Geological Survey of Norway (NGU)
Many Norwegian tunnel scandals have been prominent in the news in recent decades. The rockfalls in tunnels attracted a lot of attention. However, the biggest problems were experienced in the Romeriksporten tunnel with water leakage, drainage of the watercourses in Østmarka, lowering of groundwater levels and subsequent subsidence and damage to buildings.
The costs overruns amounted to nearly one billion Norwegian kroner. Constructing tunnels is not always easy in Norway. History shows that there have been constant problems with collapse and water leakage for more than hundred years. Despite our long tradition of mining, hydroelectric power developments and tunnel driving, problems keep arising in connection with the execution of new tunnel projects. Could the problems have anything to do with subtropical weathering which occurred way back in the history of the Earth?
During the Triassic – Jurassic Periods, more than 150 million years ago, it was hot in Norway and the dinosaurs ruled the Earth. We now believe that acidic and “aggressive” water flowed through cracks in the underlying bedrock in the Oslofjord region during the Triassic – Jurassic Periods and attacked the silicate minerals, which were slowly broken down to clay minerals.
In zones of weakness formed during previous faulting activity, volcanism or hydrothermal alteration, the water was therefore able to penetrate deep into the bedrock. The products of this weathering include the clay minerals smectite (swelling clay) and kaolinite.
The products of the subtropical weathering were probably preserved right up until the major ice ages began over one million years ago. During these ice ages, sea levels fell again. The erosion of the ice, together with the large quantities of meltwater, removed the layer of soft rocks from the Cretaceous and Tertiary which had protected the weathering products. At the same time, the uppermost layer of the weathered rocks disappeared, but the eroding forces did not reach down into the deep fracture zones where there may be more than 200m of weathering products.
This deep weathering is revealed when we construct tunnels up to 50–200m below the current bedrock surface.
Interpretation of deep weathering in the Østmarka-Groruddalen area. The deep-weathered bedrock is shown in blue and yellow. The Romeriksporten railway tunnel from Oslo to Lillestrøm is shown together with observed zones of weakness (shown in purple).
The Geological Survey of Norway is the country’s central institution for the study of bedrock, mineral resources, surficial deposits and groundwater. NGU proudly bears its long history and traditions into the future. We put geology in the centre of
the public agenda. We will ensure the transfer of knowledge needed for wise decisions and coherent actions that will benefit society and we will continue to do so.