Both NGI and Oslo municipality regularly receive messages from worried citizens who express their concern over vibrations that they experience as annoying or are afraid of can damage their homes. Such vibrations are harmless and most come from different sources, such as construction work, heavy transport or blasting. Earthquakes of small magnitude can also give vibrations.
The ground conditions in Oslo is, like in many other cities in the world, dominated by soft clay over rock. In Oslo, the depth to rock varies very much. In some places, there are several tens of metres down to solid rock, while at other areas we see rock at the surface. Such ground conditions reinforces ground vibrations considerably.
Annoying vibration is not dangerous
Humans are by nature endowed with a high sensitivity to vibrations. This explains why the difference between the lowest vibrations we can perceive, and the vibrations we experience as annoying is very small. One can speculate if this extreme sensitivity is a remnant from our ancestors who had the need to perceive a herd of elephants far away in order to be able to get to safety in time.
In connection with the development of the Norwegian Standard for vibration from transport (NS 8176) a survey was made among residents who were exposed to vibrations. This showed that already for so small vibrations that they were just barely possible to register for humans, it was about 10% who experienced this as annoying. For sound and noise the picture is very different. Sound levels substantially higher than the threshold for what we are able to hear is required in order for people to experience the sound as annoying noise.
Vibrations that are perceived as disruptive and distressing is, however, not dangerous for people, buildings, or other structures such as bridges and tunnels. In the working environment context, where one is primarily concerned about protecting health, the limiting values for vibration is many times higher than the limiting allowable values applicable to vibrations from transport in dwellings. Buildings and structures can withstand far higher vibration values than what people think is comfortable.
Vibrations and movements in urban areas are normal
In Scandinavia, there are many areas with soft soil conditions. It means that vibrations generated by rail, heavy trucks or construction activities often set the ground in fluctuations. Such vibrations can spread over long distances. These vibrations will in general not result in any risk or harm to people or buildings, even though they may be experienced as annoying.
Also in areas on solid rock, vibrations may be experienced as annoying and scary. Such vibrations are normally in connection with blasting and construction work. The Norwegian Standard for vibration from construction activities (NS 8141) has maximum limit values for vibrations from blasting in order to protect buildings against damage. It is therefore more the rule than the exception that nearby residents complain about vibration from blasting in connection with large construction projects, also when the vibrations are within allowable magnitude.
A city must live and develop, and larger cities will constantly experience heavy construction work, at several sites. Public transport by tram, metro and trains are required to increase mobility and transport capacity, and to reduce environmental impact. Mitigation measures to reduce ground vibrations are very complicated and costly, and in many cases not even possible.
Norwegian Standard has established limit values for vibrations from the transport and construction activities to protect property and buildings against damage, and residents against major disadvantages and annoyance. But residents have to endure that living in urban areas on soft ground at least once in a while will be a shaking experience.