ABSTRACT
The sewage tunnels of the Blominmäki Wastewater Treatment Plant (WWTP) consist of approximately 20 kilometers of inlet and discharge tunnels, access tunnels and shafts. The most challenging and technically critical parts of the Finnoo II contract were: The connection of the operating offshore tunnel of the old treatment plant with the Blominmäki discharge route, through a vertical shaft. Lack of information regarding the exact location, the shape and the geological conditions of the existing tunnel called for an accurate excavation design to avoid a potential tunnel collapse and rock blocking or massive waterflow during breakthrough. The precise implementation of the connection between acceleration and pumping underground caverns. The connection, in addition to the demanding excavation of complex geometry caverns, also included the implementation of six openings of square cross-section, slightly inclined, to install pipes for the flow of the treated wastewater.
