Unique solution answers miner’s big question
“Standing tall” are two words that connotate ‘pride in achievement’ and that’s exactly what J.S. Redpath Limited of North Bay feels right now after successfully answering the question: “What do you do when you have an ore body sitting under a channel in the middle of a lake?”
Goldcorp Red Lake Mines asked J.S. Redpath that very question six years ago because that was the dilemma the miner faced when it acquired the Bruce Channel Discovery from Gold Eagle Mines Inc. in 2008.
At the time, J.S. Redpath Limited had been commissioned by Gold Eagle to sink an exploration shaft on McKenzie Island on Red Lake, in northern Ontario, the only available land in their claim package to access the discovery.
With transportation limited to barge during open-water seasons or ice road in winter, logistics of such a program were less than ideal. The one ace that Goldcorp had up its sleeve was the historic Cochenour mine, sitting idle on the shore of the same channel in Red Lake.
Not without its own challenges, this historic site sits immediately behind the fences of the local Cochenour residents’ backyards. The mining program would prove to include stakeholder challenges not always common in a shaft and mine development projects; more specifically, how is a mine developed with all its potential construction, equipment, blasting, ventilation, increased heavy equipment traffic and noise, without bothering a few of the neighbours?
Stakeholder engagement would undoubtedly play a significant role in the success of this project, and still does to this day.
The new Cochenour Project is situated in one of the world’s most prolific gold camps, just south of the old Cochenour Mine site which ceased production in 1971.
The underground project area is five kilometres west of Goldcorp’s flagship operation, Red Lake Gold Mines, and approximately 230 km northwest of Dryden. The area is accessible by Highway 105 which heads north from the Trans-Canada Highway, and also by daily commercial air services which connect the local communities to both Thunder Bay and Winnipeg.
The Cochenour/Bruce Channel deposit is located down-dip from the historic Cochenour mine. It made sense that advanced exploration of the deposit could be optimized by using the old Cochenour shaft, although the program would be limited, owing to the facility’s size and antiquated state.
In May 2009, Goldcorp announced that the work would proceed utilizing the existing Cochenour facilities, while an upgraded shaft and hoisting plant would be designed and built concurrently.
The new facility would be part of a much larger plan to optimize the Red Lake assets including a 6.7 km underground drift (currently under construction by J.S. Redpath Limited) that will allow efficient ore hauling from Cochenour to the existing processing facilities at Red Lake’s Campbell milling operation.
The drift also opens up exploration, at depth, of 5 km of untested ground in one of the world’s richest gold districts.
J.S. Redpath began the task of designing a system to dewater and rehabilitate the existing shaft, with concurrent efforts directed toward the future infrastructure designs and methods. The total program included simultaneous access of both the top and bottom of the Bruce Channel ore body through the future widening of the existing Cochenour shaft and the continued construction of a 6.7-km-long rail haulage drift from the existing Red Lake underground workings.
The Goldcorp/Redpath team went to work designing a program that would allow early access to the underground workings for diamond drilling purposes, with parallel engineering and construction of the new facilities. This variant on traditional approaches would significantly reduce the overall project schedule.
By initiating the underground exploration from the existing facilities, long lead items for the new plant could be removed from the critical path, all the while valuable exploration information would get a head start, measured in months.
A pre-requisite to carrying out simultaneous work would be to find an alternate access into the old shaft, while the existing facilities were removed and new headframe and hoist were erected. The teams came up with a creative concept that utilized a “drop shaft” sunk in the position where the future ventilation plenum would be located.
From the bottom of the 18-foot-diameter drop shaft, some 75 feet below collar elevation, a “rope drift” would be driven over and broken into the existing timber shaft, with hoist and winches commissioned in the evase location at the bottom of the drop shaft.
Rope sheaves would be mounted and inverted from a temporary structure in the timber shaft just below a shaft seal/bulkhead installation. The bulkhead provided for the separation of surface works above, and the timber shaft below; the plans key component for concurrent stripping of existing shaft furnishings and the erection of the new headframe. By utilizing the drop shaft for use as both early shaft access and permanent ventilation, the multi-use concept would provide an overall savings to the project.
But what of the old open stopes just below the crown pillar where the drop shaft, rope drift and new collar house would be constructed?
Mining at Cochenour in the past had progressed to within 75 feet of surface, in the exact locations of some of the new infrastructure. Since the available footprint did not permit new areas to construct, these openings had to be surveyed, modelled and backfilled prior to commencement of construction.
