Testing the Viability of Floater Dredging in Frozen Ground

This report is an account of our work testing the viability of floater dredging for placer gold in permafrost. Floater dredging is a method of mining which uses a hydraulic excavator to feed placer gravel to a floating gravel processing plant. We wanted to see if the digging face of a dredge pond dug in permafrost would thaw fast enough to supply gravel for a production-scale mining operation. We stripped the vegetation and muck from an area on a permafrost terrace above the Fortymile River. We dug a pit in the stripped area and pumped it full of water to form a pond in which to operate our floater dredge. Water from the pond seeped into the surrounding gravel, thawing as it penetrated. We confirmed that when frozen gravel is exposed to water, it thaws faster than when exposed to air. We experimented with various methods of accelerating the rate of thaw at the dredging cut face. We found that sloping the cut face at 30? to the vertical exposed more surface area to the thawing action of the water, without sacrificing digging efficiency of the excavator. We found that equipping the excavator with spade-type abrasion teeth gave the best combination of penetration, digging efficiency, and wear. Towards the end of the experiment, we found that water had penetrated at least 12 feet back from the cut face. We believe that the artificial water-table, created by seepage from the pond, may have started thawing large blocks of the permafrost. Further work is required to test this theory. We found that the cut face thawed back at a rate of at least 2 feet in 24 hours. With a long enough face, this would provide enough thawed gravel to support a production-scale operation. A 600-foot cut face is required for a 100 yard per hour operation over a 10-hour sluicing day. The gravel did not thaw at an even rate along the face, and we dug more gravel in the thawed areas, causing the face to become uneven. This made mining difficult because the cut became disorganized. The solution to this problem was to even the cut face by using a dozer to push thawed material from the stripped area in front of the face into the sections of the cut where the deepest advances had been made. Using this technique, we were able to dredge the frozen ground successfully on a sustainable basis. Floater mining offers an option to miners working permafrost deposits. However, because there are factors limiting its applicability, a careful analysis should be made before committing resources to this type of operation.