Blominmäki wastewater treatment plant’s sewage tunnels: Challenging connection of a vertical shaft to the existing operating offshore tunnel.

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 part of this project was 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. An additional complexity related to the continuous operation of the tunnel in question, as both non-stop flow of treated wastewater and cooling water of the nearby heat and powerplant, had a huge impact to the ways that all work phases were carried out. All parts involved, including the clients, the contractor and a variety of consultants cooperated in order that one of the most demanding shaft excavations in Finland be carried out successfully. This paper aims to present an overview of the related challenges, along with the detailed design, risk assessment and respective mitigation measures, information about problem solving and lessons learned during the execution.