A massive project underground

We are excavating space in Blominmäki in Espoo for an ultramodern wastewater treatment plant.

At eight in the morning, the main arteries of the Helsinki metropolitan area are filled with traffic, but traffic is also brisk under the main routes. We are excavating space for a new wastewater treatment plant in Blominmäki, Espoo, which will begin its operations in 2020.

“We are excavating around 300 half-lorry loads of earth for the tunnel each day,” says Jarkko Meriläinen, Project Engineer at Lemminkäinen.

That’s a lot, but the area is very large, too. The area of the new treatment plant will be 10 hectares. The floor area of the halls and tunnels will be equivalent to around 15 football fields. The excavation contract is the largest individual investment in history for our customer, Helsinki Regional Environmental Services HSY.

“Blominmäki is equivalent to around two years of our water treatment investments. The planning and management of the work will employ a large group of personnel involved in water treatment investments and wastewater treatment for many years to come,” says Jukka Yli-Kuivila, Director of HSY’s Blominmäki project.

Underground is modern

The rapid growth of Espoo would have required the capacity of the Suomenoja treatment facility to be doubled. However, that did not fit in well with the other urban planning projects in the area. Expanding the plant would have taken up space for residential and service construction around the Finnoo metro station.

“This is why we looked for underground construction solutions for the water treatment plant,” says Yli-Kuivila.

The underground water treatment plant can be used without compromising stability or ease of use, regardless of the season or weather. The excavated areas do not disturb nature and they enable efficient use of the above-ground areas.

Once the treatment plant is complete, it will process around 400,000 people’s wastewater, but if necessary it can process that of up to a million people.

The space could fit an apartment block

There is still a great deal of area to be excavated in Blominmäki. During autumn 2016, we have completed around 60 percent of the excavation project, but challenges still remain. The time-consuming deepening and precision excavations are still ahead of us.
Our most important tools for excavation work are six jumbo drills manufactured by Sandvik and Atlas Copco. They are used to bore holes in the rock for explosives according to the 3D model we have designed.

We blast twice a day, sometimes even three times. Because there are no properties or sensitive city infrastructure in the immediate vicinity of the area, we are able to excavate efficiently.

“We began excavating from the top so that we could reinforce the roof with concrete. If we were to start deep excavations right away, it would be difficult to reinforce the structures, because the highest halls are as tall as an eight-storey building,” says Lemminkäinen Project Engineer Tapio Mettänen.

The Baltic Sea is grateful

Once the Blominmäki treatment plant is complete, it will have a significant impact on society. In addition to its massive capacity, it represents the very latest technology.  Along with original wastewater treatment methods, the facility also uses disc treatment for removing phosphorus and UV treatment to remove microbes. After treatment, the water is clean enough for swimming.

Around 150,000 cubic metres of water will flow through the plant each day. After treatment, it travels along the outflow tunnel into the sea, where it is mixed in with sea water.

“The load on the Baltic Sea is reduced. The treatment plant has the possibility to accommodate the growth of the eastern part of the Helsinki metropolitan area for at least the next hundred years,” says Yli-Kuivila.


  • We began our excavation and reinforcement work with HSY for the Blominmäki wastewater treatment plant in Espoo in autumn 2015. The contract will be complete at the end of January 2018. The new wastewater treatment plant will begin its operations in 2020.
  • We will excavate a total of 870,000 solid cubic metres.
  • We are excavating approximately 300-metre halls for the treatment tanks, which are over 20 metres wide and around 22 metres high. In addition, we are vertically excavating 17 elevator, ventilation and maintenance shafts as well as wastewater inflow tunnels around 700 metres underneath the treatment plant.
  • Our work during the project is reinforced by over a hundred employees each day (including subcontractors). Once the wastewater treatment plant is in use, it will employ around 30 people.
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