Murray & Roberts Cementation has introduced a series of innovations that are likely to change the way shaft sinking is done in South Africa forever. MechTech talks to Japie du Plessis, project executive designate and Jan Vermaak, mine engineering manager.
Murray & Roberts Cementation has been in the South African and global mining industry for more than a century and is a reputed leader in mine access development, contract mining, raise- and shaft-boring technology and related mining services.
“But with regards to sinking operations of vertical shafts, there have been very few game changing innovation over the years,” says Du Plessis, adding that the process has long been based on drilling blast holes on the shaft bottom using shaft drill rigs designed 30 years ago; the laborious and dangerous loading and hoisting of blasted rock; and the difficult manual handling and alignment of shaft shutter formwork before the concrete lining can be poured.
“We have carefully looked at current methodologies and technologies and put together a series of clever innovations to make pre-sink and main-sink shaft construction safer, quicker and less labour intensive,” he tells MechTech.
Murray & Roberts Cementation’s new pre-sinking method
At the heart of the company’s pre-sinking innovation was the development of what is now known as the pre-sink gantry: “This was a challenge put to our very experienced and competent Murray & Roberts Cementation team of engineers. Not only were the challenges technical, but also the goal was to fully comply with all health and safety regulations and mining legislation.
“From the outset, the team agreed to challenge the status quo at every opportunity and to focus not only on the hoisting system, but to also look at all the operations that are carried out during pre-sink operations,” Du Plessis says.
Describing the conventional shaft pre-sinking process, he explains that hand-held S25 rock drilling machines were traditionally used to drill a pattern of blast holes. Initial access to the shaft bottom is by means of ladders until a depth of 25 m has been reached, after which the working platform, or stage, is introduced to the shaft-sinking operation. This stage is normally suspended from beams on the bank level. Blasted rock is loaded into kibbles with 5.0 t capacity using Eimco 630 pneumatic rocker shovels. The loaded kibble is then hoisted out of the shaft with a Scott Derrick crane, which has the ability to slew away from the shaft to allow the rock to be emptied into a dump truck.
The new Murray & Roberts Cementation pre-sink gantry that was deployed at the Venetia project combines stage and kibble hoists and the blast cover handling operations into one rail-mounted gantry. The stage is suspended from the gantry on steel wire ropes attached to two 8.0 t stage winders mounted on purpose-built platforms to the sides of main girders in double fall.
Before blasting, the stage is raised out of the shaft to a height clear of the shaft collar. The gantry, which is physically connected to the blast cover, is then moved on its rails to the side of the shaft, rolling the blast cover into place over the shaft.
After blasting and clearing the shaft of the blast fumes by means of forced ventilation, the gantry is rolled back across the shaft. The fully equipped stage, which is automatically aligned and very accurately positioned via a fully integrated PLC, is then lowered back into the shaft to the required depth.
The main hoist of the gantry, used for kibble hoisting and slinging, were custom designed to enable a pre-sink of up to eighty meters below collar elevation. It is able to raise and lower a kibble with a 10 t payload at a conveyance speed of 0.5 m/s.
Also incorporated into the gantry system is an automatic tipping frame. “In the past, a man had to physically hook the lazy chain onto the kibble when it arrived at the surface to allow it to be emptied,” Du Plessis explains. “Now, the kibble is slewed into its docking position where it is automatically positioned and hooked onto the frame. Then, by lowering the hoist, the bucket is tipped, discharging its 10 t load into a 20 t truck below. Since 200 t of rock needs to be excavated per blast during pre-sink, this streamlined ￼￼￼loading system significantly reduces risk as well as tipping cycle times.
Since Murray & Roberts Cementation’s pre-sink gantry is rail mounted, on completion of the first pre-sink operation at Venetia, it was possible to transport the entire system on temporary rails to the position of the second shaft. “For Venetia we drove the whole set-up, including the stage and the portal cranes, across to the new pre-sink site at the mine. The set up stage for the second pre-sink was completed inside of five days, a process that used to take us up to a month,” Du Plessis reveals.
Turning attention to innovations on the pre-sink stage itself, the company has also significantly improved the safety and effort required for drilling. “We have incorporated mechanised drilling systems underneath the pre-sink stage. The system consists of six vertical drill rigs supported on swivel arms suspended underneath the stage. Each operator guides the drill and manoeuvres it to match the pattern of holes required for the blast. An inline pneumatic air leg on the rock drill extends to create thrust on the rock drill between the stage and the floor and, after drilling, for drill retraction. Much less physical effort is involved and the operator simply guides the drill’s position – and there are no longer any manual drill operations on the shaft bottom,” Du Plessis explains.
