LETTERS TO THE EDITOR — Cave-in research yields valuable

I have waited for a follow-up to the article “Cave-ins occur despite precautions” (T.N.M., Sept. 7/92), expecting some experienced mining engineer to lend a hand to those groping in the dark for the answers, but no such enlightenment has been published.

I am therefore taking it upon myself to set the record straight. I was on the Belmoral Mine Inquiry Commission in 1980 and made my point on how to prevent other such disasters from occurring. Since that time, I have carried out further research on the subject regarding the mechanics of the natural forces acting on a fragile crown pillar.

Since the time of the inquiry, my research has uncovered two most informative reports: “Memoirs of the Geological Survey of Victoria” (Australia, 1909) by Stanley Hunter and “Deep Lead and Drift Mining” (London, 1916) by M.T. Taylor. The need for further consideration of problems which can occur where crown pillars are necessary is exemplified by the fact that there are three mines in the Abitibi area of Quebec which have had crown pillar problems. Moreover, two of those experienced loss of life (a total of 12 miners). The following is an explanation of the mechanics of the natural forces acting to destroy crown pillars under certain conditions. These crown pillars are meant to protect lives but, in the past, have failed because management has not understood the basics of the destructive process. If natural forces were thoroughly understood, there would be no need to use Murphy’s Law as an excuse.

Being some 2.4 billion years old, the Belmoral gold deposit is of Archean age. It consists of a series of gold-bearing quartz lenses within a shear zone which traverses the central part of the Bourlamaque granodiorite batholith. This shear zone has repeatedly been affected by an endless number of tectonic events that have left their mark by intense shearing of the quartz lenses and the wall rock and formed mud-filled shears therein (easily leached out by percolating water), causing the pillar to lose its arching effect.

During all these eons, until the end of the Tertiary period (1.5 million years ago), a river had made its bed in the Belmoral shear zone. This type of occurrence is called a “lode-controlled type of placer deposit.” With the discovery, by geophysical methods, of a mineral anomaly in the Belmoral shear zone, the anomaly was probed by diamond drill holes to establish its dimensions and economic grade. Subsequently, a 17% decline was sunk and levels were established at 100, 200, 350 and 500 ft. The development program called for the establishment of a crown pillar at the 50-100-ft. elevation, leaving 50 ft. of Quaternary till cover above the crown pillar. This till cover consisted of: one foot of vegetable soil; 15 ft. of varved clay (an excellent water barrier); 11 ft. of silty clay; 13 ft. of sandy till (saprolite?); and 3.8 ft. of river gravel.

From the 100-ft. elevation, a crosscut was driven to enter the quartz vein and a short drift was driven into the vein. This drifting was stopped because conditions of the vein at that point of entry were bad and also because muddy water was flowing in. Later in the year, shrinkage stope number 2-7 was driven up on the vein, directly below the number 1-7 drift (crown pillar). The number 2-7 stope was mined out to some 25 ft. or less, below the 100-ft. level, leaving a sill pillar.

During this time, an escape-and-ventilation raise was being driven. The last blast broke into an open fracture in the granodiorite, through which some 1,000 imperial gallons of water per minute poured into the raise, flooding the bottom level of the mine. The pumping unit, at the time, could not handle this increased flow and so the level flooded. This fracture was directly connected to the water-bearing gravels.

There were neither lakes nor bogs within the mine area; all the water that did the damage was contained in the pore spaces in the Belmoral river gravels. These had been buried by Quaternary till, which have a 20% water-bearing capacity.

At 10:20 p.m. on May 20, 1980, the crown pillar collapsed and dropped into the number 2-7 empty shrinkage stope below and, from there, buried everything in its path, along with eight miners in various working places. From all the data presented, it is obvious that the Belmoral problem was similar to the difficulties reported in “Deep Lead Placer Mines” (circa 1860-1917, after which time deep leads came to an end). Those who did not learn quickly that uncontrolled water is fatal experienced the same problems and did not live to repeat their error. Had the water been drained from the old placer gravel bed, there would have been no accident.

The bottom 3.5 ft. of the Belmoral old river bottom, if sampled, will be found to be auriferous and will pay back handsomely the cost of the gold’s extraction.

Douglas Parent

Mining Engineer

Tertiary Consultants Inc.

Montreal, Que.