Cave-ins occur despite precautions

Hard-rock miners in Canada have an enviable record where mining accidents are concerned. They were late starters into the business and were able to build on the hard-earned corpus of mining law generated elsewhere.

Large-scale coal and iron mining were major industries in Europe 100 years before hard-rock mining came on the scene in Canada and a great deal of mining experience had been gained by then.

Despite the free flow of information between mining jurisdictions, major accidents still occur. Sometimes there is so much similarity between the events, there is the eerie sense of deja vu.

Such a case involves a 1950 coal mine cave-in in Scotland and the cave-in at the Ferderber mine in northwestern Quebec in 1980. The latter is at Val d’Or, Que. and is still operating. It is owned by Belmoral Mines.

The Knockshinnock Castle colliery (long since closed) in Ayrshire, Scotland, mined a faulted and steeply dipping coal seam. One of the headings driving updip at an inclination of 27 degrees sustained a minor roof fall on the evening of Sept. 7, 1950. The fall was accompanied by a flow of water. Though water inflows were common enough at the colliery, the mine foreman and superintendent knew the mine face was near surface (only 30-40 ft. below surface, as it was determined later). They decided to take a look at the location in the farm field immediately above the heading. While standing there, the ground beneath them started to collapse and before long there was a massive crater “as big as a football field.”

The underground workings were overwhelmed by “a black avalanche of liquefied peat and sand.” One hundred and twenty-nine men were trapped. Thirteen men died; the other 116 were rescued four days later.

The layman’s reaction was predictable. This natural reaction was substantiated by the subsequent inquiry. How reckless could the mine owners be — mining so close to surface without knowing the depth, or condition, of the overlying soils?

Management was unaware of how close to surface they really were (their surveying was in arrears) and instead of the normal sub-soil and clay overburden that they could have reasonably anticipated, they broke into an ancient lake bed filled with gravel and waterlogged peat.

The principal recommendation of the inquiry was also predictable — know the nature of the overburden and leave a barrier of untouched coal at the outcrop. In hard-rock jargon such a barrier would be called a crown pillar. The mineralized zones of the metal miner are frequently near vertical, so it is logical to create a barrier between the overburden and the mining area proper. It adds to the mine’s stability and allows the surface to be used without hindrance.

In this regard, the Ferderber mine had its crown pillar in place at the time of the cave-in on May 20, 1980.

The mine was in the early stages of production. A number of levels had been taken off the access ramp with the uppermost being a short drift 100 ft. below surface. The main workings at the time of the accident were on the 200-ft. level. On surface, the line of the outcropping mineralization (and thus the location of the crown pillar) was drilled to determine the depth and nature of the overburden; it averaged 45 ft. thick.

Despite these measures, part of the pillar caved at the 100-ft. level. There was a massive in rush of slime, clay and gravel. Eight men in the 24-man crew lost their lives.

The subsequent inquiry determined that a local depression in the bedrock had not been identified from surface drilling. It was the point of breakthrough into the mine. Significantly, the rock and mineralization composing the pillar itself was structurally weak at the critical location. Failure, it would seem, was inevitable.

As if to underscore the inability of man’s laws to circumscribe those of nature, there is another example. In May of this year, part of the crown pillar of the Casa Berardi Est gold mine in Quebec’s Casa Berardi region unraveled.

The collapse allowed 12,000 cubic metres of overburden to flow into the mine. No lives were lost. From the limited information so far released, slippage of the pillar along a graphite lubricated fault was responsible. Operator of the mine is TVX Gold.

Even to the most careful miner, there is no accounting for all of nature’s subtleties.