The first order of business was the pumping out of some 276 million gallons (418 Olympic size swimming pools) of water from the old Cochenour workings. With the existing hoisting plant re-commissioned, workers accessed the shaft to dewater and rehabilitate the three-compartment timber shaft down to a depth of 2350 feet, at which point debris at that elevation made access to final shaft bottom at 2756 feet impassable.
Dewatering was carried out with twin 125 hp submersible pumps suspended from a purpose built crosshead and lowered with a designated hydraulic winch in one of the two existing hoisting compartments. Worker and material travel via the existing Cochenour hoist was limited to the other compartment, to allow pumping to be carried out independently from other hoisting operations. Pumping stations, utilizing 500 hp multi-stage pumps, were set up as required on the newly accessed levels and pumped to a surface water treatment plant.
As levels were dewatered and cleaned of slime and debris, bulkheads were installed to close access to the old workings. In all, 18 levels were exposed, cleaned up and inspected.
The first phase of diamond drilling was carried out from the 2050 level, where Redpath crews were required to extend the existing workings.
That was a significant challenge in itself, as there weren’t any means of loading and hoisting to surface the development rock to be generated during this program.
A unique “Ejector” car was later designed and built to allow the development rock to be trammed to existing open stopes on the 1300 level and dumped. This was but one of the unique challenges that had to be overcome during the program; that at times seemed to throw a new challenge at the team daily.
With little documented information of the existing mining openings available, crews routinely came upon excavations that required backfilling prior to proceeding with development.
In early October 2010, with the drop shaft and rope drift completed to coincide with the commencement of surface construction, the diamond drills were demobilized, the surface plant decommissioned, and a shaft bulkhead installed 60 feet below the existing collar elevation.
Access in the drop shaft was by means of an Alimak HEK double drive electric personnel cage. This unit would also serve to remove old shaft timber and furnishings as it was stripped from the shaft and trammed across the rope drift on rail cars via air tugger.
Seventy-five feet below the collar, a shaft stripping hoisting plant was installed. This consisted of a single drum Timberland 300 hp personnel hoist with .75-inch diameter rope for worker transport during the shaft stripping. A pair of New Era 56-inch stage winches with 1-inch diameter rope suspended the timber stripping work stage. A Timberland 40-inch diameter emergency egress hoist outfitted with .75-inch rope rounded out this plant.
The drop shaft and hoisting plant provided for uninterrupted access to the shaft during decommissioning and removal of the existing Cochenour hoisting facilities including the headframe and hoist and buildings on surface.
By December 2010, the new concrete headframe, collar house and hoist plant were being erected. At the same time, shaft stripping commenced from the deepest point reached in the shaft pumping/rehabilitation phase at the 2350 elevation. As crews retreated upward with the progress of the removal of the shaft timber and services, the dewatering pumps were shut down and removed accordingly. The obvious outcome of the suspension of the pumping system would be encountering the recharging water elevation during shaft slashing activities, to commence some months later. A robust pumping system had to be incorporated into the designs to handle the re-pumping of the shaft during slashing activities, at a rate that would not excessively impede the shaft sinking progress.
The 2300 feet of timber and services removal in the old shaft was completed on schedule to coincide with the completion of the new headframe and hoist plant in June 2011. The drop shaft facilities were decommissioned, and a second raise climber was installed, inverted in the existing collar on surface to remove the shaft bulkhead and slash the shaft collar to permanent dimension. Slashing to full 18-foot diameter in the collar area was completed inside the new headframe to a depth of 140 feet, to allow the installation of the new shaft sinking gear.
The full sinking plant consisting of a refurbished 12-foot double drum Nordberg hoist outfitted with 1.75-inch non-rotating rope and 2 x 1750 hp motors, 10-foot double drum CIR auxiliary hoist with 1.25-inch non-rotating ropes and 2 x 700 hp motors, 3 each 84-inch diameter 85,000 pound line pull New Era stage winches complete with 1.75-inch diameter ropes, 5 deck sinking galloway, Brutus shaft mucker, 2 boom electric hydraulic jumbo, collar doors, 20-foot high by 18-foot diameter concrete shaft forms and the sinking bucket dump. This was completed in November 2011.
Further challenging the designs of the sinking plant, more specifically the galloway and shaft forms, was the requirement to accommodate three compartments and a fourth “deepening” compartment at three separate locations throughout the length of the old shaft. With the 18-foot diameter shaft designed to centre the three compartments, it meant the full area of the fourth compartment would be outside the new circular concrete shaft dimensions.
To allow for the loads presented by filling this void behind the concrete forms, a separate set of secondary hanging rods had to be installed at these locations.
