An actively refrigerated, slab on grade foundation was designed for a coal-fired electric generating facility with a total capacity of 11 megawatts. The facility was constructed to supply power to the Over-the-Horizon Backscatter site near Gakona, Alaska. The soil on which the foundation was placed consisted of relatively warm (30°F [−1°C]) permafrost consisting of ice-rich clay averaging 25%-50% total moisture. A 8.5 foot (2.6 m) thick insulated gravel pad was designed for the power plant foundation. A stress analysis for the foundation indicated that a stress of over 2900 psf (139 kPa) would be observed at the permafrost/gravel depth under the power plant boiler. Because of the temperature, properties of the soil matrix, and loading configuration, it was determined that excessive long-term creep settlement could occur. The final foundation design consisted of 55 near-horizontal thermosyphons to remove the heat transferred through the floor of the approximately 80°F (27°C) power plant. In order to add strength to the native permafrost, and thereby minimize the creep settlement, an active refrigeration system incorporating the 55 near-horizontal thermosyphons and over 30 near-vertical thermosyphons was designed to lower the temperature of the permafrost. The excess electricity generated by the power plant made active refrigeration an economically feasible solution to the creep settlement problem. The system was designed to refrigerate the permafrost from the original temperature of approximately 30°F (−1°C) to a temperature of approximately 20°F (−7°C) to a depth of 50 feet (15.2 m) below the existing natural ground level. The lower temperature will significantly increase the strength of the in situ soil, and thus reduce the amount of creep settlement. After the desired temperature is reached, the system will be operated passively to maintain the 20°F (−7°C) temperature. Eight thermistors were installed for weekly monitoring of the subgrade temperature, and weekly settlement monitoring of the foundation was recommended.

You can access this article if you purchase or spend a download.