Good fortune has smiled on Campbell

With the good fortune that seems to attend most things the Campbell gold mine of Placer Dome in northwestern Ontario does, the shaft collared in October, 1946, turned out to be within a few feet of the location that would have been the ideal for the long-term working of the mine. And, it should be remembered, 1946 was long before there was any glimmer of understanding of the many ore zones that traversed the property. The plan of the 15th level showing the shaft and lateral development demonstrates this well.

The 4-compartment shaft, however, is at its limit, and a new opening will be needed to develop below the present shaft bottom of 4,317 ft. Whether this will be a new shaft from surface or an internal winze is now under study.

The shaft has undergone many improvements over the last years to squeeze the last ton of hoisting capacity from the 40-year-old installation.

A 6-ton Kimberley skip and a 5-ton skip-cage combination run 14.5 hours per day, seven days per week, in order to hoist 427,000 tons ore and 123,000 tons waste per year (or, on a daily basis, 1,170 and 337 tons respectively).

Levels are taken off the shaft at 150-ft. vertical intervals, except for the uppermost four levels. An ore and waste pass system extends to the 27th level station (4,000 ft. below surface), the deepest level yet developed from the present shaft. Crushers are installed at two locations and feed loading pockets below the 20th and 27th levels. The lowest stoping horizon is the 23rd level (3,400 ft.),leaving four more levels yet to be developed for stoping.

Campbell is essentially a track mine and was strictly conventional until the first wheeled loaders were introduced into the stopes in 1984.

Drifts and cross-cuts are driven a nominal 8×8 ft. with jacklegs, and overshot loaders muck out into rail-bound cars. Raises, too, are driven manually; they are generally driven open and at a 49

angle; Campbell has five raise crews.

In the stopes there are 14 electric front-end loaders of 1-and-2-cu.-yd. capacity, six (also electric) in the half-to-one-yard range and a number of Cavo air driven units. Air-driven scrapers are common in narrow stopes (5-8 ft.) and all wheel loaders are captive. Ramp access to stopes has not found favor so far.

Production drilling is by jackleg or Longtom using 8-12 ft. (and occasionally 15-ft.) flat holes. Longhole drilling for hanging wall cable bolts or for blastholing is by a jumbo mounting a single machine, often remotely operated. (Two of the machines seen were of Boart manufacture.)

Breasts are 8 ft. high and the majority of stopes are 250-650 ft.in length. They may be relatively straight, narrow openings 5-10 ft. wide but much more often the veins split, reunite and send fliers off into the walls, leading to a stope form that is more like an irregular chamber with “fingers” leading away from it and quite possibly re-converging to create another chamber.

All told there are currently 23 cut-and-fill stopes and 10 longhole sill pillars in the recovery stage. There are also up to 40 other work places, including development headings.

One of the most interesting aspects of the Campbell mine is the manner in which stoping methods have evolved over time to counter the ever-increasing effects of rock pressure.

Shrinkage was the dominant, if not the exclusive, method for mining from surface to about the 1,500-ft. horizon. These stopes created long, thin horizontal sill pillars supporting the floor of the overlying drift, and island-like draw point pillars were a feature of the design. As mining went deeper, the remnant pillars absorbed, or transmitted, more and more load until a point was reached when the rock could absorb no more. The remnant subsequently released its strain energy through fracturing, often explosively, and in time deteriorated to a barely cohesive, fragmented mass.

The first burst occurred in 1965 on the 5th level (750 ft.) when sill and draw point pillars were being recovered. Occurrences became more frequent as salvaging proceeded and by the time pillar removal had reached the 7th level (1,050 ft.), pillar recovery had to be abandoned. Bursting to this time had been of a nuisance character but it was evident that the phenomenon was increasing in intensity and that it would soon become a menace to those working in the vicinity.

Systematic shrinkage mining had meanwhile reached a far deeper horizon (on the A zone this would be the 10th level) and it became evident open stoping had reached its natural limit.

Shrinkage production has continued to decline over the years and was down to 30% of the mill feed in 1984. In 1990, the figure was practically zero. It should be noted that all but a tiny proportion of this broken ore came from stopes completed several years ago but, because of the nature of shrinkage mining, the stopes still contained significant and accessible reserves of broken ore.

Cut-and-fill and longhole mining account for practically all of today’s mill feed except for development ore and residual amounts from shrinkage stoping.

A 64-channel micro-seismic system and a 5-channel macro-seismic system provide the 3-man ground control department with a comprehensive picture of what is happening underground.

The amount of time and steel devoted to holding up the stope — most of it to supporting the back — is startling and an expense of major proportions. Undercut methods would eliminate the reasons for much of this support and coincidentally, allow for mining in a more logical manner. The undercut and fill methods used in the Sudbury region in recent years are not appropriate for this purpose and new systems need to be devised.

The first lift of a standard cut-and-fill stope comprises taking down the backs to a height of at least 10 ft. and then installing a 6-ft. thick timber and cement mat. The cement-to-fill ratio is 5:1. The purpose of the mat is to permit complete extraction of the underlying ore when the stope below approaches completion.

To ready the stope for production, a drawpoint drift is driven in the footwall 30-50 ft. away and access drifts are driven to the millhole chutes and manways. The latter are timber crib in construction and spaced at 150-ft. intervals along the length of the stope. Stoping then proceeds in a routine manner with 8-ft. lifts taken until the elevation of the predetermined sill pillar is reached. Extraction of the sill pillar is by longholes drilled from the stope below, and mucking is generally by remote control.