Thermal Performance of the Permafrost Protection ...

Thermal Performance of the Permafrost Protection Techniques at Beaver Creek Experimental Road Site, Yukon, Canada Julie M-Lepage, Guy Doré Department of Civil engineering- Laval University, Québec, Canada Daniel Fort ier Geography Department- University of Montréal, Québec, Canada Paul Murchison Yukon Highways and Public Works, Transportation Engineering, Whitehorse, Yukon, Canada

Abstract To better understand permafrost degradation under roadways, 12 experimental sections were constructed on the Alaska Highway near Beaver Creek in April 2008. These sections study one or several co mbined methods of thermal stabilizat ion such as air convection embankment, heat drains, grass -covered embankment, reflecting surfaces and snow clearing on embankment slopes. This paper presents the results of the analysis of the ground thermal regime and the heat balance for each section during their first three years in service to determine the thermal impact of the construction and the short term effectiveness of th e techniques experimented. Keywords: Thermal regime; Permafrost mit igation technique; Alaska Highway; Embankment; Heat ext raction index

Introduction The main objective of this paper is 1) to analyze the ground thermal regime for each of the 12 sections during their first three years in service; and 2) to determine the thermal impact of the construction and the short-term effectiveness of the protection technique.

In the Canadian North, permafrost is thawing at an accelerated rate due, amongst other factors, to climate warming. As a result, subgrade soils underlying roadways are settling and experiencing a loss of bearing capacity. This is reducing the level of service and increasing the risks of accidents for road users. Maintenance costs are also considerably higher for affected sections of the road (Remchein et al. 2009). Furthermore, road construction techniques developed in southern Canada need to be adapted to the northern environment to prevent dramatic permafrost thawing after new road construction. Preventing permafrost warming or thawing by simply increasing the thermal resistance (e.g., by increasing the embankment height or by using more thermal-resistant materials) is a passive method and lacks long term effectiveness (Zhang et al. 2010). To find better solutions, 12 experimental sections have been constructed on the Alaska Highway near Beaver Creek, Yu kon (fig.1).

Proble ms Statement In permafrost regions, there has been very limited success building linear infrastructure that does not undergo deformations resulting from thawing of ice rich soils or frost heave induced by freezing of wet foundation soil (Kondratiev 2010). During construction of an embankment on ice-rich soils, the thermal regime can be significantly impacted causing rapid thawing of permafrost. The main types of resulting distresses are differential settlement and cracking along the road shoulder.

Fig. 2. Left: thaw-settlements causing water to pond in the depression near the centerline of the road (km 1858). Right: longitudinal cracking observed along Alaska Highway (km 1760).

Fig. 1. Schematic of Beaver Creek experimental test site

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TENT H INTERNATIONAL CONFERENCE ON PERMAFROST

This phenomenon causes an important loss of the functional and structural capacities of the Alaska Highway over a 200 km section main ly fro m Destruction Bay to the Alaska border. Every year, an estimated $22,000 per kilo metre is spent on maintenance directly related to damage caused by degrading permafrost. In fact, in these areas highway resurfacing is required every 3 to 4 years compared to 15 to 20 years for sections constructed on stable ground. The techniques used at the Beaver Creek experimental site were designed to prevent permafrost thawing by either extracting ground heat under the embankment or by reducing absorbed solar radiation at the embankment surface. It is expected that these methods will allow the control of the active layer thickness and limit the occurrence of differential settlements and cracks if thawing occurs. In the discontinuous permafrost zone near the study site, the active layer thickness is an average of 40-60 cm in peat covered soils. However, road construction is one of the main factors causing permafrost degradation, and the active layer thickness can easily reach to 3 or 4 meters locally under these embankments.

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There is no embankment.

water flow under the

road

Fig. 3. Shematic of the heat extration and induction index in the logitudinal culvert in 2009

The Fourier’s law of heat conduction (equation 1) has been used to compute heat fluxes.

