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Insufficient stabilisation suspected as cause of Norwegian rockslide that killed five

Inadequate stabilisation after blasting is believed to have caused a rockslide into flats in the Norwegian coastal town of Ålesund, killing five people on 26 March.

An investigation by Norway's Ministry of Justice will focus on blasting that took place to make way for the apartments.

A massive piece of granite that was protruding from the mountainside behind the flats dislodged and fell into the first floor of the six-storey apartment block. Witnesses described how the building seemed to leap forward and then collapse. A propane gas tank also caught fire and neighbouring houses were evacuated.

A spokesman for the Norwegian Geotechnical Institute (NGI), which is investigating buildings adjacent to the mountainside, told GE that the collapse was linked to the original blasting that took place: "[The mountainside] had to be blasted to make way for the apartments, but this probably caused new cracks in the rocks that made it unstable."

Usually, when such blasting takes place, rock anchors and possibly grouting is used to stabilise the remaining rock but "the stabilising of those cracks has obviously not been sufficient", the spokesman said.

It is believed the rock might have been made even more unstable in the weeks leading up to the collapse because of a combination of very warm days for the time of year of 15˚C and very cold nights of -10˚C.

Residents living in houses further up the mountainside reported that they noticed the cracks in the rock seemed to get wider and wider in the days leading up to the accident.

Imperial College London senior lecturer in rock mechanics John Harrison told GE that inadequate rock analysis following blasting could have seen the apartment built near unstable rock."It looks like they have excavated the slope in a previously fractured rock mass. The fact that those fractures are there is not necessarily an issue because the geotechnical investigation and subsequent design should have taken that into account," Harrison said.

"Blasting could have induced additional fractures and had the effect of disaggregating the rock behind. Water then seeps into these fractures, which can lead to instability."

Following the collapse, all homes within a 500m radius of the apartment block were evacuated. Many people living in apartments directly adjacent to the collapsed block are still refusing to return to their homes.

The NGI carried out a two-day investigation into the safety of neighbouring buildings and was due to publish its report as GE went to press.
The Ministry of Justice's investigation into the collapse will be carried out by a team of geologists and geotechnical and structural engineers.
The membership of the group is yet to be released but is expected to include representatives of the NGI and Norwegian geological society the NGU.

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