A new underground wastewater treatment plant is currently excavated at Blominmäki in the city of Espoo, Finland. The Blominmäki plant will replace the current Suomenoja plant. In addition to the wastewater management of 400,000 residents and making provision for improved wastewater treatment, the new plant will also release valuable seaside land for construction. Almost 1 Mm3 of rock will be excavated. In addition to tunnels and large halls, excavation includes 13 shafts as well as decomposition silos with diameter of 20 m. Parts of the facility will have tunnels crossing each other at different levels. The widest span is 30 m and the maximum height of the halls is 22 m. A large number of investigations and laboratory analysis have been made including core drilling, percussion drillings, seismic surveys, soil investigations and core sample stress analysis. A 3D model of the weakness zones was created and rock mechanics simulations were carried out. Rock engineering design included modelling for a multi-disciplinary 3d model of the project. Several weakness zones in bedrock caused need for pre-excavation rock reinforcement. A systematic grouting program was carried out because of the high hydraulic conductivity of the rock. Rock mechanics monitoring program has been carried out using several online extensometers.


The process basins and technical areas of a large, modern wastewater treatment plant form a complex tunnel network when excavated and built underground. In addition to large spans, excavated caverns are high and tunnels are located in several floors. In addition, in Blominmäki, the new plant has been excavated in bedrock of varying characteristics and in some places bedrock is disadvantageous for rock construction. The location, position and altitude of the tunnel network are largely determined by external constraints, such as the nearby nature reservation area. So, for example, all the weakness zones could not be avoided.

One design target has been a systematic process for reinforcement design which ensures an effective excavation work despite of design changes and variable geological conditions in tunnels. Number of bedrock investigations have been required to provide enough output data for reliable rock mechanical simulation results and to identify the most challenging excavation areas before the excavation tendering phase. The design and implementation of a successful rock mechanical monitoring program are based on reliable investigation and simulation results.

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