Caribou Mine — Tailings Dam Raise

The Caribou Mine, about 50 km west of Bathurst, is in care and maintenance — but its tailings facility still has to be raised. That meant cutting new service and auxiliary spillways into rock right beside the existing Main Dam and its concrete spillway: structures that cannot be put at risk.

The specification left little room. A 25 mm/s vibration limit within a metre of the dam and spillway structures, no subdrill anywhere near concrete, full blast mats on every shot, and pre-split final walls so the new concrete would sit on sound rock. The plan called for test blasts before production.

The rock was shallow, too, and without subdrill a cartridge-loaded hole tends to leave a hump of rock standing at the toe. So we brought bulk emulsion onto the job with our 1,800 kg mini pumper. Pumped emulsion fills the hole wall to wall and delivers full energy right down at the toe. The shots pulled good floor with no subdrill at all — something packaged product, in the same pattern and hole size, would not have done.

Every one of the 30 blasts was monitored by an independent consultant, with seismographs at the toe and the crest of the dam. The highest reading of the program was 9.2 mm/s — 37% of the limit. Just as important for an earth dam: not one of 171 frequency readings fell below 20 Hz. Low-frequency vibration is what dams are most sensitive to.

That record now does a second job. Regressed into a site-specific attenuation model, it is the design basis for the spillway work at Heath Steele.