Modern technology keeps the ground frozen year-round

Alaska Business Monthly; July 2004

Alaska’s harsh climatic conditions often have adverse effects on man-made infrastructure such as roads, buildings, railways, or pipelines. Over the years, a number of innovative solutions have been developed to help overcome these difficulties. One well-known example is that of the trans-Alaska oil pipeline, which employs a system of thermosyphon cooling devices that keep the vertical support members permanently frozen into the permafrost.

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ACE ventilated shoulder being constructed on Thompson Drive, September 2003

The design is unique, and was an outgrowth of important developments in the field of arctic engineering combined with a careful design process.

Many other types of infrastructure are affected by the presence of harsh climatic conditions and permafrost. One only needs to drive the highways of Interior Alaska for a short time to realize that challenges remain. Simply put, roadways and permafrost don’t get along, at least not in the large portion of Alaska that is underlain by ice-rich permafrost. The process of clearing vegetation and constructing roadway embankments is one that often produces local warming of the ground surface. The warming, in turn, interferes with the thermal state of the underlying permafrost causing thawing. If the permafrost has a high ice content, a common occurrence in many of Alaska’s permafrost areas, then the thawing will result in settlement (usually referred to as thaw settlement) and damage to above-ground structures. In the case of highways, the consequences of thawing permafrost are all too familiar to the residents of Interior Alaska who are often confronted with rough distorted roadways.

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Hairpin thermosyphon evaporators being installed in Thompson Drive

In 2001, the Alaska Department of Transportation began the design of a new section of roadway near the campus of the University of Alaska Fairbanks. The new road, named Morris Thompson Drive after the late Alaska Native leader, is to provide a new entrance to UAF by connecting Geist Road and Tanana Drive. The road has a length of a little more than a half mile and includes a bridge over the Alaska Railroad, as well as concrete curbs, gutters, and sidewalks. The project area includes two sections of previously undisturbed permafrost and, thus, designers were concerned with the possibility of thaw settlement. Because of the inclusion of the concrete improvements, the consequences of any thaw settlement were more serious than usual. Re-leveling of distorted sidewalks, curbs and gutters represents an expensive maintenance issue. In order to avoid these difficulties, the decision was made to include advanced cooling technology in the project in an effort to avoid permafrost thaw. Construction began in March 2003 and the project is scheduled for completion in 2005.

KEEP IT COOL
Two different types of cooling technology are being incorporated into the Thompson Drive project. The first uses the same cooling principle as the thermosyphons on the trans-Alaska oil pipeline, although a new configuration is being utilized. The devices, known as hairpin thermosyphons, are completely buried beneath the roadway surface and do not have exposed fins like those used on the pipeline. These thermosyphons work by pulling heat from the permafrost beneath the roadway during the winter months. The heat evaporates a refrigerant in the lower portion of the thermosyphon, known as the evaporator. Refrigerant vapor then travels upward to the upper portion of the hairpin where it condenses in the condenser, releasing heat just beneath the roadway surface. Heat from the condenser is then dissipated to the cold winter air above the road surface. The enhanced winter cooling that results from the operation of the thermosyphons lowers the permafrost temperature during winter in order to keep it from thawing during subsequent summers. Arctic Foundations Inc. of Anchorage has manufactured the 150 thermosyphons that are being used in the project.

A second type of cooling technology, known as an air convection embankment (ACE embankment), has also been incorporated into Thompson Drive. ACE embankments are constructed of rock with a size range of roughly 6 to 12 inches. Ideally, all fines are excluded from the ACE material resulting in highly porous layers that allow air to circulate freely. ACE layers can be incorporated in the center portion of the embankment beneath the asphalt or can be included on the embankment side slopes. In either case,
these layers promote air circulation during winter months when the pore air tends to be cold and heavy in the upper portion of the layer and relatively warm and light below. The warm air rises, carrying heat with it, while cold air from above sinks and cools the underlying permafrost. In this way, ACE layers can provide the same type of enhanced winter cooling as described previously for thermosyphons. If designed properly, ACE layers can also prevent permafrost thaw.

Two different types of ACE systems are being employed on Thompson Drive. The first is referred to as a ventilated shoulder and uses ACE material only on the side-slope of the embankment. In this case, cold ambient air is drawn into the lower portion of the ACE side-slope layer and warms as it travels upwards through the ACE rock. By circulating cold winter air directly through the embankment shoulder, a large cooling effect is achieved which chills underlying permafrost thus preserving its frozen state year-round.

During January of 2004, evidence of ACE air circulation was observed at the site. Holes were formed in the. snow layer by the warm air exiting the upper portion of the ACE side slope. Intense vapor plumes were observed exiting these holes as the warm moist air flowed upward into the cold January environment.

Another type of ACE system employed on Thompson Drive uses an ACE layer that extends all the way across the road from toe to toe. In this case, the pavement is installed directly on the top of the ACE layer using a fabric separator to keep the fine material
below the asphalt from falling down into the ACE rock. In this type of a system, air circulates internally in a circular fashion beneath the asphalt surface. The internal circulation transports heat upward in the winter and sub-cools the underlying permafrost. As with the other systems, this enhanced winter cooling allows the permafrost layer to survive through the summer without experiencing any thaw.

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Material is placed, compacted and allowed to freeze in place, November 2003

WINTER WORK
Another unique feature of the Thompson Drive project involves the construction schedule. In order to give the cooling system a helping hand, some portions of the project were scheduled for winter construction. By utilizing the low air temperatures available, it was possible to ensure that the lower portions of the embankment structure were in the frozen state. Material was placed thawed in thin layers and allowed to freeze after compaction. A temperature monitoring system ensured that the base of each layer was completely frozen before the next layer could be placed. Constructing the embankment in this fashion helped to avoid introduction of heat into the permafrost layer, a common problem with summer construction activities.

Professors from UAFworked closely with AKDOT to carry out thermal design calculations for both the ACE and Hairpin Thermosyphon design features. Numerical models were run in order to ensure that each system could accomplish its goal of chilling and protecting the permafrost layer year-round. An extensive measurement and instrumentation system will be installed at three locations within the project in an effort to monitor the thermal performance. Temperatures measured within the embankment will be monitored to ensure that the permafrost remains frozen. The data will also provide new information that will allow university and AKDOT researchers to enhance their knowledge base, thus improving their ability to predict the performance of these systems. Installation costs for the ACE and thermosyphon enhancements were covered by the Experimental Features in Construction program of the Federal Highway Administration, while design and data monitoring costs are being covered by research funds from AKDOT.

Research concerning the interaction between infrastructure and Alaska's harsh climate has been one of the central themes of a large research project that was recently funded at the University of Alaska by the EPSCoR Program (Experimental Program to Stimulate Competitive Research) of the National Science Foundation. Collaborative research programs such as these, combined with real-world application, are sure to lead to future
advances in the state of the art when it comes to dealing with Alaska's permafrost challenges.

By Douglas J. Goering
Article (C) 2004, Alaska Business Publishing Company