Excavation of cave facilities
The board of Helsinki Region Environmental Services Authorities HSY chose Lemminkäinen Infra Oy as the prime contractor for the excavation of in-rock facilities of the Blominmäki wastewater treatment plant. The contractual period was 16.9.2015–5.2.2018. According to the contract plan, a total of ca. 950 000 cubic metres of rock was excavated by the end of 2017. The task began with excavation work. The contract also included major construction work which could be carried out - and it was only natural and sensible to do so together with the excavation work, such as the four digestion silos and three emergency exit shafts. Skanska Infra Oy’s preparatory contract of nine months was prior to the Lemminkäinen contract and included excavation work on access and maintenance tunnels from Mikkelänkallio to the two sides of the wastewater treatment plant, plus a vertical shaft that could be used for ventilation. Some of the small-scale excavation work on channels and pits were not done but will be performed in the next contractual stage.
Spoil from Blominmäki was used for preparatory contractual works (e.g. in soil replacements of Blominmäentie) and constructing the wastewater treatment plant of tens of thousands cubic metres. Crushed aggregates were produced on site for the construction site needs using a movable crusher for thirty days. Most of the spoil was transported for treatment in HSY's Ämmässuo Waste Treatment Centre and in the crushing plants located on the south side of the airport, near Ring III (ca. half of the amount of spoil was used in both of these locations). From there, the crushed aggregates have been or will be transferred for further utilisation in construction in the Helsinki Metropolitan Area.
Monitoring of blasting work
Before and after blasting work, Kalliotekniikka Consulting Engineers Oy carries out structural building inspections and vibroprotection of sensitive equipment on the surrounding properties. Additionally, monitoring measurements will be conducted with continuous vibrographs in the adjacent buildings throughout the blasting work period. The allowed vibration limits are based on the norms set by the Ministry of Employment and the Economy for the field of blasting work and defined by external experts. The excavation progress and size of one-time detonations are regulated according to the measurement results gained so that no damage will occur.
Excavation work on sewer tunnels in Mikkelä
Throughout May−January 2017, the excavation work on sewer tunnels went on from the treatment plant’s contractual borders continuing underneath the Mikkelä residential area. That was done as extra work. It was added to Lemminkäinen Infra Oy’s excavation contract concerning the Blominmäki wastewater treatment plant as it was desired to lower the risk of the works in question remaining a limiting factor for the treatment plant’s commissioning. The original contract included excavations of 1 100 metres of sewer tunnels and the extra work covered a good 1 100 metres more. The width of the tunnels was approximately 4.5 metres and the height was ca. 5.5 metres.
The excavation work on sewer tunnels, successive to the treatment plant excavation, was also continued from Mikkelä. A short summary of this is provided elsewhere on the Blominmäki webpages in connection with the sewer tunnel excavations.