Performance of a Thermopile (Long Thermopile) Foundation—50 Years Later

A four-legged, 250 foot tall, communication tower located in Glennallen, Alaska is supported on Long Thermopiles (thermopile) installed in 1960. This foundation was the first two-phase thermopile foundation installed in Alaska. The foundation remains in operation today, 50 years later. However, differential movement of the foundation was recently documented. This case history provides an analysis of the foundation performance throughout its life. A geotechnical exploration and site characterization was conducted at the site in October 2010 by Golder Associates Inc. (Golder). Each of the four tower legs is supported by three, 12-inch diameter thermopiles arranged in a triangle, to an embedment depth of approximately 18 feet below the ground surface. Design drawings show a 5-foot square steel plate (bearing surface) attached at the base of the three piling. The depth to the permafrost surface near each of the four tower legs ranged from 10 feet to 35 feet in October 2010. Soil temperature measurements collected weekly from January 2011 to October 2011 confirmed the permafrost depths and thermal state of the soil. A single control borehole, located away from the influence zone of the thermopiles, identified permafrost at a depth of 44 feet. The thermal degradation of the permafrost has extended beyond the influence zone of the thermopile foundation for two of the four support members, resulting in significant differential tower settlement due to a combination of thaw consolidation of the previously frozen soil and minor consolidation of the clay after the soil thawed. Many factors attributed to the thermal degradation of the permafrost at the site, including site clearing of vegetation around 1960 or earlier; maintenance directly related to the thermopile performance; changes to regional climate conditions over the 50 year facility operation life; and seasonal variations of site conditions.