CRADLE TO GRAVE •
Supplied by Murray & Roberts Cementation
SINKING PALABORA ’ S 1 200M VENTILATION SHAFT – A TRIUMPH OF COLLABORATION
Murray & Roberts Cementation was awarded the contract to sink and line a ventilation shaft at PMC .
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 .
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 . •
www . africanmining . co . za African Mining Publication African Mining African Mining • October 2023 • 33