Air Convection Embankment Research Articles

Thermo-mechanical Stability Analysis of Hollow Cellular Concrete Block Air Convection Embankments for Cold Regions

Crushed-rock air convection embankment (ACE), as a highly open-graded porous medium, has been used to mitigate the thaw settlement of pavement structures in permafrost regions. Previous studies have revealed that cellular concrete has significant potential as a cost-effective material for ACE to solve the problem of crushed rock shortage in interior Alaska and improved the thermal stability of pavement structures in cold climates. The design configurations of the hollow cellular concrete block ACE were further designed to maximize the performance benefits and facilitate future implementation and field construction. However, the mechanical behaviors of the proposed structures and ice-rich subgrade were still unknown. In this study, a thermo-mechanical coupling model was developed to evaluate the thermal effect on the long-term mechanical stability of the selected pavement structures and subgrade soils. The results indicated that the thermal stability of the proposed hollow cellular concrete block ACEs was superior to that of the conventional crushed-rock ACE. The maximum thaw settlement of the proposed hollow cellular concrete block ACE was only 13.4% that of the conventional crushed-rock ACE. Thermal and mechanical analyses showed that the proposed cellular concrete ACEs have a significant advantage over conventional crushed-rock ACE.

Innovative Air Convection Embankment for Cold/Arctic Region Low-Volume Roads

Roadbed thaw settlement is a unique challenge for the durability of low-volume roads (LVRs) in permafrost regions. Air convection embankment (ACE) is an effective technique that acts as a semi-heat-transfer system to control temperature variation and reduce the thaw depth of subsoil. However, limited by the shortage of the necessary crushed rocks within a short distance, building an ACE in Alaska, U.S., is prohibitively expensive. Previous studies identified the feasibility of using cellular concrete for ACE and determined the optimized thickness of the cellular concrete aggregate interlayer for ACE. However, the economic efficiency and thermal and mechanical performance of the optimized, innovative cellular concrete aggregate ACE need further investigation. Therefore, two innovative cellular concrete ACEs with reasonable heights for cold/Arctic region LVRs were evaluated in this study. A thermal-mechanical coupling model was created using ANSYS Fluent and ANSYS Mechanical to evaluate the thermal and mechanical stability of the two optimized innovative cellular concrete aggregate ACEs by comparing them with a typical Alaskan flexible pavement, a silty sand/gravel embankment, and a conventional crushed-rock ACE. The fatigue damage was predicted using the elastic-based Alaska Flexible Pavement Design (AKFPD) program. A life-cycle cost analysis was conducted using AKFPD to evaluate the overall long-term economic efficiency of the cellular concrete ACEs. The results showed that cellular concrete ACE could achieve better thermal and mechanical performance with much lower embankment height than crushed-rock ACE. The cost analysis showed that the proposed cellular concrete ACEs had a significant cost advantage over the conventional crushed-rock ACE.

Thermal performance analysis of hollow cellular concrete block air convection embankment for cold regions

Crushed-rock air convection embankment (ACE) is an excellent passive cooling technique that uses open-graded crushed rocks as a “thermal semi-conductor” to prevent roadbeds from thawing in summer and enhance the cooling effect in winter in permafrost regions. However, the desired crushed rocks needed for ACE are not readily available in interior Alaska, resulting in extremely high construction costs. Previous studies indicated that using the cost-effective cellular concrete for ACE could effectively enhance the cooling performance of ACE and mitigate moisture warping and temperature curling of the asphalt concrete layer. It is promising to be an alternative to crushed rocks for ACE to mitigate pavement distresses. However, the design of cellular concrete ACE needs to be further studied to maximize performance and facilitate future implementation. Hence, two design configurations of cellular concrete block ACEs were proposed in this study. The thermal performance of pavement structures with these two designs was investigated by comparing them with the other four. A total of six pavement structures were analyzed, including a typical flexible pavement in the Northern Region of Alaska and five pavement structures reinforced with different paving interlayers, i.e., silty sand/gravel, crushed rocks, cast-in-place cellular concrete (full insulation), and two types of precast cellular concrete blocks. The thermal profiles, air pressure gradients, and velocities were numerically analyzed in ANSYS Fluent. The numerical results indicated that the two proposed cellular concrete ACEs exhibited a significant heat insulation effect in summer and a desired cooling effect in winter, which raised the permafrost table significantly. The maximum thaw depth of the two proposed cellular concrete ACEs was only 15% of the thaw depth of traditional silty sand/gravel embankment.

