Straight up & down
The use of low to medium capacity hoisting systems installed within raised shafts in deep mines is not new, but now two significant technical obstacles have been overcome to enable the concept of a higher capacity borehole hoisting system for both mine service and primary production (300 to 600 tonnes/ hr) to become a reality.
One essential requirement for a higher capacity borehole hoisting system is that it be truly vertical; within tight tolerances.
Massive payloads travelling at high speeds within a hoisting shaft would generate significant lateral accelerations due to excessive deviations, resulting in undesirable swaying and lateral dynamic forces.
Apart from causing undue wear to guides and shoes, such dynamic forces and swaying could soon damage conveyances, supports and shaft furnishings and most of all, could result in safety issues insofar as the entire system is concerned.
The second requirement is to provide sufficient space for larger, high-capacity conveyances to pass by one another safely while travelling in the shaft, while at the same time allowing the conveyance to be dimensioned appropriately for efficient handling of rock materials. The necessary cross sectional area requirements point to borehole diameters of the order of 5m to 6m.
The two enablers of a higher capacity, deep borehole hoisting system are: (i)the technology to drill vertical pilot holes with high accuracy; and(ii)the realization of large diameter raise boring capabilities. In hard rock for longer raises up to 1,000m in a single leg.
What is Raise Boring?
As the words imply, Raise Boring is technically a method where a drilling machine working from above will drill a pilot hole (typ. up to 380mm dia.) down through rock to intersect a mine opening at depth, say 1000m; whereupon the pilot drill bit is removed and replaced with a large diameter reaming head (typ. up to 5m dia.).
The drilling machine now applies rotation and upwards thrust to the reamer through torque and tension in the drill rods, and commences excavation from the bottom up: rock cuttings fall by gravity to the bottom, from where the muck is continually removed. A pre-requisite for this method is for there to be access to both top and bottom locations.
What is Borehole Hoisting?
Borehole Hoisting is the use of hoisting plant to hoist rock, workers and materials or both, vertically through a borehole, between a mine level and surface or between two levels of an underground mine.
The borehole is part of an overall hoisting system that comprises: the hoisting plant (including the hoist, power supply, ropes, and sheaves); the headworks at the top of the raise (including the dumping and/or off-loading arrangements); the borehole including the conveyance guide system: the conveyances for service hoisting or for carrying the hoisted material; and the loading system at the bottom of the raise.
A borehole hoisting system can be configured for mine service, or primary production (rock and ore) hoisting, while still providing a ventilation function.
For operating efficiency, hoisting systems should be vertical, and consequently, these systems traditionally have had to be installed in blind sink shafts because of: the ability to ensure vertical alignment in excavation; or the ability to excavate large diameters required to accommodate the hoisting conveyances and conveyance guide system(s).
A variant on the traditional blind sink method, where mine access is available also at the bottom, has been to provide a near-central small diameter (typ. 2m to 3m) raise bored hole (or other raise excavation method), and then to sink at the larger diameter by slashing from the top-down using traditional sinking methods, except mucking out is achieved down the central raise.
This has advantages: mainly being faster than blind sinking, but one that requires mobilizing for two phases of work; the initial raise bore and then the sinking set-up.
There is now a single-pass raise bore alternative. The evolution of larger diameter raise bore excavations together with increased pilot-hole drilling accuracy means that these boreholes can now be considered for hoisting applications.
Borehole hoisting is not a new concept: however, because of borehole size and drilling accuracy limitations, in the past it has typically been confined to small diameter boreholes for low capacity rock hoisting applications. Up to now there have been few, if any, primary mine production high-capacity borehole hoisting applications installed.
Raise Bore Drill Developments
Early raise bore drills were primarily employed for the excavation of open raises such as ventilation and ore or waste pass raises. These raises were limited in diameter (typically 1.8 m) and length due to raise drill equipment limitations.
As raise drill equipment evolved for hard rock (>150 MPa), large diameter raise boring was typically considered to be 3 to 4m in diameter and 300+m in length. Mine and ventilation engineers were challenged to design raise systems within these limitations or slashing and modifying to suit.
The raise boring of vertical raises/shafts to be equipped for service or muck hoisting was not generally considered because of the limitations in diameter, length and pilot hole verticality.
As the demand for longer, larger diameter raises developed, so too did the evolution of more powerful raise drills, and in-hole hardware. These technical developments are today delivering boreholes larger than 6m in diameter and 1000m in length.
