Inspection of the Kiruna mine continues and rock technicians are working all out to solve the seismic puzzle. Damage is being surveyed and documented and rock reinforcement work is under way in preparation for a successive re-opening of the mine. This is a very extensive undertaking and safety is always the first priority.
A major tremor, which occurred in the early hours of Monday, May 18th, has left few untouched. This was the most dramatic seismic event ever recorded by LKAB. “I realized immediately that this was something very out of the ordinary and I went to work straight away,” says Oskar Niva, technician in the rock mechanics department. The event had a magnitude 3.3 on the local magnitude scale, causing extensive and widespread damage to the mine. “I’ve met a lot of people who have questions and want to discuss the event. Some are very concerned, which is understandable. However, at this point, we do not yet know the full extent of the damage. We are still working with inspection,” says Oskar Niva.
A seismic event of this magnitude is unusual and many people will undoubtedly recall the sensation caused by the tremor on that early Monday morning. The rock mechanics department responded quickly, establishing a task force to systematically and methodically piece together what actually happened and how the event and its aftermath will be addressed. “We decided to work in pairs, due to the unusual nature of the event,” says rock technician Anders Wälitalo.
It soon became apparent that the damage to the mine was both widespread and extensive. Powerful vibrations, caused by the seismic event localized at level 1146, resulted in both large and small rockbursts and fracturing in walls and ceilings.”We don’t know how long it will take to inspect the mine and we are proceeding carefully. The rock mass has to be allowed to rest and the rock mass has the final word. Experience tells us that the greater the magnitude of the event, the longer we have to wait,” says Anders Wälitalo.
As mining progresses deeper, rock stresses increase. These stresses can never be eliminated but they can be reduced, for example, by adapting the mine layout, i.e., how roads and drifts are placed in relation to factors including the geology. When an orebody is mined out, rock stresses must find new paths around and under the mined-out area. This is when rock stress concentrations are induced and, if the stresses exceed the strength of the rock mass, the rock mass ruptures. Seismic activity, sudden failures in the rock mass, releases enormous amounts of energy.”We know from experience that the ore normally withstands seismic events better than the surrounding barren rock, but that the barren rock is affected,” explains Anders Wälitalo. Most events, sometimes numbering several hundred or thousand over a 24-hour period, cannot be felt but only measured with sensitive instruments – geophones. Normally, some 30,000 seismic events occur during a week, but energy sometimes builds up and is released in major events, as in this case. “Last week we recorded just as many events in ten hours as we normally do over a week,” says Oskar Niva.
Rock technicians are working all out to survey, cordon off, and successively re-open the affected areas. Since the increased seismic activity persists, primarily in the middle of the mine, it has not been possible to survey much of the mine. “When we inspect we look at even the smallest fractures and areas of damage. It is very important that everything is done strictly by the book. It’s a matter of safety – my safety and everyone else’s,” says Anders Wälitalo.
Reading and understanding the rock mass is something of an art and requires considerable experience and expertise. A trained eye and ear can often discover structures, fractures and deviations that cannot be detected by the uninitiated. A rock technician must have a good grasp of theory, as well as considerable practical knowledge and experience.”Our eyes are everywhere and we listen carefully for tell-tale sounds. Often, we ask that ventilation fans be switched off while we are doing inspection, so that we can hear the rock mass,” adds Anders Wälitalo.When a large seismic event occurs or a rockburst happens, we face a difficult puzzle that must be solved. Not until every puzzle piece that may contain an important bit of information is in place can a reliable overall assessment be made. This involves everything from damage survey, photo documentation and outcome studies to geological and structural analyses. In addition, reinforcement work, shotcreting, rock bolting and rock reinforcement mesh must be inspected down to the last detail.”When you work as a rock technician, the job has a lot to do with relationships, trust and confidence. We have a great responsibility for safety in the mine; therefore, trust means everything. We must be able to cooperate and help each other,” says Oskar Niva. When the puzzle pieces have been laid and a comprehensive overview of the situation is in place, the mine planners, who are responsible for the layout of the mine, must have all the facts in order to plan how mining will proceed. Therefore, it will take some time before mine production can start again, because safety must always come first – every time. And in this context, geophones play a decisive role. They provide information about activity in an area.”This is step one of our work; determining what it actually looks like in the area. In this case there is an elevated level of activity over a relatively large area, which means that we can’t go in yet,” says Oskar Niva.
As the days pass, seismic activity in the mine decreases. This means that the rock technicians can proceed further in towards the epicentre of the event. “It looks very bad in some areas, but in many areas we are seeing vibration damage and minor fracturing,” says Oskar Niva. The major damage has been identified mainly around the production areas, in blocks 22 and 26. In some cases rockbursts have occurred. What is certain is that inspection and rock reinforcement work will continue for the time being. Four of 11 production blocks are temporarily closed. “The closer we get to the epicentre, the more reinforcement we need,” concludes Anders Wälitalo.
Anders Wälitalo, rock technician, is working with inspection. Rockbursts have been noted in some places.