The sinking of a 1 200 metre ventilation shaft as part of the Palabora Mining Company (PMC) Lift 2 expansion is proving to be a partnership success story, based on an unshakeable commitment to safety.

Murray & Roberts Cementation was awarded the contract to sink and line a ventilation shaft at PMC. Image supplied by Murray & Roberts Cementation

Murray & Roberts Cementation was awarded the contract to sink and line a ventilation shaft at PMC. Image supplied by Murray & Roberts Cementation

PMC commissioned this new ventilation shaft – which measures 8.5 metres in diameter – to service its Lift 2 block cave, awarding the project to Murray & Roberts Cementation in February 2019. Now in its final phases, the project has earned an impeccable safety record; it has been fatality-free, and last year achieved 574 days without a Lost Time Injury (LTI).

“Feasibility studies indicated that a blind sink was the optimal method, despite its higher cost and longer timeframe,” says Jas Malherbe, Murray & Roberts Cementation’s project manager. “Ground conditions were among the reasons why raiseboring was not an option, as the side walls needed immediate support to prevent scaling.”

Traditionally, the shaft would be lined to within 12-18m of the shaft bottom, with the sidewalls being temporarily supported with split sets and mesh. However, the difficult ground conditions led to high levels of scaling that made this practice unviable.

“With ground conditions being such a key challenge on the project, we responded in an innovative way by taking the shaft lining right down to the blasted face,” he says. “The shaft sinking methodology in this project is therefore based on the Canadian shaft sinking method pioneered by Murray & Roberts Cementation – but has been adapted to ‘the PMC way.’ This has involved lining the shaft to within 1.5m of the shaft bottom, after mucking out the waste.”

Murray & Roberts Cementation use a specialised concrete mix for rapid setting and early strength which hardens to 3MPa within four hours – and this would be in place for at least eight hours before blasting. This solution requires that the blast is conducted while the shutters are still in place – so the shutters are strengthened and a toe added that would better handle the blast. The exposed concrete above the shutter is able to withstand the blast, as it has already cured for 48 hours.

Malherbe explains that drilling is undertaken by two twin-boom electro-hydraulic jumbo drill rigs. These are slung down the shaft from surface and nested in the four-deck stage for drilling the shaft bottom, a procedure which is repeated for each 48-hour blast-to-blast cycle. Waste rock is lashed using an excavator with a 0,36 cubic metre bucket, which is lowered from surface through the stage to shaft bottom.

After blasting, an excavator is used for loading rock from the shaft bottom, which is safer than the conventional cactus grab. Ground conditions lend themselves to the generation of large rocks during blasting, which can be difficult to handle. These are broken up using an hydraulic breaker, which can be coupled to the excavator. An 11-tonne kibble transports the waste rock to surface.

“Lashing a shaft with an excavator is not a new idea, but it is usually a back-up method to the cactus grab,” he says. “In this project, we decided that the excavator would be the primary lashing method, to further enhance safety on site.”

Steps have also been taken towards automating the headgear, to avoid the safety hazards of manually hooking the kibble to tip out the waste rock. This includes the winding engine driver being able to use a camera to check for correct hooking.

“We also opted to use electric actuators in this project, rather than the traditional pneumatic and hydraulic cylinders on equipment such as the bank doors, swing chutes and tipping chutes,” he continues. “This has allowed us to mitigate the risks such as hearing loss from the noise of certain actuators, and contamination from oil leaks.”

According to Sechaba Letaba, PMC’s package manager on the project, PMC took a deliberate decision to prioritise safety in the sinking of this ventilation shaft.

“By taking the PMC way, we have accepted that the pace of sinking would have to be compromised,” says Letaba. “This has proved to be a positive approach, as we have an outstanding safety record on the project. This is in stark contrast to the history of shaft sinking, which would often claim lives and cause injuries. We are therefore very proud of what we have achieved to date.”

He highlights the pivotal roles played by Sam Ngidi, PMC senior manager Operation & Lift 2 Project, and Aidan Schoonbee, senior project manager Construction – in driving the project and ensuring its success.

Fred Durand, Murray & Roberts Cementation’s senior project manager, points out that unexpected challenges tend to have an impact on scheduling, so the strong relationship of trust with PMC was vital to solving any issues as they arose.

“Our approach has always been to work closely with customers on solutions, and to ensure they are regularly updated on progress,” says Durand. “As a team, for instance, we decided that the rock breaker was the right solution for the issue of oversized rocks, and it was accepted that this would have an impact on the cycle times.”

Similarly, encountering more challenging ground conditions than expected required a significant mind-set change about how the team approached the project.

“With the supportive relationship between PMC and Murray & Roberts Cementation, we were able to agree on the necessary remedies and adjust the timeframe to suit our priority – which was safety,” he says.

Letaba emphasises that relationships between the client, the contractor, suppliers, employees and surrounding communities are vital to success. This project is testament to this fact, he says, and to the professionalism of Murray & Roberts Cementation.

“It is a pleasure to be serviced by a company like this, who proactively put world class safety measures in place,” he says. “We look forward to completing the final phases of the shaft in the same safe manner.”

Malherbe concludes that, for Murray & Roberts Cementation, the lessons learnt on this project have shown the industry a viable alternative method of shaft sinking that takes safety to a new level.