Methodology The results presented in this paper are based on the analysis of thermal data collected at each test section. Temperature mon itoring at each of the test sections was done using thermistor strings. As a minimu m, a vertical thermistor string was installed in the middle of the section at mid-slope (south side) to a depth of 15 m fro m the surface. Sections including protection systems extending across the embankment also included a thermistor string positioned at the center of the cross section of the embankment. Finally, a few sections included an additional thermistor string located at the toe of the embankment slope (south side). These sections are the reference section, the full air-convection embankment and the full emban kment heat drain. The thermal data record starts in October 2008 and includes 6 daily readings. Data was made available to researchers by Yu kon Highways and Public Works (YHPW). The raw data has been analyzed to extract useful informat ion including evolution of temperature with depth and time, thermal regimes at specific times and thermal gradients using EXCEL and SURFER software. To characterize the effect iveness of the sections, a new method based on heat flux measurements at the interface between the embankment and natural soil has been used in this study. Heat flu xes have been calculated to obtain the heat extraction Hx and induction Hi indices for each protection technique at the road/embankment interface (fig.3). The proposed method is based on three assumptions: 1- The first 2 m of the natural ground under the embankment is homogenous. 2- Only conduction governs heat transfer into the natural ground.

Where: q: heat flu x [W/ m2 ] k soil : thermal conductivity [W/m°C] (Type of soil at the test site: gravelly muddy sand)

T: temperature measured at 0m and -2 m [°C] Z: depth [m] Based on field measurements (De Grandpré 2011), k frozen has been set at 1.337 W/ m°C and kunfrozen at 0.907 W/m°C. The definit ion of the heat extraction/induction indices is based on the quantity of heat extracted Hx or inducted Hi during a year. The heat induction index is computed by adding all positive heat flu xes obtained based on thermal gradients measured every 14 days (equation 2). The heat extraction index is obtained by adding all negative heat flu xes (equation 3).

Where Δt = 14 days = 1 209 600 seconds

The heat balance at the interface is obtained by adding Hi and Hx. A negative heat balance means that more heat is extracted than heat is induced. The upper portion of the natural ground should then be cooling.

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Control section The control section has been constructed using standard Alaska Highway construction practices. This section is used as a reference to compare the thermal performance of the other sections and to assess the relative effectiveness of the protection techniques. The kriging interpolation graph shown (fig. 4) has been created using the daily mean temperature data fro m the control section between October 2008 and October 2009. The dashed line located at the 0 meter depth illustrates the interface between the embankment and the natural ground. The dark solid line corresponds to the 0°C isotherm. The shaded region represents a critical temperature zone where the soil has temperature values between -1°C and 0°C. In 2008 and 2009 the 0°C isotherm confirm that the active layer is at a depth of 1.4 meters. This graph shows the importance of heat intake in a standard bituminous surface treatment road during summer season.

effectiveness of the convective system, two ventilation pipes have been placed at the base and at the top of the convective layer. Section 9 is similar to section 3 but the ACE material is exposed at the surface.

Fig. 5. Schematic of the air convection embankment sections

Fig. 4. Kriging interpolation of the control section thermal regime (°C)

Mitigation technique tested Sections protected with air convection embankment This thermal stabilizat ion method uses natural air convection to activate heat loss during winter, increasing winter cooling rates. The section was constructed using 150 mm to 300 mm d iameter crushed rock for the embankment slopes or across the full width of the embankment to form interconnected convective cells. During winter, air cooled in the upper part of the convective material, sinks down into the embankment displacing warm air upward. Heat is then extracted at the top of the embankment by conduction. During summer, the 5 meters thick porous material insulates the ground and reduces warming by keeping warm air near the surface and cool air at the base of the air convection embankment. Three ACE test sections were constructed. Section 1 is characterized by the presence of a convective layer across the full width of the embankment. The embankment’s slopes are also covered by an organic soil layer to impede heat intake and reduce warming of the ACE material during summer. Section 3 differs fro m section 1 as the application of ACE material was limited to the embankment slopes. In an attempt to maximize the

Longitudinal culvert This method uses 750 mm-diameter waterproof plastic culvert, buried under the embankment slope and parallel to the road, to generate air circulat ion and convection cooling. During winter, warm air tends to move upward through the vertical outlets drawing cold air in at the base of the embankment through the inlets. During summer the inlet and outlet are shut by lids to impede warm air circulat ion in the embankment.

Fig. 6. Inlets and outlets of the longitudinal culvert system

Snow/Sun sheds This method provides embankment slope protection during winter (snow accumulat ion) and summer (solar radiation). In the winter the technique enhances cooling by preventing snow accumulation on the ground thereby eliminating the insulating affect of snow and allowing cold air circulat ion over the ground surface. The technique reduces solar radiation on the embankment slopes during the summer.

