Siloquakes – Problematic Ground Borne Vibration

Synopsis

LKAB's SILA facility at Narvik, Norway, is an automated unloading station where iron ore is unloaded in the form of pellets from trains onto ships in Narvik harbour. The facility consists of twelve storage silos excavated up to 60 metres deep into solid rock. The silos allow iron ore from trains to be stored and very quickly loaded onto ships. Each wagon unloads 100 tonnes of ore into a silo in a few seconds, allowing trains of up to 68 wagons to be unloaded in several minutes. Iron ore is then discharged, at rates of up to 4,000 tonnes per hour, onto a conveyor belt in a tunnel under the silos. This conveyor belt carries the ore hundreds of metres out to the deep water harbour where it is dropped into the holds of waiting ships.

Residents near the facility have complained of shaking in their homes for some time and accelerometers were installed in nearby houses in 2013. These indicated that the vibration was related to silo discharge, but it was not possible to locate the precise source of vibration or elucidate the source mechanism. In 2014 LKAB commissioned a team comprising the British Geological Survey (BGS), Edinburgh University and the engineering consultants SWECO to describe the source of the vibration – hopefully providing information that could be used in its reduction.

A network of 10 seismometers was deployed on the SILA site for two weeks and recorded during many instances of silo loading and unloading. A complex seismic signal, comprising many discrete wavelets, was clearly recorded across the entire network. The data was correlated to very detailed logs of silo activity as well as analysed in more conventional ways, and several properties of the seismicity have been identified that will help in understanding the source.