In 1946, the Planning Office for Oslo Central Station, led by Fin Hvoslef, was established by the government. In 1950, they presented a new report, which recommended that a new route for the tunnel be considered to ease construction and increase safety. This was in part based on an engineering report from 1949, which had made the first detailed plans for the line. It was a compromise between the Norwegian State Railways' need for a line as straight as possible, and Oslo Municipality's demands that the construction should not be a risk to buildings in the area. The ground in Oslo consists of clay-filled grooves up to deep. The area also contains alum shale, which expands when it comes into contact with air and water and can attack concrete, argillaceous schist, cracks with clay and water, and hard volcanic rocks.