OREA – Extension of an Existing Sewer Treatment Plant in Strømmen, Norway
OREA is the extension of a large underground sewer treatment plant (figure 1), owned by four municipalities outside Oslo. The purpose of OREA is treatment of excess flood water that today runs via a spillway directly out to the Nitelva river. The main rock cavern of OREA will be 60m long, 20m wide and 20-25m deep, rock quality variable and rock overburden shallow. The location is in an urban area, and the cavern is lying straight underneath the Strømmen College. The excavation method is conventional drill and blast with great environmental challenges concerning noise and vibrations from drilling and blasting. Besides, safe performance of tunnel excavation, stability and rock support is a great challenge. Another great challenge of the excavation, will be the breakthrough from the intake cavern to the sewer tunnel, which is in full operation, both technical and with respect to health and safety aspects.The paper describes ground investigations, rock modelling and stability analysis, environmental impact and mitigation measures as well as methods and procedures for safe performance of rock excavation.