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PBO Installs First of Four Permafrost GPS Stations in Alaska

Overview

The PBO Alaska Regional Office installed the first of four Permafrost GPS monuments planned for Alaska in the city Kotzebue on March 7th 2007. Kotzebue is located on the northwest portion of the Baldwin Peninsula on Kotzebue Sound in Northwest Alaska. The permafrost monument is a new one of a kind monument that was designed by Duane Miller and Associates LLC for UNAVCO using a thermopile manufactured by Arctic Foundation, Inc. to build and maintain a stable GPS monument in Arctic areas with permafrost subject to seasonal freeze thaw cycles. The station was designed to replace the traditional Deep Drilled Braced Monument (DDBM) commonly used by UNAVCO, Inc in unfrozen conditions. A standard DDBM would not have remained stable in permafrost because the angled legs of the DDBM would have been subject to extensive vertical frost action forces as the active layer of the permafrost froze and thawed on a seasonal basis. The active layers of the permafrost in the Kotzebue area is typically 3 to 4 feet deep and with the annual freeze thaw cycles and constant movement of the entire ground surface, the legs and entire monument would have permanently deformed in only a few years.

The thermopile consists of a single pile constructed of sealed 4.5-inch outside diameter schedule 40 pressure sealed steel casing that is 21 feet in length and containing carbon dioxide kept under pressure to keep both liquid and vapor present at all times. When the top of the two-phase thermopile is cooled by winter air below the temperature of the warmer liquid, the vapor condenses and the condensate flows by gravity towards the lower section of the pipe. The condensation reduces the vapor pressure, causing the warmer liquid to boil. The resulting cycle of condensation and vaporization is a highly effective, on-way heat transfer system and is passive with no need for external energy. Thermopiles have been used in Alaska since 1960 with great success and are largely responsible for the stability of the Trans-Alaska Pipeline.

Permafrost consists of two layers: the active layer which starts at the surface and extends 3-5 feet below the surface, and the permafrost which can extend more than 1000 feet below the active layer. The active layer thaws each summer and refreezes each winter. The freeze-thaw action of the active layer results in extreme upward forces known as ‘frost heaving’. In summer, when the air temperature is warmer than the active layer, no heat transfer occurs in the thermopile. However, in the late summer and fall, as the air temperature becomes cooler than the active layer, heat is transferred from the active layer to the air. This results in radial freezing of the active layer from the center of the thermopile, sooner than would be possible with a simple pile or post. A frozen active layer minimizes frost heave forces on the monument. The permafrost below the active layer stays frozen year round.

Alaska Field Engineer Max Enders and PBO Materials Manager Chuck Kurnik installed the station with the assistance from the KIC Drilling Company. In order to minimize disturbance of the permafrost, the monument had to be installed during the winter while the ground was completely frozen. Temperatures at the time of the install ranged from -28° to -15°F, and 20 to 30 knot winds, making the installation less than enjoyable for the crew and any exposed skin. Fortunately, the drilling took only one hour to complete using a 12 inch auger bit that drilled to a depth of 16.5 feet. With the aid of a large fork lift, the thermopile was lifted up and set into the hole and leveled. After the thermopile was placed in the hole, slurry consisting of warm potable water and natural soil cuttings and gravel was poured back in to the hole around the thermopile. Given the cold temperatures present at the time of the install, the slurry began freezing and locking the thermopile in to place with 20 minutes of being poured in the hole.

The top of the thermopile was mounted with a standard SCIGN leveling mount, a Trimble Choke Ring Antenna and enclosed in a SCIGN tall radome. The base of the antenna is approximately 50 inches above finish grade. The entire process from start to finish took only a matter of hours to complete. The Kotzeube PBO Station AB18 was installed on land owned by the Kikiktagruk Inupiat Corporation (KIC) that is currently leased by the Kotzebue Telephone Cooperative (OTZ) who operates and maintains a cellular phone tower on the property. Engineers from the OTZ helped to establish data communications on site to allow for real time data downloads from the site and provided storage of the GPS receiver and access to free AC power to power the GPS receiver and DSL modem used for data communications and downloading.

Additionally, it should be noted that this installation would not have been possible, had it not been for the cooperation we received from KEA electric, KIC construction, and OTZ telephone. Special thanks, go out to Matt Bergan with KEA electric, Doug Neil, Pierre Lonewolf and Mark Iten with OTZ telephone, and Bish Gallaher of KIC construction.

Three additional permafrost monuments are planned for installation in 2007 and 2008 in the villages of Arctic Village, Sagwon, and Fort Yukon. Arctic Village and Sagwon will be installed in April of 2007 and Fort Yukon will be installed in March of 2008.


Last modified: Sunday, 11-May-2014 18:01:10 UTC

 

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