Successful shaft sinking and design are highly dependent on the selection of a suitable method and equipment, requiring comprehensive information about rock mass in the area, its properties, and behaviour in different stress conditions.
Conventional shaft sinking is typically carried out using controlled blasting, where a series of blast holes at the base of the shaft are loaded with small charges, and the top of the shaft is covered with blast mats or other protective devices.
Once the charges have been detonated, the resulting pieces of rock (or muck) need to be removed while the shaft’s walls are adequately supported.
The main disadvantage of this method is that all personnel must retreat to a safe distance during the firing of explosives and the ventilation of the shaft face, which can significantly slow the rate of shaft sinking at depth.
Another concern with this method is the impact of the explosives on the seismology of the area, particularly the rock mass response and potential for increased strain-bursting – much of the rock activity does not actually occur until blasted muck is removed.
Similarly, the main factor determining the selection and parameters of mechanised shaft sinking systems is the physical and mechanical properties of the rocks, along with economic and organisational capabilities.
Mechanical techniques for shaft sinking include all methods that involve mechanised excavating with hammers, full diameter boring, or cutting with various types of cutterheads, which can all be carried out manually as well as with a machine.
The loading and transport of rock, however, is carried out with a series of shaft loaders or in parallel with continuous mechanical conveyors or pneumatic/hydraulic transportation systems.
Mechanised shaft sinking greatly enhances mine development by substantially accelerating shaft sinking rates, while reducing the risk to operators.
Another benefit is increased efficiency as mechanical methods allow for more process steps to be taken simultaneously, such as excavation at the same time as muck removal or, in some cases, the installation of rock support (shaft lining).
An analysis by Costmine Intelligence examined the costs and considerations related to shaft excavation and construction, leading to the development of detailed cost models intended for scoping and pre-feasibility studies.
The results showed labour costs were the single most significant factor, accounting for an average of 65 per cent of total expenses. Meanwhile, construction materials and equipment operation contributed 20.7 per cent and 7.3 per cent, respectively.
These were followed by drilling and blasting supplies (4 per cent), temporary utility materials (2.4 per cent), and ground support (0.1 per cent).
Costmine said: “Shaft depth is another key driver of cost – shallower shafts tend to have higher unit costs due to fixed expenses, such as sub-collar and headframe construction.
“Meanwhile, deeper shafts face incremental increases in costs associated with hoisting, pumping, and equipment leasing.”
A study examining the effects of preconditioning blasting for shaft sinking showed that it had immediate positive impacts, significantly enhancing operator safety.
This was because seismicity was significantly reduced due to the bench floor being exposed.
The method and effectiveness of blasting are heavily influenced by the mine site’s lithology, specific rock mass characteristics, and stress conditions. Adjustments may be necessary throughout a single shaft, as initial preconditioning can become ineffective at depth.
In the study, it was determined through field observations and numerical modelling that stronger rock was causing high stresses to flow closer to the excavation, rendering the initial preconditioning design less effective.
Additional preconditioning holes were introduced, and the existing holes were repositioned to more effectively disrupt the flow of high stresses. This adjustment aimed to push stresses further away from the excavation, thereby reducing overall seismicity, spalling, and strain-bursting.
The researchers said: “As preconditioning is implemented, a constant feedback loop must be established to monitor changes in the rock mass conditions and determine if changes to the pattern are required.”
Another study published last year in the journal Scientific Reports investigated the influence of rock properties and joint structure on rock-crushing efficiency when using a single ring of tipped cutters, with the broader aim of controlling the stability of the drilling machine during shaft sinking.
The results showed correlations between compressive strength, tensile strength, hardness, brittleness, specific energy consumption, and the vibration of thrusting forces.
The researchers indicated that the results suggested the spacing, dip angle, and continuity of joints significantly influenced failure patterns and crack propagation during rock fragmentation, as well as the frequency and amplitude of the cutter’s breaking force and its energy consumption.
They said: “In the exploitation of deep resources, mechanical drilling can not only significantly reduce the cost, but also prevent the occurrence of geologic disasters.
“Therefore, efficient mechanical drilling has become the primary problem in the exploitation of deep resources.”
Other research published this year in Scientific Reports aimed to address the low rock-breaking efficiency of milled-tooth rolling cutters used for shaft sinking via drilling methods in the Jurassic strata, by conducting rotational cutting tests using a mechanical rock-breaking test platform.
Rock-breaking mechanisms and cutter efficiency were investigated by first preparing rock-like specimens that reflected the physical and mechanical properties of weakly cemented sandy mudstone from the Jurassic system. Rotational tests were then conducted using a milled-tooth rolling cutter.
The study found that the average rock-breaking force had a positive linear correlation with both the penetration and rotational speeds of the rolling cutter. Notably, when penetration exceeded 1.5 millimetres and the rotational speed fell below 0.19 rotations per second, the drillability of the rock improved.
The milled-tooth rolling cutter achieved the highest efficiency within a penetration range of two to three millimetres.
The researchers said: “These findings could provide theoretical support for selecting rock-breaking cutting tools for shaft sinking… and for optimising the power parameters of drilling rigs under [weakly-cemented Jurassic strata] conditions.”