292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



sary since it was first encountered. Particularly in placer mining, 

 frozen ground has been the factor that has made many operations 

 uneconomical (Wimmler, 1927). 



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 unpublished manuscript) .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. In 

 the technique used holes are drilled in or through the permafrost at 

 regular intervals of possibly 10 to 30 feet, depending on the depth 

 and types of material, and cold water is forced through the perma- 

 frost into underlying permeable foundations or out to the surface 

 through other holes. Hot water and steam, formerly used, are un- 

 economical 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. In many places refreezing of permafrost around cased 

 holes produces pressures great enough to collapse most casing. 

 Cementing of casings is costly and very difficult, as ordinary 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 

 around the casing and create a settling hazard in the foundation of 

 the rig and also a disposal problem. In some foundations refrigerat- 

 ing equipment must be used to prevent settling. 



Permafrost also may act as a trap for oil or even have oil reser- 

 voirs within it. The cold temperature adversely affects asphalt -base 



