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  Hard rock wastewater tunnel excavation with a sustainable focus in Stockholm

  A challenge faced today when building new underground facilities is to fully respect the surrounding nature. To create a 15 km long rock tunnel including access tunnels and shafts without making a big impact on the environment is a challenge. To interfere with the surroundings to the minimum both visually and in the terms of not dewatering the area has been of great importance during the design of a new wastewater tunnel in the northern parts of Stockholm, Sweden. On behalf of the Käppala Association, one of Stockholm municipalities’ two large wastewater treatment companies, WSP has carried out the design having sustainability and value in mind. The tunnel will be an extension of existing sewage systems, making it possible to connect several municipalities, strengthen their systems and secure a long-term capacity in the region. The extension starts as a rock tunnel and continues as pipelines along the sea bottom. The rock tunnel will pass under rural areas, roads and railways. In an early stage of the design, a feasibility study on tunnel excavation was carried out comparing drill and blast, the most common excavation method in Sweden, to a tunnel boring machine (TBM). Four vertical evacuation shafts will be excavated by raise boring. Four access tunnels for maintenance will make it possible to access the tunnel by vehicle. The location of the shafts and access tunnels have been chosen carefully in order to have a minimal impact on surroundings. Landowners also have had many opinions on how the tunnel will affect their properties. In addition, locations were optimized for production such as availability to existing roads, better rock quality and sufficient rock cover. The new tunnel will connect to a new pumping station, which will be connected to an existing rock tunnel by a 100 m long horizontally borehole. In the other end of the tunnel, the pipelines from the sea connect to an above ground chamber before continuing in a 40 m long borehole to the tunnel.The design of the tunnel was done using 3D-models. Tunnel layout, investigations and rock- and ground surfaces were added into the model to validate the future tunnel alignment. The model has successfully been used during the design phase and helped the client in discussions with landowners. Along the main tunnel, the rock cover varies from 5 to 40 meters. The concept for rock support is shotcrete and rock bolts. In order to minimize water leakage in the tunnel and dewatering, pre-grouting will be needed. The stringent requirements for sustainable facilities in Sweden has puts increased pressure on tunnel designs of this length. Continuous discussions with the client about the tunnels impact has been of great value in order to find the best solution when it came to the choice of the tunnel alignment route and placement of entrances.

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