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Hollinger open pit operational support

SRK News | Issue 54: Rock Engineering and Slope Stability

A4   |   Letter

The Hollinger Open Pit, one of Goldcorp’s Porcupine Gold Mines, is situated in the gold-mining City of Timmins, in northern Ontario, Canada. The Porcupine Camp, located above a sequence of Archean basaltic volcanic rocks, led to the establishment of Timmins. The Hollinger underground and surface mines, discovered in 1909, operated from 1910 to 1968 and produced approximately 20 million ounces of gold. Before 1930 shrinkage mining was used, followed by cut-and-fill stoping (backfilled with unconsolidated sand) through the late-1960s. In the 1970s and ‘80s numerous small open pit mines scavenged the crown-pillars, and some underground mining continued until 1984. The mine workings were flooded in 1988.

SRK completed a Feasibility Study in 2010 for an open pit mine design, and since 2013, helped implement the design.

There is sufficient grade at specific gold price-points to mine a 150m deep pit, with the rock being processed at the nearby Dome Mine mill. The voids (sub-vertical stopes, shafts, and connecting drifts/drives) are sometimes spaced nearly 20-50m apart extending several hundred metres deep in a 2km square area beneath the City. The pit design was constrained by a regional highway (and the City’s water tower) along the northern wall, with homes and businesses on all other sides of the Property.

The Hollinger is being mined using three 6m and two 9m cuts making up 18m high benches in mostly FAIR to GOOD ground. All blasting is done with blast-mats to reduce fly-rock and noise. Because the mine is in-town, damage needs to be minimised. Earlier, blast trials were conducted to determine the depth of blast-damage induced by varying sizes of confined blasts. These trials indicated that a three-row trim shot onto a pre-sheared final wall would not induce excessive damage. To reduce the damage further, the pre-shear drill-hole spacing is adjusted for ground conditions within the immediate vicinity of voids.

This strategy sufficiently decreases the back-wall damage to less than 4m. The multi-use of probe holes to presupport the rock mass before blasting is part of the production cycle. Grouting cables into the final slopes, above and alongside voids, before blasting reduces the amount of final wall crown and wall rehabilitation required. This pattern has 8m cable-bolts on 4m spacing, drilled at -45° and -05° from the pre-shear line. On the North Wall, the dominant fabric (72/155°) is the main control on the bench face angle. This highly foliated rock mass tends to delaminate when blasted. To reduce the bench-crest attrition from delamination, pre-support is applied using 6m cables on a 6m spacing, drilled at -45° into the crest from the pre-shear line.

For each of the larger multi-bench voids encountered, individualised support designs are being implemented.

Anton Bloem:

SRK Africa