Borehole Ground Support
The borehole reaming process generally results in a smooth, circular hole which receives no ground support during excavation, and, with limited perimeter disturbance to the rock usually requires minimal permanent ground support as compared to a drill and blast blind-sink methodology. The latter requires temporary support at the shaft bottom to protect miners working at the shaft bottom during the excavation followed closely by the permanent support which is usually in the form of a concrete lining.
Borehole permanent ground support systems can only be installed after completion of the excavation (reaming) process. The ground support would generally be installed from the top down using a temporary equipping stage suspended from winches located at the borehole collar. When required the complete range of ground support system alternatives are available: – full concrete lining; concrete rings; shotcrete or fiber-reinforced shotcrete; rockbolts and screen.
An advantage of a borehole over a blind sink excavation is that the installation of temporary support is not required i.e. only the permanent support system is installed on completion of reaming, and then within a period measurable in weeks. Post-construction videos of raise walls are also possible to compare predicted and actual conditions. Selected wall support can be tailored to suit actual conditions and installed from the work stage, or can be provided by remotely spraying fiber-reinforced shotcrete.
Ground conditions need to be appropriate to allow the use of the raise bore method. The rock mass can be assessed using the widely accepted rock mass ratings, for example, (RMR) by Bieniawski (1989) and the Q method of Barton et al. (1974) together with stability analyses such as the method by McCracken and Stacey (1989). The RSR (raise bore stability ratio) used by McCracken and Stacey is a function of the raise bore diameter and the excavation service life; which in the case of borehole hoisting would be either the intended service life, or the sho
rt period prior to installing ground support. Where there may be specific concerns about wedge type block failures from the excavation walls, assessments of joint patterns using wedge analysis can be done.
Borehole Accuracy Requirements
The vertical alignment accuracy of pilot holes for bored raises has vastly improved with the development of equipment such as MICON’s Rotary Vertical Drilling System (RVDS) utilized throughout the entire length of the pilot hole.
The verticality of the borehole is governed by the pilot hole and there are two deviation components in the accuracy of the pilot hole drilling process: corkscrew deviation; the spiraling travel of the pilot hole bit within an imaginary circle plotted around the pilot hole centre at the collar; and directional deviation; this is related to rock structure and is generally in one direction and usually beginning at some distance down the hole.
The corkscrew deviation results in side-to-side undulations in the circumference of the final borehole wall during the reaming process. These wall undulations will impact conveyance clearances when the hoisting system is installed in the borehole. They typically range in dimension from 190-380mm (a half to a full diameter of a the pilot hole bit) and the required conveyance clearances can be achieved by reaming the borehole with a hole diameter which includes an allowance to accommodate these undulations.
The directional deviation component impacts overall borehole alignment and is influenced by the length of the borehole.
Benefits of Borehole Hoisting
There are several benefits derived from the borehole excavation of hoisting shafts or winzes compared to the more conventional blind-sink shaft excavation: safer excavation methodology as operating personnel are not exposed at shaft bottom to unsupported freshly blasted rock faces; excavation methodology causes less disturbance to the surrounding rock; the installation of temporary support is not performed during excavation of the borehole i.e. only the permanent support is installed on completion of reaming; overall borehole excavation rate is faster than sinking rate; overall project time schedule can be significantly reduced because of ability to carry out concurrent work activities; reduced excavation costs; reduction in overall capital costs; reduced engineering requirements for excavation as compared to a blind-sink shaft; and long-term delivery/procurement items such as a permanent hoist can be removed from the critical path.
Conclusions
Large diameter borehole hoisting systems for rock hoisting are a viable, practical alternative to conventional shaft hoisting systems which can offer benefits in both development and installation time as well as overall capital costs.
The deviations experienced in boreholes with today’s raise boring equipment are relatively small and vertical hoisting systems can be readily designed to accommodate this deviation. Borehole deviation is not an impediment to the installation and operating efficiency of vertical hoisting systems.
Raises for borehole hoisting systems such as these should be integrated into the mine planning process to exploit the opportunities for efficient ventilation services.
Where underground access is available and ground conditions are suitable, raise boring development for the installation of mine hoisting systems should be considered and evaluated against conventional shaft or winze options for individual projects based upon project specific criteria and parameters.
*This Special Report provided by Dennis G. Martin, Manager, Raise Boring, Roland Hunt, Discipline Specialist, Mining, and Alun Price Jones, Technical Director, Cementation Canada Inc., North Bay, Ontario
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