Duct-ventilated Embankment Research Articles

Analysis of asymmetric temperature fields for the duct-ventilated embankment of highway in permafrost regions

The heat capacity and polythermal effect of highway roadbeds with wide asphalt pavements have become increasingly significant, which causes their influences on engineering and permafrost issues to become prominent. A duct-ventilated embankment is a potential mean to proactively cool ground temperatures. This method is applied experimentally in a highway for the first time. According to a field testing in the Tibetan Plateau Highway Pilot Project, a duct-ventilated embankment has excellent cooling effect. The mean annual ground temperature reduction at the embankment/natural ground surface interface is 1.62°C. The mean annual temperature difference between the south- and north-facing slopes reaches 2.39°C. This difference results in the asymmetry of the underlying temperature field and longitudinal roadbed cracking. The asymmetric distribution of the duct-ventilated embankment temperature field is caused by differential sun exposure and the heat transfer process and intensity within the duct. Differential sun exposure is a major factor between the south- and north-facing sides, accounting for 84%, whereas the remaining 16% is ascribed to the different heat transfer processes within the duct. The amount of asymmetry is increased to 20% by the duct and slightly accentuated by the ventilation duct. To avoid this effect, future projects should focus on solving problems related to roadbed direction and the effect of slope temperature difference on ground temperature. Measures are recommended to be taken to regulate both slope temperature adjustment and ventilation ducts, and thus, effectively combine the advantages of both measures in embankment design.

Calculation theories and analysis methods of thermodynamic stability of embankment engineering in cold regions

The calculation theories and analysis methods of thermodynamic stability of embankment engineering in cold regions are systematically summarized. The engineering theories and methods taken to control frost heave and thaw settlement in seasonal frozen soil regions may not be applicable in permafrost regions. The active cooling technology of roadbed should be utilized to limit the effects caused by both climate changes and human engineering activities. The paper mainly discussed the calculation theories and analysis methods of four kinds of embankment structures, i.e., the crushed-rock embankment, duct-ventilated embankment, thermosyphon embankment, and composite embankment. It is expected that a scientific basis could be provided for the theory, design, and application of embankment constructions in cold regions.

Cooling effects study on ventilated embankments under the influence of the temperature differences between the sunny slopes and the shady slopes

Based on theories of heat and mass transfer, three-dimensional theoretical and numerical models were presented to analyze the temperature characteristics of embankments in permafrost regions. Conditions of air convective within crushed rock layer and the background of the climate warming in the future were considered. In addition, temperature differences between the sunny and shady slopes of embankments, as well as the uneven distribution of air temperatures inside the ventilated duct were also considered in the models. Cooling effects and temperature field characteristics of three kinds of embankments were analyzed and compared. Results of traditional embankment indicated that there were significant differences in permafrost tables between the sunny and shady slopes, and the underlying permafrost was in serious degradation. A large asymmetric 0 °C melting bulb was formed in and beneath the embankment over winter time. The analysis indicated that the duct-ventilated embankment could lower the temperature of underlying permafrost, and elevate the permafrost table under the embankment. The duct-ventilated embankment could also adjust the differences of permafrost tables, and improve the asymmetry of the temperature fields between the sunny and shady slopes. Due to climate warming, however, a 0 °C melting bulb still developed at the sunny slope foot of the duct-ventilated embankment. To prevent the 0 °C melting bulb from occurring, a closed crushed rock revetment was applied on the sunny slope, and the results indicated that the cooling effect of duct-ventilated embankment with the closed crushed rock revetment was better than that without the closed crushed rock revetment. Soils in and beneath the duct-ventilated embankment with closed crushed rock revetment could refreeze completely over winter time and this kind of embankment could also effectively adjust the differences of permafrost tables between the sunny and shady slopes.

Three-Dimensional Nonlinear Analysis for the Cooling Characteristics of Crushed-Rock Interlayer Embankment with Ventilated Duct along the Qinghai-Tibet Expressway in Permafrost Regions

The crushed-rock embankment and duct-ventilated embankment have been used as effective cooling measures to protect permafrost underlying the Qinghai-Tibet Railway from thawing in China. These two cooling techniques are not directly applied to the Qinghai-Tibet Expressway, however, due to the large width and higher temperature of pavement surface. Therefore, considering the heat transfer characteristics of crushed-rock interlayer embankments and duct-ventilated embankments, we designed the crushed-rock interlayer embankment with ventilated duct. For cold regions engineering projects, the thermal regime is the most important factor that determines the stability of construction. To investigate the thermal stability of this new type of embankment, a three-dimensional numerical model was developed based on heat and mass transfer theory. The model includes coupled heat transfer between the airflow and the duct wall, air convective heat transfer within the crushed-rock interlayer, and heat conduction with phase ...