Impacts of snow cover on the pattern and velocity of air flow in air convection embankments of sub-Arctic regions

In northern regions, air convection embankment (ACE) has been implemented to mitigate road subsidence caused by permafrost degradation. The seasonal snow cover largely alters the surface energy balance and adversely affects the thermal stability of the road. However, thermal characteristics of a closed ACE and the quantification of their impacts on talik development remain largely unknown, especially in the present and future context of global warming. Here, impacts of snow cover on the pattern and strength of the air circulation were investigated and quantified. Snow cover greatly weakened the cooling effect of the closed ACE side slopes. The pore air velocity of the crushed-rock layer was reduced by half and the mean annual heat flux increased by approximately 1.6 W/m2, in comparison to the snow-free case. Conversely, the U-shape closed ACE had a better cooling performance due to a stronger air convection in winter, which increased heat loss from the snow-free surface of the embankment center. Our simulations suggest that the U-shape ACE embankment is a potential long-term solution to mitigate permafrost thaw over the life service (50 years) of road embankments in sub-arctic regions. The net results of this study are imperative to improve engineered design and road maintenance in cold regions.

Impacts of Lightweight Aggregate Interlayers for Air Convection Embankment on Pavement Thermal Profile and Pavement Performance in Alaskan Permafrost Regions

Crushed-rock air convection embankment (ACE) has been used to mitigate pavement distresses caused by climatic extremes in permafrost regions for more than three decades. This study proposes using lightweight aggregates as cost-effective alternative materials to overcome the shortage of desired crushed rocks needed for ACE in interior Alaska. This paper presents an investigation of the performance of six selected pavement structures: an asphalt pavement typical of the northern region of Alaska and five pavements reinforced with different paving interlayers (i.e., silty sand/gravel, crushed rocks, cellular concrete aggregate, foam glass aggregate, and lightweight expanded clay aggregate). Pavement thermal analyses using Temperature Estimate Model for Pavement Structures (TEMPS) program and ANSYS Fluent software were performed to predict heat transfer patterns and thermal performance for each case. Pavement performance analyses using viscoelastic-based FlexPAVE™ and the elastic-based Alaska Flexible Pavement Design (AKFPD) program were conducted to evaluate the long-term performance and structural stability of subgrade soil with different types of interlayers. The results showed that the lightweight aggregate ACEs improved cooling performance more effectively than the crushed-rock ACE. The lightweight aggregate interlayers could maintain desired thermal insulation in summer and enhance the cooling effect in winter. The modeling results identified the high potential of using lightweight aggregates as alternative ACE materials to improve pavement service life and subgrade stability.

Field and numerical studies on the thermal performance of air convection embankments to protect side slopes in permafrost environments

Shoulder air convection embankments (ACEs) are sufficient to counter the effect of ground staying relatively warm due to the insulation of snow accumulation on side slopes of northern transportation infrastructure. However, little information is available for the design procedure. This paper aims to propose new design guidelines and an engineering design chart to quantify the heat extraction capacity of shoulder ACEs. The tested shoulder ACE, constructed on the Alaska Highway at Beaver Creek, Yukon, in 2008, provided valuable data for the assessment of field performance and model development. Thermistor strings were installed under the centerline and under the side slope. Measurements indicated that the cooling of ground soils was significant mainly under the side slope during the monitoring years from 2009 to 2013. The data collected were also used to calibrate a coupled conduction-convection model to reproduce the thermal conditions and the changing temperature trend in the subgrade. An engineering design chart was developed based on the calibrated model, and was successfully validated using the thermal data from the Puvirnituq airstrip in Northern Quebec, Canada.

