As LKAB’s underground mines get ever deeper, seismic activity is increasing. This is managed with considerable technical effort, well-developed procedures and an increasingly safety-aware staff. Consequently, the number of accidents has also decreased in recent years.
LKAB’s vision for the systematic safety work is no accidents. This is a realistic vision because, for more than two years, more than 80 per cent of the company’s work sites have reported no accidents with absences. This includes work sites both above and below ground, from every part of production and the support functions.
One challenge is the safety underground linked to the rock’s strength and the seismic events, mine quakes, that arise as a result of the mining. Seismic events occur when the natural tension in the bedrock changes. The greatest rock stress is horizontal – lateral – rather than vertical, i.e. top-to-bottom as we might expect. When iron ore mining in the underground mines continues deeper and ‘cuts off’ the bedrock, the stress must find new ways around and under the extracted ore. Stress concentrations are formed, and if the stress is greater than the strength of the rock mass, the rock will give, i.e. fracture. Seismic activity is the rapid fracture of a rock mass that releases a lot of energy. Although the events are smaller than natural earthquakes, they can cause damage to the mine’s infrastructure.
To address the greater seismic activity, LKAB works on a broad front with a number of measures:
- - To monitor and analyse the activity, LKAB has built monitoring systems that are among the largest and most advanced in the world. There are so-called geophones placed in and around the mines that can also register very small events: 180 in Malmberget and 212 in Kiruna. Every day, thousands of readings are registered, of which most are never felt in the mine or above ground.
- In the planning of an underground mining area, advanced analyses are done of the structures of the rock that can affect the seismic activity. The order that the various locations will be mined is adjusted so as to minimise to the furthest possible extent the build-up of energy in the bedrock that can trigger large events.
- Procedures are applied to reduce the risks that large events will be triggered by blasting, for instance. This may involve restrictions on the order in which or how close to one another the blasting rounds may be triggered.
Lastly, a great deal of resources are devoted to rock reinforcement. LKAB uses a so-called dynamic reinforcement system, technical solutions intended to reinforce the rock and prevent rock collapses in seismic events. The rock reinforcement itself consists of several components:
- Spraying of concrete. A layer of around 10 cm of sprayed concrete with fibre reinforcement mixed in is sprayed on the walls and ceilings in a site.
- Bolting. Dynamic steel bolts around 3 metres in length and, when necessary, even longer cables are cast in drill holes to reinforce the rock in the ceiling and walls.
- Netting. A steel net is bolted on so surface reinforcement will withstand large movements and hold rock in place together with the sprayed concrete.
All employees at LKAB are trained to be aware of and manage the risks that exist at their work sites. The systematic safety work is based on risks being discovered early on and all incidents being reported so that the events can be investigated and steps taken. Anyone reporting a safety risk is not just doing him or herself and the company a favour, but rather acting to protect the safety of his or her colleagues as well. Such action is always worthy of praise.
When a major seismic event occurs and is registered in the seismic system, the information is taken care of by the mining engineer on call. Text messages are rapidly sent out to recipients inside LKAB and an initial report is posted both on the intranet and on the external website. Underground areas affected are cordoned off and inspected by mining engineers, and re-opened only after the activity has decreased. After every major event, a report is sent to the supervisory authority and other government agencies concerned.
The efforts made to manage the seismic activity are providing results. The number of accidents caused by falling rock has decreased even though the number of seismic events has increased. Sometimes, LKAB is asked if it will be more dangerous to work underground as mining reaches ever deeper. The answer is no, it will not be more dangerous, but increasing resources will be needed to maintain the same high level of safety. Research is continuously being conducted on how mining technology and rock reinforcement can be further developed. LKAB is also looking into whether the rock can be pre-fractured with various techniques, which would mean that energy could not build up in the same way and the magnitude of the seismic events could thereby be reduced.