of a tunnel lie in geological and hydrogeological aspects and the resulting geotechnical challenges. The depth of a tunnel makes above all rather difficult and expensive the acquisition of an adequately sound and reliable picture of the geological and hydrogeological conditions, in which the tunnel has to be built. Of special relevance are zones with poor rock quality and difficult geomechanical behavior. Nevertheless, due to the complex design of modern underground infrastructures, zones with favorable and particularly favorable geological conditions are also of importance – as for the determination of the geologically most suitable locations for complex structural parts, like emergency and evacuation rooms and galleries, cross-overs with large cross-sections etc.

2. REQUIREMENT FOR INVESTIGATION

The more information there is available about the rock and the rock mass, the better the geotechnical engineer will be able to assess the behavior of the ground. Nevertheless, there will remain a certain degree of uncertainty. When it comes to long and deep tunnels, investigation has a clear economical limit, beyond which the investment in further investigation in relation to the results which might be obtained is no longer justified. This means that deep tunnels are to be built with a higher degree of uncertainty in the geological forecast and higher remaining risks.

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