Modeling the thermal response of air convection embankment in permafrost regions

Permafrost degradation under transportation infrastructure often results in thaw settlement due to thawing of the ice-rich subgrade. Climate change is associated with permafrost-related engineering problems. Air convection embankments (ACE) have been proven to be an effective method to prevent permafrost thawing, in response to climate change. Poorly-graded aggregates are used to facilitate the air flow in an ACE, especially during winter when the air density gradient is unstable. A large-scale ACE test section was constructed along the Alaska Highway in 2008 at Beaver Creek, Yukon, Canada, to investigate the heat extraction capacity of ACEs. Boreholes under the toe, the side slope and the centerline were drilled and instrumented. Temperature data collected at this site were used to investigate the thermal performance of the ACE and to calibrate a 2D thermal model that was developed based on the Beaver Creek experimental site. Specific site characteristics, such as air temperature, foundation soil properties and embankment dimensions, were measured and used as input parameters to improve the accuracy of the 2D model developed. A relatively new approach based on heat balance at the embankment-soil interface has been proposed to investigate the heat extraction capacity of ACEs. After satisfactory calibration of the model at the Beaver Creek site, an engineering design chart has been developed and is proposed to assess the heat balance at the embankment-soil interface for different embankment thicknesses and site conditions.

Feasibility study on use of cellular concrete for air convection embankment on permafrost foundations in Fairbanks, Alaska

Abstract The crushed-rock air convection embankment (ACE) is an excellent technique to protect the permafrost from thawing in road construction in cold regions due to its superb cooling effect in winter and excellent thermal resistance performance in summer. However, the desired materials needed for ACE are not readily available, which prevents its extensive use in Alaska. To overcome the limitation (shortage of suitable crushed rocks) of traditional ACE and further improve the cooling effect of ACE, this study investigated the feasibility of using cellular concrete as an alternative material for ACE in cold regions. The finite element method was used to predict the heat transfer patterns of the sand/gravel embankment, the crushed-rock ACE, and the cellular concrete ACE. The results of the present study showed that the cooling performance of both the cellular concrete ACE and the crushed-rock ACE was superior to the conventional sand/gravel embankment. The cellular concrete ACE had better heat insulation property in the summer, and the crushed-rock ACE had stronger natural convection in winter. For the yearly cooling efficiency of the two different ACE techniques, the proposed cellular concrete ACE had a better cooling effect on the foundation soil than the crushed-rock ACE. The results presented in the isotherms and velocity vectors plots for different months of the year also indicated that the thermal conductivity and specific heat capacity of the construction materials had a significant impact on the performance of the ACE. The preliminary cost analyses showed that the cost of the cellular concrete ACE was close to that of the conventional sand/gravel embankment but much lower than that of the crushed-rock ACE.

Long-term thermal and settlement characteristics of air convection embankments with and without adjacent surface water ponding in permafrost regions

Surface water is an important local factor in engineering geological conditions in permafrost regions. In this paper, the impacts of surface water ponding on the long-term thermal and settlement characteristics of an air convection embankment (ACE) were investigated based on 13 years of data records from field observations along the Qinghai-Tibet Railway (QTR). The time series of the ground temperatures and the settlement of two ACEs with and without adjacent surface water ponding were investigated and compared. The results indicated that surface water ponding could significantly affect the thermal performance of the ACE. Without surface water ponding, the ACE showed a satisfactory cooling effect; the underlying permafrost table rose, and the shallow permafrost layer cooled significantly after construction of the embankment. However, with surface water ponding, the great heat capacity and latent heat of the ponded water and related capillary action impeded the thermal interaction between the ACE and the underlying permafrost. Before the drainage of the ponded water, no permafrost cooling occurred beneath the embankment. The settlement of the two ACEs also differed significantly. The creep of the ice-rich permafrost layer immediately beneath the permafrost table was inferred as the main contribution of the settlement. The temperature of the layer was the main factor determining the rate of settlement. The results can help the hydro-thermal analysis of engineering foundations built over permafrost and provide a reference for embankment construction and maintenance in permafrost regions with rich surface water.