Numerical study on temperature characteristics of expressway embankment with crushed-rock revetment and ventilated ducts in warm permafrost regions

The crushed-rock revetment embankment and duct-ventilated embankment, as effective cooling measures, have been used to protect the underlying permafrost from thawing in Qinghai–Tibet Railway construction of China. However, the two cooling measures cannot be directly applied to the expressways with a wide and high-temperature pavement surface in warm permafrost regions. Therefore, considering the heat transfer characteristics of crushed-rock revetment embankment and duct-ventilated embankment, we designed a new-type embankment with crushed-rock revetment and ventilated ducts. For cold regions engineering, the temperature is the most important factor that determines the stability of construction. To research the thermal stability of the new-type embankment, according to the related theories of heat and mass transfer, a three-dimensional theoretical and numerical model for the embankment, which is composed of the coupled heat transfer problem between airflow and duct wall, the air convective heat transfer problem within crushed-rock layer, and the heat conduction problem with phase change in soil layers, was presented to analyze the temperature characteristics of the new-type embankment. Three points can be concluded from the numerical results based on the assumption that the air temperature will be warmed up by 2.6 °C in the next 50 years. First, the crushed-rock revetment embankment would be unstable because the crushed-rock revetment can reduce only the ground temperature at the side-slope foots of embankment, but the permafrost table descends severely in the middle. Second, the ventilated-duct embankment can effectively cool the soil at the middle of the embankment but not at the side-slope foots, so that the uneven temperature distributions would be deleterious to the stability of expressway. Third, the new-type embankment structure is effective to decrease the underlying ground temperature and to ensure the stability of expressway with a wide (multi-lane) and high-temperature upper surface in warm permafrost regions.

Laboratory investigation on cooling effect of duct-ventilated embankment with a chimney in permafrost regions

In construction and maintenance of road construction in cold regions, permafrost has been the main influence on embankment stability. The cooling effect of duct-ventilated embankments on the underlying soil layers has been recognized as one technique but if the embankment is located such that there is little or no ambient wind, its effectiveness may be limited. In this paper the addition of vertical “chimneys” to the horizontal ventilated ducts was presented and two duct-ventilated embankments with and without a chimney were examined with the similarity theory, and it is found that the chimneys result in more effective cooling effect under windless conditions than if they are not present. The effect is straightforward: when the ambient air temperature is lower than the internal temperature of the embankment, cold air flows into the embankment from the ventilated ducts and out from the chimneys as a result of warming of the air by the internal temperatures in the embankment. When the ambient air temperature is above the internal temperature of the embankment, no strong air flow occurs within the ventilated ducts and the chimneys. The overall result is a net cooling effect through several cycles of varying temperatures and greater than that achieved by horizontal ventilated ducts alone. Therefore, the special duct-ventilated embankment with a chimney should be considered as an effective measure to protect embankment in warm and low wind permafrost regions.

Monitoring study on the boundary thermal conditions of duct-ventilated embankment in permafrost regions

Abstract Duct-ventilated embankment is a roadbed construction method for cooling the underlying soils in permafrost regions. However, numerical simulation methods to predict the long-term permafrost-protecting effectiveness of the roadbed encounter difficulties in selecting boundary conditions, because no monitored data has been collected. In this paper, based on monitored data of field experimental embankments in the Beiluhe section of the Qinghai–Tibet Railway, air temperatures in the ducts and the walls were analyzed and simulated. The results indicated that the annual air temperatures in the ducts were higher than the environmental air temperature by a value of 1.6 to 1.8 °C. This value was less than 1.0 °C in thawing periods and 2.0 °C in freezing periods. On the embankment, the ground temperature at a depth of 0.5 m was higher on south-facing slopes than on north-facing slopes, with a value of 3.5 to 5.5 °C. The monitored data showed that the natural ground surface temperature was about 2.5 °C higher and the embankment surface temperature was 4.0 °C higher than the environmental air temperature, indicating that some values used in previous numerical analyses of surface temperature of the natural ground and embankment were reasonable. To better predict the long-time effect of duct-ventilated embankment on the underlying frozen soils, a physical model and the relative boundary conditions need to be incorporated into numerical analyses.

