The project has two parts. Section 1 comprises 1.7km of 2400mm diameter pipe, 400m of 2000mm diameter pipe and 1.2km of 300mm-1200mm diameters of pipeline laid in cut and cover excavation. Eight shafts up to 12m deep provide the starting and target pits for the tunnel boring machines. One of the shafts, bored through limestone, will be used for a future pumping station. Connections between the main sewer and existing 100-400mm diameter sewers are made using ‘no dig’ methods. Section 2 consists of 980m of sewers with diameters of 1.2m, 1.6m and 2m. It is an optional part of the tender for Section 1.
The concrete sewer is tunnelled 6-8 m below ground surface, or 5-6 m below groundwater table. The tunnel route lies mainly within fissured limestone, but also passes through areas of permeable sand containing layers of clay till, peat, gyttja, silt, sand and fill. The preferred tunnelling method is with an open shield using compressed air to stabilize the cutting face.
A network of 150 boreholes connected by 3km of 200mm diameter pipes has been established to facilitate the maintenance of groundwater levels. The groundwater level must not be lowered during normal tunnelling operations. However, it can be lowered temporarily while the start and targets pit are being constructed or when a tunnelling machine passes the wall of a pit. Any water removed is pumped back into the ground again afterwards.
Most of the tunnelling takes place under existing roads. The sewer also crosses a four-lane motorway and a railway, following approval by the Danish Railways Authorities.