Study of the time-dependent thermal behavior of the multilayer asphalt concrete pavement in permafrost regions

Asphalt concrete is one of the widespread pavement materials. The multilayer asphalt concrete pavement (MACP) has strong heat-absorption capacity. The thermal behavior of the MACP can affect the energy exchange between atmosphere and embankment, especially for the permafrost ground which is sensitive to temperature. To investigate the thermal behavior of the MACP, an in-situ experiment was performed firstly to investigate the effect of MACP on the cooling performance of an air convection embankment used in permafrost region. Subsequently, a mathematical model was developed and validated to describe the thermal behavior. Finally, series of simulations were carried out to evaluate the time dependence of the thermal behavior. The results indicate that, (1) the asphalt pavement can alter the energy exchange between permafrost foundation and atmosphere, and reduce the cooling performance of the embankment; (2) the construction interval of MACP can strengthen the convective cooling capacity and accelerate the heat dissipation of the permafrost foundation; (3) an optimized construction interval was proposed for the simulated air convection embankment to maintain its long-term stability. The study can contribute to the construction control of air convection embankment with MACP in permafrost regions.

Long-Term Thermal Effects of Air Convection Embankments in Permafrost Zones: Case Study of the Qinghai–Tibet Railway, China

AbstractLong-term thermal effects of air convection embankments (ACEs) over 550-km-long permafrost zones along the Qinghai–Tibet railway were analyzed on the basis of 14-year records (2002–2016) of...

Modeling the performance of an air convection embankment (ACE) with thermal berm over ice-rich permafrost, Lost Chicken Creek, Alaska

Construction of roadway embankments over permafrost often results in settlement due to thawing of ice-rich foundation soils. The air convection embankment (ACE) is a relatively new design developed to reduce thaw settlement. In 2012 an ACE and an adjacent thermal berm were constructed as part of a realignment of the Taylor Highway near Lost Chicken Creek, Alaska. The Taylor Highway, a minor roadway with a gravel surface, is closed and not maintained during the winter months. To evaluate the thermal performance of the newly-constructed ACE and thermal berm, we performed field work and laboratory testing to determine foundation soil and embankment properties, measured temperatures at the base of the embankment, and developed two-dimensional finite element models to estimate long-term stability. Measurements indicated temperatures beneath the ACE were significantly colder than beneath the thermal berm. In the modeling, we simulated plowed (PC) and snow-covered (SC) conditions. Both models indicated that the ACE experienced density-driven air convection during the winter months. The PC scenario produced significantly colder temperatures within the ACE and underlying foundation soils. The SC model results more closely matched measured temperatures beneath the ACE, which is reasonable considering the lack of maintenance on the Taylor Highway during winter. The modeling indicates that ACE performance is improved through plowing of the surface, and that an ACE is more effective in maintaining frozen conditions in the foundation soils than the thermal berm; in fact, the modeling and measured temperatures indicate that the thermal berm actually raises the temperature in the foundation soils. Based on the results, we expect that thaw settlement will occur beneath the thermal berm until thermal equilibrium is reached, whereas the majority of the ACE will remain stable.

Review of Effectiveness and Costs of Strategies to Improve Roadbed Stability in Permafrost Regions

This paper reviews the effectiveness and costs of strategies to improve roadbed stability in permafrost regions, based on a synthesis of literature findings. Roadbeds in permafrost regions experience instability when the embankment loading and its heat absorption properties degrade the permafrost foundation. A variety of engineering strategies are used to mitigate this effect. The review summarizes the rationale, effectiveness, and costs of four types of strategies, namely those that control roadbed thawing, cool the roadbed, insulate the roadbed, and reduce roadbed fill weight. The literature reveals that strategies to control roadbed thawing, insulate the roadbed, or reduce roadbed fill weight do not reverse the long-term degradation of permafrost foundations. Strategies that cool the roadbed by implementing air convection embankments, ventilation ducts, thermosiphons, heat drains, or combinations of these are effective in reducing embankment temperatures and stabilizing the roadbed. Costs vary by geographic and climatic conditions and the proximity of materials to the construction site. Reported data suggest that conducting normal maintenance is less expensive than implementing roadbed cooling strategies, but maintaining serviceability may not be feasible.

