The existence of permafrost causes unique engineering geology problems for the development of Arctic regions. Permafrost is a naturally occurring material that retains a temperature colder than 0°C continuously for more than 2 years; this layer of frozen ground is designated exclusively on a basis of temperature. It is estimated to underlie 20%, or about 12 million sq mi, of the land surface of the world. It occurs in 80% of Alaska and 50% of Canada and the USSR. It is 5,000 ft thick in northern Siberia, 2,000 ft thick in northern Alaska, and thins progressively toward the south. The ice content is probably the most important feature of permafrost affecting human life in the north. Ice exists as pore fillings, segregations, wedges, pingo ice, and buried ice. The segregations are the most extensive but least spectacular type of ground ice. Ice wedges are widespread and may occupy 10% by volume of the upper 10 ft of permafrost. Jerry Brown of CRREL estimates that 350 cu mi of ground ice exists in the permafrost of the Arctic coastal plain of Alaska. The most dramatic, widespread, and economically important examples of the influence of permafrost on life in the north deal with construction and maintenance of roads, railroads, air fields, buildings, dams, sewers, pipelines, and communication lines. Engineering problems are of 4 fundamental types: (1) those involving thawing of ice-rich permafrost and subsequent subsidence of the surface under unheated structures such as roads and airfields; (2) those involving subsidence under heated structures; (3) those resulting from frost action, generally intensified by poor drainage caused by permafrost; and (4) those involved only with the temperature of permafrost causing buried sewer, water, and oil lines to freeze. Normal building techniques commonly must be modified at additional costs because of permafrost, but despite special problems unique to the far north, development in permafrost regions will continue at an ever-increasing rate. Improvements in scientific and engineering approaches, plus careful geologic site selection and study of the permafrost problem, will allow successful expansion into polar areas. End_of_Article - Last_Page 2501------------