Willoughby: Mother Nature’s revenge on miners

Tim Willoughby
Legends & Legacies

Tunnel timbering being crushed by underground forces, circa 1900. Aspen Historical Society/Courtesy photo

Miners invaded Mother Nature’s underground treasures but she fought back crushing their tunnels. All that stood between miners’ deaths and safe underground passage was mine timbering.

You have seen photographs of mine entrances and most have elaborate stonework, a curved cast iron ceiling or wood timbering. You may think that the whole length of a tunnel would use those same devices to prevent cave-ins, but for most Aspen mines it was only the first 50 to 100 feet. That is because it was top soil above the tunnel entrance, loose dirt and rock that had to be held back, just as it does in building basements.

Once they tunneled into solid rock, timbering was not necessary as solid rock is … solid. But minerals are found along fault lines, especially where faults intersect other faults. Because the ground is moving where there are faults, the fault zone is composed of crushed rock, not solid. That is where timbering was necessary and since tunnels often followed the fault lines, since that is where ore was, there were long sections of timbering.

Faults moved, and are still moving today, exerting pressure on that material or, for timbering, pressuring the timber. Just the weight of material above squeezed the timbering. As an example, in the Comstock Mine in Nevada, twelve-inch thick caps at the top of 18-inch square posts over a short time would compress from 12 inches to just 5 inches. After that they would often collapse. Timbering in moist underground environments rotted wood causing it to lose strength. Miners had to retimber frequently to keep up with nature’s forces.

Some timbering was simple. Props, called stulls by miners, were cut to the tunnel height and were set under an area of concern. They would be held in place with wood wedges. Often that was temporary. Timbering termed post and cap dealt with bigger challenges using a frame similar to building framing but made usually with twelve by twelve inch timbers forming a square with three or four inch thick slabs of wood placed between the frame and the tunnel ceiling and walls.

Shafts were also timbered using cribbing that interlocked in squares or rectangles. What was unusual was that they were often constructed from top-down rather than bottom-up. A section would be attached to the one above and material (rocks) squeezed behind them to eliminate empty space and to hold the framing in place. Neither tunnels nor shafts used nails or spikes if possible as that was expensive. The timber preparers fashioned interlocking timber pieces that stayed in place though the design and wedging using wood and rock wedges.

Timbering was done in larger mines by specialists who spent all their hours doing the work. In Aspen, they were aided by timbering crews outside the mine who cut the trees and milled them, or a mine contracted with a sawmill to deliver pre-cut timbering. In smaller mines miners who drilled the tunnel did their own timbering, self-motivated to build the safest protection.

Some of Aspen’s earliest tunnels, especially on the backside of Aspen Mountain used aspen trees that were old growth and had very thick trunks. Larger mines used local fir.

The final, but most spectacular form of timbering, was for the large empty spaces known as stopes, the area where ore had been taken out. Some of those, like some in Cripple Creek that were two hundred feet high and twenty feet wild, were in good rock and did not need timbering. But others needed timbering especially because mines often mined uphill so as they worked their way higher and higher, without timbering, a ceiling collapse could fall hundreds of feet crushing anything or anyone below.

For those spaces miners used what was termed “square sets,” looking much like building framing today only with short four to six foot squares in all three dimensions using very thick wood timbers.

With a thousand, or thousands, of feet of rock above you, Mother Nature would swallow you up quickly without timbering. It would appear that Aspen miners were very good at timbering as even though there were cave-ins there were few fatalities due to faulty timbering.

Tim Willoughby’s family story parallels Aspen’s. He began sharing folklore while teaching Aspen Country Day School and Colorado Mountain College. Now a tourist in his native town, he views it with historical perspective. Reach him at