Investigation of embankment with temperature-controlled ventilation along the Qinghai–Tibet Railway

Abstract The temperature-controlled ventilated embankment (TCVE) is an improved engineering measure developed based on the duct-ventilated embankment (DVE). The improved cooling effect of the TCVE is a result of enhanced heat exchange through controlling air convection in the ventilated ducts of the DVE. The field observational data from a TCVE testing section at Beiluhe, about midway in the Qinghai–Tibet Railway permafrost zone, indicates that restricting the air convection in the ducts during warm season can reduce the embankment heat absorption in the embankment. The results also indicate that the temperature in the wells of the ducts of the TCVE was about 1.0 °C lower than the temperature in the walls of the ducts of DVE. Especially it was found that in the seasonal ground temperature-decreasing period, the temperature of the permafrost at a depth of 3.5 m below the base of the embankment in TCVE decreased faster than at the same depth below the DVE. Two years after the TCVE was implemented, the temperature at a depth of 3.5 m below the embankment base was found 0.45 °C lower than at the same location below the DVE. Thermal modeling of the embankments shows that the mean annual heat loss of a TCVE is about twice that with the DVE, which means a doubled cooling effect of TCVE over DVE if think in this way. In addition, the cooling period with a TCVE is about 40 days longer than with a DVE, which contributes to its cooling effects.

Field experiment study on effects of duct-ventilated railway embankment on protecting the underlying permafrost

Abstract Cooling principle is adopted in roadbed constructions in permafrost regions on the Qinghai–Tibet Plateau for protecting the underlying permafrost. One way is to use duct-ventilated embankment. Ground temperature data were collected along experimental embankments in Beiluhe section of the Qinghai–Tibet railway. Analyses of ground temperature changes and heat influx indicate that ventilation ducts effectively cool the soils surrounding the ducts in the embankment. Permafrost behavior beneath conventional embankment and test embankments with ducts installed at high and low positions was studied over 3 years. The permafrost under the conventional embankment and the duct at a high position in the embankment decreasingly absorbs heat during the period. The ground temperature of the permafrost under the conventional embankment rises and that under the duct at a high position embankment decreases slightly. The temperature of the permafrost under the duct at a low position decreased remarkably. These findings demonstrate that a ventilated embankment with a low position duct produces beneficial effects that actively cool the subgrade soils and keep the roadbed thermally stable. Accordingly, they were strongly recommended for use in the construction of embankments in the permafrost regions on the plateau.

A numerical model of the coupled heat transfer for duct-ventilated embankment under wind action in cold regions and its application

The duct-ventilated embankment, as a special embankment structure, has been used to protect the underlying permafrost from thawing in Qinghai-Tibetan railway construction of China. For cold regions engineering, the temperature is the most important factor that determines the stability of construction, and wind can directly affect the temperature distribution in the construction. To research the stability of the duct-ventilated embankment under wind action, based on the wind, temperature and geology conditions of Qinghai-Tibetan Plateau, a three-dimensional numerical model of the coupled heat transfer for duct-ventilated embankment, which was composed of the air convective heat transfer problem between inside and outside duct, the coupled heat transfer problem between airflow and duct wall, and the heat conduction problem with phase change in roadbed, was analyzed to determine the temperature and velocity characteristics in the ventilated duct, as well as the temperature fields of the duct-ventilated embankment. The numerical model can reasonably solve the coupled heat transfer problem for duct-ventilated embankment; furthermore, from the results, it was found that, on the assumption that the air temperature will be warmed up by 2.2 °C in the future 50 years and in the areas where the mean annual air temperature is − 4.0 °C, the duct-ventilated embankment structure can be an effective measure to decrease the ground temperature beneath it and ensure the stability of permafrost roadbed.

Ground-temperature controlling effects of ductventilated railway embankment in permafrost regions

Based on observed data from field-testing embankment of the Qinghai-Tibet Railway, ground-temperature controlling effect of duct-ventilated embankment is studied in this paper. The results show that ventilation ducts can effectively cool the soils surrounding the ducts of the embankment, and the heat budget of the ambient soils in a year shows as heat release. Temperature status of the permafrost below the embankment with ducts buried in the relatively high position is similar to that of the common embankment. The permafrost processes warming all along in the two freezing-thawing cycles when the embankment was constructed. However, the temperature of the frozen soils below the embankment, in which the ducts buried in the relatively low position, rises a little in the initial stage. After that, it cools down gradually. At the same time, ventilation ducts can effectively reduce the thermal disturbance caused by the filled soils. The frozen soils below the common embankment and that with high-posited ducts absorb heat all along in the initial two cycles. While the soils below the embankment with low-posited ducts begin to release heat in the second cycle. This phenomenon proves that the ventilation embankment with low-posited ducts shows efficient temperature-controlling effect. Such embankment can actively cool the subgrade soils and therefore keeps the roadbed thermally stable.