The 9-pm blast is the most noticeable

Local residents have been mostly disgruntled with the 9-pm slot in the blasting schedule.

The machine's red lights shine in the dark rock tunnel.
Blasting holes are drilled in the rock walls using a crawler drill. Photo taken in the underground areas in the Laakso Hospital area in September 2023. Photo: Anu Åkerman

The last daily blasting window in the underground excavation schedule is between 8.50 pm and 9 pm from Monday to Friday. Residents report feeling the vibrations from the blasts a long distance away. 

‘The 9-pm blasts are no bigger than those that take place earlier in the day. However, the neighbourhood is quieter in the evenings and there is, for example, less traffic, which makes the blasts more noticeable,’ explains Project Manager Marja-Liisa Hynynen. 

The human body also senses the vibration from underground blasts more acutely, as the reaction takes longer to complete than in open-face excavation. When rock is broken up in the confines of a tunnel, it expands and spreads out around the detonation site. This is why the vibrations from a single blast can be felt for as long as ten seconds.

The human body is sensitive to vibration and can react acutely to even small movements that have no effect on buildings. Vibrations resulting from the tunnelling works have been noticed by residents in at least the neighbourhoods of Ruskeasuo, Länsi-Pasila and Töölö.

Vibration levels under constant scrutiny

The Alliance prepared for the blasting work well before excavation began. The nearby properties were surveyed, and the blasting work was planned to avoid damaging the neighbouring buildings.

Several buildings were fitted with seismographs, which record the intensity of each blast during the works. If there is no seismograph, vibration levels can be worked out mathematically.