Modern Mining April 2023

the lining requirements – is undertaken in two phases. During the lashing cycle (traditionally one of the riskiest tasks in shaft sinking), the operator, work ing from within the machine’s protective cab, is the only worker at shaft bottom. Within five minutes, the excavator can be equipped with a hydraulic breaker via a quick cou pler to allow it to break up the bigger rocks prior to them being loaded into the 11-tonne kibble used to transport the blasted material to surface. Because there is no temporary support, the excavator also rakes the side walls after each blast to ensure that they are safe. Unusual features of the project include the use of a self-contained emulsion machine – or ‘pod’ – which charges up the shaft bottom in preparation for blasting, and the use of slick lines, as opposed to kibbles, to deliver concrete to the shaft bottom from the batch plant on surface. The concrete mix being used has been specially designed for the needs of the project. “The mix gives us 3 MPa within four hours which is essential given the way we’ve sequenced our operations,” says Malherbe. According to Malherbe, the methods being used at Palabora represent an adaptation of the Canadian shaft sinking method that Murray & Roberts Cementation pioneered in South Africa at its Venetia Underground Project (VUP) for De Beers.

Above: A view showing the location of the ventilation shaft project at PMC. Left: PMC’s Lift II project, a new mining block which will eventually replace Lift I, will extend the mine’s life by more than 15 years.

contract

to handle,” he says. “However, careful attention to our blasting techniques has allowed us to manage the problem successfully.” He adds that the difficult ground conditions also resulted in Murray & Roberts Cementation tak ing the shaft lining right to the bottom of the shaft. “Consistent with conventional shaft sinking practice, we tendered on the basis of lining the shaft to within 12 to 18 m of shaft bottom and we envisaged using split sets and mesh as temporary support, but this did not prove effective,” he explains. “We therefore took the decision to dispense with temporary support and line right down to the blasted face. Some of our shaft-sinking veterans were doubt ful that we could do this successfully, but we’ve proved them wrong!” The key production machines deployed on the project are two twin-boom Komatsu jumbo drill rigs specially acquired for the contract, and which can be remotely operated. The jumbos are slung down the shaft from surface and nested in the four-deck stage for drilling the shaft bottom, a procedure that is repeated for each 48-hour blast-to-blast cycle, including a 3 m shaft concrete lining. Before each blast, the stage is raised to a safe position. Another vital piece of equipment is a Komatsu excavator with a 0.3 m 3 bucket (roughly equivalent to a handling capacity of 750 kg). It is lowered from sur face through the stage to shaft bottom and is used for lashing, a procedure which – to accommodate

Concrete lining of the 8.5 m diameter ventilation shaft.

April 2023  MODERN MINING  21