Sammendrag
This paper describes work carried out as a part of the R & D project �Tunnels for the citizen�, which was initiated in 1998-99 after severe problems, particularly with respect to groundwater control, were encountered during excavation of the railway tunnel Romeriksporten in Oslo, Norway. The aim is to develop and improve the Norwegian tunnel technology, to make communication tunnels more cost efficient and to minimize the environmental consequences. This effort is financed by Norwegian owners, contractors and the Research Council of Norway.
This part of the project described here has main focus on pre-investigations, the optimal extent of preliminary investigations is sought. Different investigation methods are tested in different geological conditions. In this paper results from the site investigations carried out at the Lunner- and Skaugum tunnel are discussed and compared with what have been encountered of geological conditions and water leakage during excavation. Geologically both tunnels are located in the so-called Oslo Region. The rock types are, however, quite different.
The owners have carried out �traditional� investigations, and in addition some extra investigations, which have not been in common use in Norway, have been done as part of the R & D project. The pre-investigations carried out by the owners have been: desk studies of available geological information, detailed surface mapping, refraction seismic and core drilling. The main extra investigations have been geo-electrical methods and geophysical measurements including flow measurements in boreholes.
During excavation of the Lunner tunnel large water leakages were encountered connected to a major weakness zone consisting of a fault and the boundary between hornfels and syenite. High water leakages were also encountered in syenite with open joints. A total inflow of 16,325 l/min in a section of 341 m was measured in pre-grouting holes. The water leakage was much higher than expected. The locations of major leakages were however as expected based on results from the site investigations. The resistivity measurements gave good indications of the water bearing zones.
In the Skaugum tunnel the water leakages until now (15th July 2003) have been less than in the Lunner tunnel, but in some sections water leakages of 100 l/min or more have been encountered for one pre-grouting round. The rock mass in the Skaugum tunnel is alternating shales and limestones. The highest water leakages have been encountered at densely folded areas and boundaries between geological layers. In a thoroughly investigated section of the tunnel, conditions encountered generally were better than expected. The zone encountered was smaller and had no major water leakage, especially compared to the low resistivity (20 ohmm) registered in this section.
The main conclusion based on this study is that the importance of understanding the geology can not be overestimated. Thorough study of available geological information and detailed geological mapping is the first and most important investigation. In the planning of site investigations and in interpretation of results, it is of great importance to be aware of what the different investigation methods are actually measuring. It is important to combine the resistivity measurements with other investigations to understand what the anomalies indicate. More data and testing in different rock types are needed to fully understand the results from the resistivity measurements.
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