Better bolting in the heart of Lapland
Published 2010-08-16 00:00:00 | Updated 2010-08-08 21:34:23
At the LKAB mine in Kiruna, northern Sweden, the latest bolting technology is making rapid progress on a massive reinforcement project. Careful planning, effective tools and a comprehensive training programme got the project up-and-running with remarkable efficiency.
A major rock reinforcement programme is under way at LKAB’s Kiruna iron ore mine that lies within the Arctic Circle in the far north of Sweden. The work is divided into three main projects; reinforcement of new drifts, reinforcement of a new haulage route at the 1365 level and complementary support work for the mine’s older drifts and tunnels.
Referring to the complementary support project, Nils Stenberg, Project Manager, LKAB says: “So far, in just one year, we have installed more than 40 000 bolts. And we expect to put a further 100 000 in place before we finish. After that the only bolts being installed will be for newly constructed drifts.”
Given the significant amount of bolts to be installed, ease and speed of installation were key factors in selecting both the type of bolts to use and the equipment and manpower to install them. In this regard, LKAB has taken an holistic approach, not only in the selection of effective equipment, but also in planning the training of operators to make sure the work could be done according to schedule.
Dedicated fleet
To meet the requirements of the programme, a range of Atlas Copco products were chosen including 10 Boltec LC fully mechanized bolting rigs, a Cabletec cable bolting rig, a Boomer E2 C rig for bolthole drilling and Swellex bolts.
The Boltec rigs feature two booms; one for drilling, injecting and bolt installation and one for handling and holding wire mesh in place during installation. The drilling boom features a ten-bolt carousel and can install a wide range of bolts including Swellex, rebar and split set types.
Before the bolts and mesh screens are installed, scaling is performed followed by the application of shotcrete that is reinforced with metal or plastic strips.
At Kiruna, the bolt of choice is the Atlas Copco Swellex coated bolt. Both the 3- and 2.4 m variant of this inflatable rock bolt – designed to provide instant loading capacity along its entire length – are used. The variant used here is the Mn24 Manganese Swellex bolt that has a breaking load of 240 kN.
The size of the bolt used is determined by the cross section of the tunnel, with 2.4 m bolts used in the smaller drifts (where the tunnel’s height and width is under 5 m). At the end of each bolt there is a faceplate that holds the wire mesh in place and evenly distributes the pressure from the lower section of the bolt to the rock face.
The LC version of the Boltec rig series is designed to install bolts of between 1.5- and 6 m in length in roof heights of up to 12 m. It features heavy-duty booms for fast and accurate positioning between holes, which at Kiruna are placed every 1–1.5 metres apart depending on the expected load on the rock being reinforced. The mesh and bolts are normally placed from 1 metre above the floor.
Although the cementing capability is not required for the Swellex bolts, the rigs feature an automatic cement handling system. The ratio of water to cement in the mix can be controlled via the Rig Control System. The cement handling system provides quality assurance through statistics logging and features a cement silo with agitators.
In other parts of the mine, the Boltec installs Kiruna bolts. The changeover from Swellex bolts to the Kiruna bolt requires an adjustment of the magazine to accommodate the new bolt length.
Target for production
“The productivity target we are aiming for is 1 600 bolts per Boltec per month,” says Nils Stenberg, “but we are achieving an average of around 1 200 today. This is mainly due to time lost adjusting the boom when we change over from one bolt size to another. In the best months however, we have installed 4 000 bolts using one rig, so we know what the rig and operators are capable of.” The operators work in two shifts covering 06.00 to 22.00.
Boltec operator Tom Grundell is working on the complementary rock programme and says he is pleased with performance of the rig and the Swellex bolts: “I am really satisfied with the rig’s performance – the cabin is comfortable and the air quality is good.
In my eight-hour shift I can usually install 40 bolts including the drilling and net installation. The Swellex bolts are easy and fast to install and give instant support.”
After the bolt hole has been drilled by the Boltec rig, the unexpanded bolt is transferred from the boom’s carousel to the hole. Once in place, the boom’s water injector connects to the protruding end of the bolt and water is injected at 300 bar. This pressure forces the bolt to expand into the walls of the hole providing instant support.
The bolts can withstand high levels of deformation and so are able to accommodate ground movements and are not sensitive to blasting vibrations. Further benefits include chemical and grout-free installation and the fact that the Swellex bolt is not to be affected by voids, water or joints in the host rock. Although LKAB is using standard 2.4 and 3 m bolts, connectable Swellex bolts are available to create longer lengths.
LKAB operates its own comprehensive training programme, the goal of which is to ensure that well trained operators are ready to use equipment whenever and wherever needed.
The programme is divided into modules and begins with safety and underground orientation training before moving on to drilling, bolting, explosives handling, shotcreting and scaling. The training programme is intensive, with one trainer conducting one-on-one or one-on-two training sessions. It also benefits from having one Boltec rig reserved purely for training purposes.
Effective training
The operators chosen for the rock reinforcement programme were drawn from KGS, a subsidiary of LKAB and were new to operating the bolting rigs. “Since the new rigs started to arrive in spring 2009, some 90 people have been trained to operate Atlas Copco rigs including the Boltec, Cabletec and Boomer rigs,” says Roger Jatko, Trainer, LKAB.
“Some of these people had never worked in the mine before and were new to operating rigs. Atlas Copco supplied comprehensive training material, delivered in time before the arrival of each rig. After training, the operators are assessed and well educated.
“The collaboration with Atlas Copco has worked very well in terms of commissioning the new equipment when it arrived. In fact, when they arrived, the rigs worked ‘straight out of the box."
Service and maintenance of the fleet is provided around-the-clock by an Atlas Copco technician and there is an underground workshop stocked with spares. To date, the overall availability of the fleet is estimated to be 80–85 percent.
When the programme is complete, LKAB will have achieved its goal, with all the currently-used areas of the mine benefitting from the latest rock reinforcement technology and products.
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