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REFRACTION SEISMIC FOR MAPPING OF LIMESTONE SURFACE IN A TUNNEL PROJECT IN COPENHAGEN (2016)
The Damhusledningen (Damhus pipeline) project in Copenhagen will improve water quality in and around the Damhus river and secure the area from flooding at downpour. The tunnel is constructed with “Pipe Jacking” in the top of the limestone. In the summer of 2014 the tunnel front reached an area where it unexpectedly broke through the top of the limestone into a permeable gravel layer. This led to loss of air pressure and inability to keep stability in the tunnel front. Drillings in the area indicated the possibility of a dramatic topography of the limestone surface, but none had indicated depressions into the tunnel alignment. To map the extent of the depression with sufficient lateral resolution it was decided to use refraction seismic tomography in combination with drillings. The seismic results showed very good correlation with available drillings and with the level of the limestone surface as found in the tunnel front. After grouting of the permeable layer the construction could continue successfully. Based on these encouraging results it was decided to use refraction seismic also at a crossing of a large railway embankment where it had been difficult to place enough drillings. The Danish Railway Administration’s strict requirements on vertical displacement of the tracks required further knowledge on whether there exist any depressions in the limestone surface under the embankment. The embankment and the railway constituted a serious challenge to the refraction seismic method and great care was taken to ensure good results. It was finally concluded that the limestone surface under the embankment did not show any dramatic drops. With this information the project could go on with acquiring permission for the construction from the Danish Railway Administration.
NGM 2016 Reykjavik, Proceedings of the 17th Nordic Geotechnical Meeting, Challenges in Nordic Geotechnic 25th – 28th of May
Rambøll Danmark, Danmark