Assessment of the effectiveness of two heat removal techniques for permafrost protection

Two mitigation techniques, an air convection embankment and an embankment of a granular material with an integrated heat drain, have been tested for the implementation in the shoulders of road and airfield embankments in permafrost regions. Both techniques will allow cold air to penetrate the embankment from the bottom, while warm air is dissipated at the top. The techniques have been tested in the laboratory, where a small-scale embankment (SSE) was build and placed in a cold room to measure the embankment temperatures during winter conditions. A numerical modeling has been developed and calibrated on the SSE to verify the effects on the thermal regime of full-scale embankments. The results have shown that both techniques will cause a decrease in temperature, which will minimize or even possibly avoid permafrost degradation underneath the embankments. The laboratory results have also shown that the effectiveness of the air convection embankment technique can be increased during winter conditions by ventilating the top and the bottom of the embankment shoulders. Installation of air intakes along the shoulders will facilitate air flow into the system during winter and will trap the cold air in the bottom of the embankment through the summer period. This solution has been verified using the numerical model.

Experimental validation of passive permafrost cooling systems

In areas of warm permafrost, the thawing of interstitial ice in the permafrost layer can cause man-made infrastructure to suffer from thaw settlement damage and failure. For highway and railway embankments, thermosyphons and air convection embankment (ACE) layers are two methods that can be used to reduce embankment and foundation soil temperatures and prevent thaw settlement. These techniques utilize the low air temperature during winter to enhance freezing of the foundation soils and maintain the thermal integrity of the permafrost foundation. In the present paper we report on the application of these techniques in a full-scale field application, carried out as part of a new road building project near Fairbanks, Alaska. Three separate cooling techniques were utilized in the project, including hairpin thermosyphons, horizontal ACE layers, and ventilated shoulders in three different configurations. Each of the three configurations was instrumented with a thermal monitoring system in an effort to evaluate the cooling effectiveness and predict the long-term impact on underlying permafrost temperatures. This paper describes the cooling systems that were used in the project including the thermal monitoring system, and presents performance results for the first year of operation.

In Situ Test on Cooling Effectiveness of Air Convection Embankment with Crushed Rock Slope Protection in Permafrost Regions

An experimental air convection embankment (ACE) was constructed in Beiluhe on the Qinghai-Tibet Plateau during 2001–2003, using coarse (5–8 and 40–50 cm), poorly graded crushed rock fill material on the slope of embankment with thick ground ice permafrost foundation, which should be called the air convection embankment with crushed rock slope protection (ACE–CRSP). The highly permeable ACE–CRSP installation was designed to test the cooling effectiveness of ACE–CRSP concept in an actual railway project. Ground temperature data were collected from test sections on the railway with thermistor sensor strings. The results showed that the mean ground temperature under the layer of the crushed rock with coarse particle diameter of 40–50 cm was lower than that under one with finer particle diameter of 5–8 cm, and the fluctuating range of temperature under the former was bigger than that under the latter. It was obvious that the maximum thaw depth was raised under the layer of crushed rock with coarse particle diameter of 40–50 cm, which resulted from the stronger cooling effectiveness of air convection during the winter. The amount of heat exchange also showed that the absorbed cooling energy of the foundation, under the layer of the crushed rock with coarse diameter, was larger than that with finer diameter.So, we believe that the cooling effectiveness of the crushed rock layer with coarse diameter was stronger than that one with finer diameter.

Air Convection Embankment for Roadways: Field Experimental Study in Alaska

During 1996—1997, an experimental air convection embankment (ACE) was constructed in Fairbanks, Alaska, using very coarse [1- to 6-in (25- to 150-mm)], poorly graded, crushed rock fill material on top of icerich permafrost foundation. The highly permeable ACE installation was designed to test the cooling effectiveness of the ACE concept in an actual roadway project. Using thermistor sensor strings, temperature data were collected from the ACE test section and an adjacent control section. Results showed that the ACE technique can be used to generate a passive cooling effect. This effect manifested as a depression of mean annual temperature at the base of the ACE cross section compared with mean embankment surface temperatures. The data indicated that this thermal offset is on the order of 7°F (Δ = 4°C) for this installation. The measured data also showed that mean annual temperatures are generally near or above the freezing value in the upper portion of the ACE embankment but significantly below freezing at the base. The temperature offset caused a gradual cooling of the foundation soil beneath the embankment with maximum base temperatures that have decreased from more than 40°F (4.4°C) to near freezing during the 5-year measurement period. Within a few more years of operation, it is anticipated that the permafrost table will move up into the base of the ACE cross section, completely eliminating any seasonal thaw in the foundation soil below and thus preventing future subgrade thaw–consolidation.