The sinking cycle was far from standard through the upper portions of the project on account of the frequent encounter with previous shaft loading pockets and finger raises that broke through into the various workings.
The bottom deck of the galloway required provisions for covering the opening of the rectangular shaft excavation so that no worker was exposed to a fall hazard. Initial shaft slashing had the broken rock filling the existing shaft void, negating the need to utilize the shaft mucker and hoisting this rock to surface.
In March 2012, the old shaft void was eventually filled and rock mucking/hoisting commenced using the Brutus mucker and surface bucket dumping system.
Seeing as the shaft had 18 existing levels to contend with, each station brow presented fresh challenges, and very close coordination and planning to execute their excavation safely. At some of these old stations, voids were encountered below the station floors that required extensive rehabilitation prior to the resumption of sinking activities.
As slashing proceeded, it was imperative not to allow the recharging water to impede sinking productivities. The recharging water was kept lower than the advancing shaft bottom via pumping through the existing ore/waste pass system. Cage protected submersible 60 hp pumps were lowered by tugger to next approaching level. The 60 hp units pumped to inline 150 hp pumps that in-turn, pumped to pre-existing sump stations (475L, 1175L). Often a series of 150 hp in-line pumps at intermittent levels (~200 feet apart) were required to transfer the water to sumps where the 500 hp pumps would handle to surface. Once out of the reach of accessible ore/waste pass systems, it was deemed necessary to drill dewatering holes level-to-level with a ‘bar and arm’ style drill. Once drilled through, the holes were reamed to eight inches where a 40 hp pencil pump was lowered behind the pre-existing bulkheads/dams to pump out the up-coming level.
The bar and arm holes were used to monitor and pump behind the existing bulkheads. In some cases the ore/waste passes were blocked but they were accessible at each level,
Shaft slashing eventually reached the lowest point accessed during shaft rehabilitation and dewatering at the 2350 elevation. From here, some 400 feet of slime, shaft timber and debris from decades of operations were removed systematically in advance of slashing the face. Pumping of the water and slimes proved to be a challenge through this 400 foot section.
The majority of the slimes were diluted with water and handled via the pumping system. The old rectangular shaft bottom at 2756 was eventually reached in August of 2012. A minor conversion to the galloway configuration was required to accommodate the transition to full-face shaft sinking methodologies at this point.
Full-face/blind shaft sinking starting in September of 2012.
During the slashing phase, 50 blast holes two inches in diameter x 16 feet long were required to excavate to the final circular shaft dimension. Full-face sinking consists of 90 holes x two inches diameter in five circular radiating rows, drilled to a depth of 14 feet with a two-boom electric/hydraulic jumbo.
The jumbo is outfitted with Montabert 109 drills. Holes are loaded with 1.5-inch x 16 inch nitro-based explosive cartridges, 90 gram boosters and nonel caps initiated with detonating cord. All blasting is initiated electrically from the surface central blast unit.
Mucking is carried out utilizing three x eight-tonne sinking buckets and a pneumatic Brutus telescopic boom type shaft mucker. Average bucket count is in the range of 75 buckets per bench. Bolting consists of 6-foot split sets and screen, with bolting being carried out as shaft mucking exposes sufficient wall area to bolt. Shotcrete is used where required. Unique to the project, mucking was limited to day-shift only, to minimize noise of the dumping activities for the Cochenour residents, until the dump area could be noise attenuated.
A standard 300 mm monolithic concrete liner was poured utilizing concrete buckets behind 20-foot-high shaft forms with concrete inserts casted in the A ring. Galvanized shaft steel sets with a single cage and two skip compartments and an auxiliary hoisting compartment, are installed behind the advancing shaft bottom. Sinking buckets and crossheads run on the newly installed shaft guides.
Currently crews are excavating the 3400 level station where a lip pocket will be installed. The lip pocket will handle development rock as crews drive development headings out to the orebody for further definition drilling. A ramp will be completed from this elevation to link up with the high-speed haulage drift excavated below.
Final shaft bottom elevation will terminate at the 3660-foot elevation in the first quarter of 2014, where provisions for loading pocket installations will be made. As exploration of the zone continues, the shaft bottom arrangement will be flexible enough to allow a quick resumption to shaft sinking if so required.
With the shaft portion of the program complete, it is clear the old Cochenour shaft presented significant options for early access to underground drilling, while planning and construction of permanent facilities was carried out. Not without significant challenges, it is clear that with careful planning and a bit of creative thinking; a perceived liability could be turned into a tremendous opportunity to prove up the next great orebody in the Red Lake camp.
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