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TENT H INTERNATIONAL CONFERENCE ON PERMAFROST Light-coloured Bituminous Surface Treatment This section uses light-colored aggregates in the bituminous surface treatment to reduce heat absorption fro m solar radiat ion.

Heat balance from the embankment slopes

Fig. 7. Snow/Sun shed on west side of the Alaska Highway

Snow clearing The objective of this technique is to reduce the insulating effect of snow accumulation on the embankment and allow heat to be removed more effectively in the winter. This is acco mplished by mechanically removing the snow from the embankment slope. Heat drains This protection technique uses a 25-mm-thick geocomposite drainage layer (dimp le board/drain board) to induce cold air circulat ion by natural convection through the embankment. In let and outlet pipes are attached to the geocomposite at regular intervals along the road. During winter, warm air moves upward in the drain and is expelled by the outlets. This movement draws cold air into the inlets and through the heat drain at the base of the embankment. The heat drain technique was used in three sections. Section 2 was constructed using a heat drain extending across the full width of the embankment. In section 4 the heat drain was limited to the embankment shoulders. Finally, the system used in section 8 includes the use of a 50-mm-thick insulation layer above the heat drain.

Fig. 8. Schematic of the heat drain sections

Figure 9 represents the heat balance computed for selected sections during the 2010 monitoring year. The heat balance represents the sum of the heat induction and extraction indices for a given section.

Fig. 9. Heat balance in the embankment slope in 2010. (Positive index means net heat intake and negative index net heat outtake).

Based on temperature measurements under the embankment, five sections appeared to have cooler ground temperatures than the control section in 2010: the longitudinal culvert, the snow/sun shed and the 3 sections using air convection embankments. Ho wever, based on heat balance measurements, only 3 of those sections have successfully cooled the natural ground and show negative heat balance: the snowshed, the longitudinal culvert and the ACE uncovered (9). For the ACE uncovered, the upper half of the slope is being contaminated by sand and gravel fro m winter maintenance operations (fig 10). This might cause the section to be less effective in the future if the voids become obstructed.

Fig. 10. West slope of the ACE uncovered contaminated with gravels by snow removal in M arch 2011

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TENT H INTERNATIONAL CONFERENCE ON PERMAFROST Table 1. Freezing and thawing index registered at the Beaver Creek test site in 2009-2010 compared to the 30 year-average

The thermal regime in the embankment slopes of only one section was not affected by the winter climate variation recorded during the monitoring period. The longitudinal culverts ext racted more heat fro m the ground while the mean air temperature was 1°C h igher. In fact, this technique extracted 1626 KJ/ m2 more in 2010 than in the previous year (fig.12). Fig. 11. Thermal regime at 0, 1 and 2 meters depth into the natural ground before and after the snow/sun shed construction

The snow/sun shed was constructed in fall 2010, one year after the construction of the other sections. Fig.11 shows how quickly the ground temperatures are reduced right after construction of the shed. The poor performance of the heat drain sections at the Beaver Creek site was not expected based on the good performance reported for the heat drain section tested on the Tasiuaq airstrip in northern Quebec (Ficheur & Doré 2010). A ll of the systems tested at the Beaver Creek site have a higher heat balance than the control section. It is suspected that the heat drain sections are not functioning based on their lack of performance. YHPW completed smoke tests in August 2011 to determine air could be circulated through the heat drains. Smoke was in jected in section 4 using 2 of the air in lets. Smoke was detected in the other inlets but not at any of the outlets. The results of the test suggest that there may be a blockage or break in this section of heat drain that is preventing it from working as expected. It is possible that some part of the geocomposite was crushed or intake/outlet connections were damaged during construction. The problem may also be associated with air leakage at the jo ints of the ventilation pipe system.

Fig. 12. Induction index, extraction index and heat balance in the longitudinal culvert section in 2009 and 2010

Heat balance under the centerline of the road Only seven techniques are represented in fig.13 because thermistor strings are not located at the centerline of the road for every section. Therefore, knowing the cooling effect induced by the longitudinal culverts and the snowshed under the center of the road is not possible. However, an interesting effect can be observed with the ACE sections.

