CROSS CUTS TOO LITTLE, TOO LATE

Last year the south face of the Afton open pit mine, near Kamloops, B.C., experienced what is known to mine engineers as a “severe toppling failure.”

In other words some 7.3 million tons of rock broke loose from the upper part of the wall and slid into the open pit, covering the mine’s access ramp.

Catastrophic as this may sound, the event didn’t come as a complete surprise to the mine’s operators and no one was injured. Cracks began forming in the wall two years ago and the company installed a number of extensometers to monitor the wall’s movement.

What they found was a period of constant movement of 0.24 inches per day, lasting for several months, followed by a phase of acceleration to velocities of 2.4 inches per day.

At this point, according to G. Reid, a geological engineer for Afton, the company believed a significant failure was imminent. On May 27, miners were notified and the haulage road was blocked off. But work continued on the pit floor until quitting time on June 5. On June 2, company engineers predicted a major failure would occur sometime between June 6 and 8.

As engineers continued to take measurements, the failure “window” was narrowed down to a matter of hours — between 12:00 and 4:00 a.m. on June 6.

“By 2:30 a.m.,” Reid says, “the noise of almost continuous spalling was such that we felt disinclined to approach an extensometer located directly beneath the slide.”

The failure occurred at 3:00 and lasted about 45 minutes. It occurred despite the fact that the company drilled some 10,000 ft of horizontal holes to drain gr oundwater and relieve pore water pressures from the rock wall.

Suffice it to say that the amount of drilling was insufficient to prevent failure — and the old maxim of “too little, to late” probably applies here.

Mining is continuing at the Afton under a new, revised mine plan.