The largest sewage treatment plant in Norway is being constructed underground at Bjerkås, 30 km south of Oslo. The plant will serve 300 000 persons and is designed to treat a dry weather flow of 3.0 m3/s. The main reason for the subsurface location is environmental. The plant is being placed in 11 parallel caverns of 16 m span with 12 m wide pillars between. The rocks in the area are Cambro-Silurian shale and nodular limestone. The conditions revealed by excavation were very similar to those predicted from the combined results of surface mapping, drill core analyses and seismic measurements The support is systematic bolting and shotcrete, partly mesh reinforced. Some injection work has been undertaken to prevent inflow of water. The rock mass underlying an industrial area nearby is fed by water through several boreholes to prevent lowering of the ground water table in the overlying clay.
Norway has long traditions of building underground. Our hydro-electric projects make up the corner stone of Norwegian expertise regarding major subsurface constructions. This technology has been converted into use for several other purposes: air raid shelters combined with sport arenas, subway systems, oil storages, general storage and public utility facilities such as water and sewage treatment plants. Although representing a rather unique feature it was not out of the ordinary when the communities of Oslo, Bærum and Asker in 1976 decided to go ahead and build a large regional sewerage facility consisting of 40 km full-face bored tunnels and a mechanical/chemical treatment plant excavated into the hill of Bjerkås in Asker, 30 km southwest of Oslo - adjacent to Oslofjord (Fig.l). During the early surveys - and the preliminary reports - several alternatives were considered - also surface locations. The main reason for choosing the subsurface alternative was of an environmental nature. A surface plant was considered less expensive. However, the difference was eventually considerably less than originally expected. The Bjerkås area was finally chosen for the plant mainly because of political considerations. The rocks were estimated to be of poorer quality here than at the other possible localities, but Bjerkås was already an established industrial area with few private homes. GEOLOGY The project is situated in the geological formation called the Oslo Graben. The bedrock at Bjerkås consists of sedimentary rocks of Cambro-Silurian age, mainly shale and limestone, cut by some Permian dykes. The Bjerkås area is a hill about 70 m high. Due to limitations of space the orientation could not be chosen in the most favorable direction and the minimum rock cover is approx. 15 m. Geological mapping showed that the northern slope of the hill consisted of nodular limestone, in the southern slope there was shale. Seismic refraction measurements showed velocities of about 5000 m/s in the nodular limestone and 2500 - 4000 m/s in the shale. Some crushed zones gave 2000 - 3000 mis, the same as a near surface weathered zone of about 10 m thickness.