Also simplified is the shaft lining process: “Previously, we had to handle the shuttering and formwork from rigger mounts drilled into the shaft lining. But we have now developed proprietary shuttering, suspended from the sinking stage. The shutter depth is 6.0 m, thus after each 6.0 m of excavation, the shutter is positioned 12-18 m above the shaft bottom – and this shutter can be left in place during drilling and blasting operations,” says Du Plessis.
The shuttering is suspended from the lining above using suspension rods, which remain embedded in the concrete lining after pouring and protrude through the shutter at the bottom. To seal the bottom end of the shutter, scribing support bars are pushed out towards the sidewalls, curved scribing planks are laid and fine steel mesh is used to seal to the rock interface. A series of steady brackets between the formwork and rock holds the shutter securely in place to contain the self-levelling concrete while it sets.
Air hoist are used to hold the top section of the shutter in position in preparation for concrete pouring. The concrete is then poured into the shuttering using concrete kettles, or kibbles, to supply the concrete from surface. Once the concrete has settled, the shuttering is ready to be moved to the next position. The key￼-plate bolts are loosened, the curb ring is removed and the whole system is lowered a further 6.0 m. Via screw couplings, a new set of suspension rods are attached to those protruding from the set concrete and the formwork is re-assembled in the new position.
“Both pre-sink operations for the Venetia Underground project have now been successfully completed,” says Du Plessis.
Main sink innovations
Murray & Roberts Cementation’s mine engineering manager, Jan Vermaak, proceeds to tell MechTech about the company’s additional innovations for main sink operations. “Once we are finished with pre-sinking, we erect the full headgear for the main sink. For the main sink we use a five-deck stage, which is 22mhighanditweighs75to80t, unequipped.
“With the introduction of stage- mounted drill jumbos, the stage does not need to be moved up the shaft for the traditional excessive distance during the blasting operation. We can now drill a burn cut round as opposed to the traditional V-cut round. Thus, the stage is only raised to a level 50 m above the blast area since we do not create the fly rock we experienced with the V-cut drilling pattern,” Vermaak says.
Conventionally, in Africa and all around the world, shaft-sinking companies make use of cactus grab muckers to lift and load blasted rock into kibbles. “This is another high-risk machine, be- cause the cactus is on a steel wire rope that has to be swung and dropped onto the rock pile by operators. It is, therefore, inherently unsafe as it requires many people in the bottom of the shaft for the manual manipulation of the kibbles,” Vermaak says, adding that it is also labour intensive.
Murray & Roberts Cementation there- fore decided to abandon this technology in favour of a more modern technology developed by colleagues in Canada. Called a vertical shaft mucker (VSM), the big advantage of this technology is the use of clamshell muckers to lash the rock. “The operator is positioned vertically above the mucker, so the whole unit is aligned for easy and accurate positioning of the clamshell and the safe transfer of rock to the kibble. The opera- tor has full visual cover of the working area below him. Two systems with full manoeuvrability are used simultaneously, to push kibbles into position, and only one person is now needed on the shaft bottom to manage the connection and disconnection of the kibbles. The two VSMs are used in tandem to position the kibbles correctly in the bottom,” Vermaak says, adding: “the use of VSM systems at Venetia is a South African first.”
Also differentiating Murray & Roberts Cementation from its competitors is the drop-down fifth deck. “While all other main-sink stages have the cactus grab suspended underneath the fifth deck, we have added an additional flat concrete deck, that can be dropped down and covered to simplify shuttering and lining processes,” he continues.
“When we do the concrete lining, we need to scribe the whole area around the walls. The scribing bars and other equipment have to be passed down through the kibble opening. This is traditionally done off the third deck, but this space is very cluttered and work has to be done around kibble holes, guides and screens.
By removing the cactus grab from below the stage and making use of VSMs instead, “we have been able to add a flat deck that can be lowered with covers for the kibble hole openings”. All of the holes are covered to create a flat unrestricted surface to work from. While assembling the concrete deck, the workmen “go down with full body harnesses, which are attached to dedicated life lines provided for this purpose”. But once the shutter plates are lowered and positioned, these act as safety barriers for the lining process.
“We also use a self-blinding mesh to blind the gap and prevent concrete leakage. In addition, we have developed our own self-levelling concrete to avoid having to use vibrators for compacting,” Vermaak informs MechTech. “And we bring this concrete down in a concrete kibble instead of having to use concrete supply pipes from the surface, which cause the concrete to arrive at the shaft bottom under very high forces,” he says.
Du Plessis concludes: “While key advantages of these innovations are that fewer people are needed and the shaft sinking can be completed more quickly and economically, we have also managed to significantly reduce the safety risks and improve the working conditions of our workers in the shaft.
“For us, this constitutes engineering excellence by a special group of people that, under huge pressure to meet specific project deadlines, has managed to conceptualise, design and fabricate a system that is way beyond previous generation technologies,” he says.