Microwave Thawing of Frozen Soils and Gravels

Winter conditions in northern climates and permafrost regions make mining and excavation in frozen ground very difficult. Current methods of weakening frozen ground by thawing with combustible materials or electric heaters are slow and need improvement. Instantaneous volume heating is achieved by radiation heat transfer with microwave energy. The objective of this work by the U.S. Bureau of Mines was to determine the ability of microwaves to thaw frozen soil and gravel. The thawing rates and penetration depths as a function of soil types were experimentally determined at field test sites using a 6-kW microwave system operating at 2.45 GHz. Five types of soil were tested ranging from washed sand to clay with moisture levels from 7 pct to 44 pct. Field thawing tests were performed in Minnesota in winter, with ambient temperatures down to −23°C. The lowest specific energy obtained during field tests was 18 kW$\dot{s}$h/m ³ for a run of mine gravel.