The houses were built for the purpose and placed in a quarry in Våler, south-east of Oslo. Students in technical construction at Solør Vgs. (high school) drilled holes, filled them with explosives, and blasted the houses until they showed cracks.
-The main question we are going to examine is how powerful vibrations a building can withstand until cracking actually occurs, tells Karin Norén-Cosgriff, who is technical lead for vibration and earthquake engineering at NGI.
From the 1950's and 60's
The limiting values for vibration used today are, for a large part, based on research from the 1950's and 60's. Recent experience shows that it almost never occurs blasting damage that can be associated to vibrations, even when the limit values are severely exceeded. The vast majority of the damage is caused by debris from the blast or a deformation and lift of the rock between the point of blasting and house construction. Therefore, the investigators and the industry would like to test out the damage limits under controllable conditions.
Four years ago, a revised edition of the Norwegian Standard NS 8141 – "Vibration and shock" was issued with some major changes from the previous edition. In order to make the limiting frequency values independent of the type of foundation and distance, a so called frequency weighted measurement was introduced. However, it proved difficult to get this to work as intended, and the discussions on limit values was actualized. The part of the standard which deals with the effect of vibrations from blasting on buildings/houses, was omitted in 2016, in accordance with wishes from the industry.
Identical levels in Sweden and Norway?
-We still believe that it is a good idea with frequency weighted measurement, says Norén-Cosgriff. She is a member of the Audit Committee for NS 8141, led by Multiconsult.
-Therefore we work further with the case, and we have a close cooperation with Sweden. They also consider transferring to frequency weighted measurements. We have a goal to introduce the same limit values in Sweden and Norway, and we hope that the blasting project in Solør can contribute to this, says Norén-Cosgriff.
The students made a compacted base of gravel above firm rock, and here the two houses were placed. They were built with typical walls with loaded wooden beams on top, one of the houses had regular concrete walls and the other with Leca-brick walls.
The blasting tests were made in November 2018. The houses were exposed to increasing vibration from consecutive blasting tests of increasing strength. The resulting damage was carefully registered after each blasting.
NGI and Multiconsult collaborated on the data acquisition of extensive fiber-optic strain measurements and vibration measurements on both houses during the blastings. In addition, vibrations were measured in the rock to determine which vibration-reducing effect a putty pad has and how the vibrations decrease with distance. A thorough mapping of the bedrock between the houses and the blasting area has been done.
NGI's Karin Norén-Cosgriff inspecting the houses after a blasting test
New standard in 2021
A huge amount of data will be processed both from these blasting tests and from other test sites where a lot of measurement data has been collected. The analysis work will continue in 2019 and 2020 and will, to a large extent, be carried out within the research project REMEDY (see below).
The plan is for a revised vibration standard to be completed in 2021.
For the high school students, the tests provided valuable practical experience. They also got additional knowledge about vibrations from blasting, which is normally not part of the school's teaching program.
Several actors in the construction industry have contributed financially to the project, which made it possible to carry out the advanced blasting tests.
Begrens Skade II / REMEDY
is a research project with the aim to reduce the risk of damage caused by groundwork (excavation, foundation and construction work) during and after execution of a project, and the risk of damage to neighbouring structures. Read more about the project here. Read more about the project here.
Norwegian Code - NS 8141
establishes a method for determining recommended limits for vibrations in structures due to ground work such as blasting, piling, digging, construction, etc., and how the vibrations can be measured. The limit values apply to vibrations on the building's foundation.