Variation in air temperature between 2009 and 2010 The air temperature is recorded at the test site by a weather station located adjacent to the Alaska Highway. The annual mean air temperature reg istered in 2009 and 2010 were -3.9°C and -2.9°C respectively. The Beaver Creek climatic normal fro m 1971 to 2000 is -5.5°C (Environment Canada 2011). However, it can be noted that the warming main ly occurred in 2010 during the winter period when most of the technique are active (table 1).

Fig. 13. Heat balance under the centerline of the road (2010)

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Using the convective material across the full embankment helps to significantly cool the embankment in the middle of the road (1). For the uncovered ACE (9), the cooling of the slopes appears to extend under the centerline of the road. Finally, for the A CE with the vegetation cover on the slopes combined with the ventilation system (3), there is a significant warming effect under the road. This warming could be associated with malfunctioning of the ventilation system. The light coloured aggregates bituminous surface treatment has a heat balance of 104 KJ/ m2 which is 5338 KJ/ m2 lower than the control section. This high albedo surface appears to significantly reduce the embankment heat intake caused by absorption of solar radiation by darker surface material. Equilibriu m is reached throughout the year between heat induction and extraction. The beneficial effect of the light-coloured aggregated surface has also been measured with a FLIR infrared camera (fig.14). This infrared image was taken at the beginning of August on a sunny day around 4:30 p m. The dark b ituminous surface treatment zone on the left had a mean temperature of 36.2°C. The right zone that is the light-colored aggregates had a mean temperature of 33.7°C. During March in the morn ing, a difference of 1°C was measured between the two surfaces. It appears to be a promising technique to prevent permafrost differential thawing on the road and could be comb ined with others mitigation technique in areas highly affected by permafrost degradation.

Fig. 14. Infrared image comparing dark and light-colored bituminous surface treatment

Conclusion This analysis of the temperature data and heat balance fro m the Beaver Creek experimental site has led to preliminary conclusions on the thermal performance of

permafrost mitigation techniques. Some permafrost protection systems have shown good potential during their first two years in service. This is the case for sections 1, 3 and 9 (air convection embankments), section 7 (longitudinal cu lverts), section 6 (snow/sun shed) and the section 12 (light-colored aggregates). More tests will be required to understand the non-effectiveness of the heat drains sections. It is necessary to continue the thermal monitoring and analysis for a few more years to assess section performance in various climatic conditions . The durability of these sections as well as their long-term economical potential must also be assessed.

Acknowledgme nts The authors would like to acknowledge the technical and the financial support of Yukon Highways and Public Works for this project.

References Environment Canada 2011. National Climate Data and Information Archive, Beaver Creek A (Yukon Territory), www.climate.weatheroffice.gc.ca Ficheur, A. & Dore, G. 2010. Expérimentation de techniques de mitigation des effets de la fonte du pergélisol sur les infrastructures de transport du Nunavik : Aéroport de Tasiujaq, Rapport final, 177 p. Kondratiev, V.G. 2010. Some geocryological problems of railways and highways on permafrost of Transbaikai and Tibet, 63rd Canadian Geotechnical Conference and the 6th Canadian Permafrost Conference: 541-548. M-Lepage, J., Doré, G., & Fo rtier, F. (2010) Experimentation of mitigation techniques to reduce the effects of permafrost degradation on transportation infrastructures at Beaver Creek experimental road site (Alaska Highway, Yukon) , 63rd Canadian Geotechnical Conference and the 6th Canadian Permafrost Conference: 526-533. Remchein, D., Fortier, D., Dore, G., Stanley, B. & Walsh, R. 2009. Cost and Constructability of Permafrost Test Sections Along the Alaska Highway, Yukon. Proceedings of Transport Association of Canada Annual Conference, Vancouver, October 2009, 120. De Grandpré, I. 2011. Impacts de l'écoulement souterrain sur la dégradation du pergélisol, mémoire de maîtrise, département de géographie, faculté des Arts et Sciences, Université de Montréal, 132 p. Zhang, M., Lai, Y. & Dong, Y. 2010. Three-Dimensional Nonlinear Analysis for the Cooling Characteristics of Crushed-Rock Interlayer Embankment with Ventilated Duct along the Quinghai-Tibet Expressway in permafrost Regions, Journal of Cold Regions Engineering, Vo lu me 24, Nu mber 4: 126141.