YLE News recently reported on puddles at the Koivusaari and Lauttasaari metro stations. Water dripping from bedrock is a natural phenomenon.
“West Metro was built into excavated Finnish bedrock, which always contains water. The bedrock is shaped by soil movements and thermal expansion. Water seepage is common in all excavated bedrock, it is a typical phenomenon in the hard Scandinavian bedrock that West Metro was built into,” explains Jari Heikkilä, a geotechnical expert working on the West Metro project.
Excavated areas are reinforced by pumping cement slurry into the rock in a process called grouting. In addition, storm drains will be built into the tunnel at five-metre intervals to control water flows. The drains will collect any residual water that drips from the bedrock.
Storm drains can be found in all stations and metro tunnels, from ceiling to floor. The water is guided from the drains to pumping stations, and from there to the Helsinki Region Environmental Services Authority’s network.
If necessary, Helsinki City Transport (HKL), which is in charge of service and maintenance for the stations, will install evaporation pans in the station’s roof, allowing the water to evaporate.
Water seepage volumes are measured by calculating the volume of water in litres over a distance of 100 metres for a period of one minute. Normal water seepage ranges from 5 to 10 litres over a distance of 100 metres. Additional reinforcing of the rock is also possible afterwards, if necessary.