Ch. 14] MINING 267 



Rockie, 1942). The best solution is to select farm lands in those areas 

 free of permafrost or free of large ground-ice masses (Tziplenkin, 1944) . 



Mining 



In Alaska, placer miners particularly, and lode miners to a lesser 

 extent, have utilized permafrost or destroyed it as necessary since it 

 was first encountered. Particularly in placer mining, frozen ground has 

 been the factor that has made many operations uneconomic (Wimmler, 

 1927) . These problems cannot be treated lightly. 



In the early part of the century, when gold was being mined so 

 profitably at Dawson, Fairbanks, Nome, and other places in northern 

 North America, it was common for miners to sink shafts more than 

 100 feet through frozen muck to the gold-bearing gravels (P. S. Smith, 

 unpublished manuscript). These shafts were sunk by steam jetting or 

 by thawing with fires or hot rocks. If the muck around the shafts or 

 over the gravels thawed, the mines had to be abandoned. 



Now, with the advent of dredges, such ground is thawed, generally 

 with cold water, one or more years in advance of operations. The tech- 

 nique is one of drilling holes through the permafrost at regular in- 

 tervals of possibly 10 to 30 feet, depending on the material, and forc- 

 ing cold water through the permafrost into underlying permeable 

 foundations or out to the surface through other holes. Hot water and 

 steam, formerly used, are uneconomical and inefficient. Where thick 

 deposits of overburden cover placers, they are removed commonly by 

 hydraulicking. Summer thaw facilitates the process (Patty, 1945). 



Permafrost is commonly welcomed by the miners in lode mining, as 

 it means dry working conditions. Its effect on mining operations other 

 than maintaining cold temperatures in the mine is negligible unless it 

 contains aquifers. Because of cold temperatures, sealing such aquifers 

 with cement is difficult, and other techniques must be used as the 

 situation demands. 



Some well drilling in permafrost requires modifications of existing 

 techniques and more careful planning for possible exigencies (Fagin, 

 1947). Difficulty may be encountered in getting proper foundations 

 for the rig. In rotary drilling, difficulty may be experienced in keep- 

 ing drilling muds at the proper temperature, in finding adequate water 

 supplies, or in finding proper local material for drilling muds. In 

 shallow holes particularly, the tools will "freeze in" after a few hours 

 of idleness. Cementing of casings is costly and very difficult, as con- 

 crete will not set in subfreezing temperatures. Deep wells below the 

 permafrost may encounter high temperatures (100° to 150° F.) , and 

 the hot drilling muds on returning to the surface thaw the permafrost 



