SHAFT SINKING region ,” he said . “ Verticality challenged the approach to the production shaft , as the useable envelop remaining in a bored shaft if the hole isn ’ t perfectly vertical will reduce the useable area of the hole in most cases if hoisting is to be conducted in it .” Options exist to directional steer or drill the hole oversized to account for this misalignment , but the trade-offs must be considered to understand the impact to cost and schedule , Melong added .
“ In most cases , a winding system and headworks must be installed to support the ground and install guidance systems , further bringing the option back closer to the cost and schedule of blind sinking ,” he added .
With the company having completed a second record-breaking raisebore in the Americas this year – a 1,010-m-long raisebore at Macassa no less – IM asked Melong : is large-diameter raiseboring technology close to hitting a ‘ ceiling ’?
“ We have completed some near world record holes recently , in terms of depth and diameter ,” he said . “ As the depth and size of holes are absolute functions of ground conditions , that is difficult to say , but limits on current raisebore machines seem to be near , or at their capacities .
“ In this equation , depth and diameter equate directly to higher risk to machine and hole success .”
“ It can offer significant capital and schedule advantages under the appropriate conditions ,” he said . “ Certainly , the proven ability of longer , large-diameter raisebored shafts gives mine planners and engineers increased options to optimise brownfield operations , whether through increased ventilation or increased capacity of waste / production hoisting , or more efficient mine access as the mining centre of gravity moves away from established infrastructure .”
This was the case at the Northgate shaft at AuRico Gold ’ s ( now Alamos Gold ’ s ) Young- Davidson gold mine in Ontario , according to Kohtakangas .
On this project , the company separated the new raisebored shaft into legs – with a mid-shaft
loading pocket – to allow the client to smooth the spending profile and realise early production , he said .
This shaft , commissioned last year , was designed to boost underground production at Young-Davidson from 6,000 t / d to 8,000 t / d as the existing mid-mine infrastructure was replaced with the new lower mine infrastructure .
Kohtakangas sees potential for using largediameter raiseboring in combination with blind sinks in projects too .
“ This has always been the case when developing a new shaft-accessed mine – consideration of raises ( more so bored raises now ) to complete the ventilation circuit in parallel with a shaft ( or decline ) and to provide second egress ,” he said . “ Existing mines have
The benefits of groupthink
For a large diameter raisebore to be successful , at a basic level , one must provide positive answers to the following four questions , according to Cementation Canada ’ s Kohtakangas : n Is the size and length of the opening within current capabilities ? Broadly speaking , you can ask : is your raise ( or sections of raise ) no longer than 1,000 m long ? n Is the ground suitable ? ie , is the rock suitable for excavation by raisebore in terms of sidewall stability and stand-up time ? Are water inflows not likely to be an issue ? n Do you have access to the shaft bottom ? Access is necessary to the lower horizon for installation of the reamer head as well as for the mucking of cuttings ; and n Can you handle the waste generated underground ? Do you have capacity to handle the additional muck volume that will be generated and the different muck size to development or production muck ? With Cementation continuing to test the limits of this mechanised technology for shaft sinking – including an upcoming bore of about 8.5-m diameter – Kohtakangas is continuing to see this option considered in studies and trade-offs when the company ’ s engineering studies group is engaged early on in a project .
Cementation Americas Group of Companies
Mine Development & Construction • Shaft Sinking • Raise Boring Contract Mining • Mining Material Handling Solutions Crushing & Conveying Systems • Mobile Stacking Systems Feasibility & Design Engineering • Project & Construction Management
SEPTEMBER 2021 | International Mining 111