Investigation of construction site explosion accident: Challenges in implementation of comprehensive safety work on collaborative large-scale contracts

The Safety In­ves­ti­ga­tion Au­thor­ity, Fin­land (SIAF) has com­pleted the in­ves­ti­ga­tion into the ex­plo­sion ac­ci­dent at a con­struc­tion site in Helsinki on 9 Au­gust 2024, where an ex­ca­va­tion ex­plo­sive det­o­nated un­in­ten­tion­ally. This re­sulted in se­ri­ous in­jury to two work­ers, as well as ma­te­r­ial dam­age and dan­ger due to stones fly­ing into the sur­round­ings. In ad­di­tion, there were sev­eral peo­ple in the area af­fected by the ex­plo­sion.

“In terms of public safety, one of the conclusions of the investigation is that uncontrolled explosions in urban and densely populated areas pose a high risk of personal injury,” says Investigator in Charge Timo Naskali.

“The Safety Investigation Authority has previously investigated accidents at Friisilä and Matinkylä excavation sites, in 2006 and 2012 respectively. In both cases, people were injured and the material damage was considerable. The limited space, working methods and self-monitoring of the site, as well as the handling of explosives were highlighted as key contributing factors in the accidents. In the accident at Laakso Hospital now investigated, perhaps the most important observation was the difficulty of organising comprehensive safety work in collaborative large-scale contracts, as there are many operators, plans and instructions, and these are difficult to harmonise with each other. The processing of previous incidents had weakened the interaction between the parties to the construction site. The incident during the blast area charging, where explosives ended up in an incorrect location, was not communicated in accordance with the project’s reporting procedures”, adds Naskali.

The immediate cause of the construction site explosion accident was an explosive that had ended up in the wrong location approximately two months prior to the accident and remained there. Not all explosives had been removed, but the information about this remained known only to a limited group of people and was not communicated to the operators. After the removal of the blasted rock, the blast area was cleaned, and a hydraulic rock hammer attached to an excavator hit the hidden dynamites, which caused them to explode.

“It should be noted that although there are challenges in the implementation of comprehensive safety work in collaborative large-scale contracts, the occupational safety for the workers had been well ensured. The problem was more in the larger whole, as the risk assessment of hazardous work phases, such as post-excavation hydraulic hammering, was not carried out. There was also friction in the interaction and flow of information between the parties. Blasting and transport of explosives are strictly regulated in Finland, for example in transport and storage, but when explosives are moved within the construction site, the same instructions and regulations do not apply. Simply put, explosives or dynamite can be transported within the site even with an excavator from one place to another without clear instructions,” says Naskali.

“When there is a lot of blasted rock after an explosion, it is impossible to know if something remained unexploded. The blast area can be over 10 metres deep. In this case, all operators and workers should have clear procedures for dealing with unexploded explosives. Here, communication and, in particular, the importance of reporting deviations is emphasised,” adds Naskali.

As a result of the investigation, the Safety Investigation Authority, Finland issues three safety recommendations, concerning the promotion of good practices in the safety management of collaborative contract forms, the systematic identification and management of accident risks at construction sites, as well as the planning of work phases including use of explosives and the consideration of risks at construction sites.