Abstract
The construction of roadways in permafrost regions modifies ground-surface conditions and consequently, negatively varies thermal stability of the underlying frozen soils. To avoid the thawing of the permafrost layer under the scenario of global warming, roadways are usually laid on a built-up embankment, which not only disperses the traffic loads to underlying layers but also minimize the thermal disturbance. In the embankment, duct ventilation, or called air duct, can be embedded to further cool the underlying permafrost. While the thermal performance of duct ventilations has been well documented, the long-term structural stability of duct ventilation remains unknown. This study examines the structural stress of ventilation ducts that are placed in harsh weather such as the Qinghai-Tibet Plateau. The ducts are currently buried in the embankment filler, with the wind-outlet and -inlet ends exposed and cantilevered out of the embankment. Field studies found that the exposed parts have plagued cracking and even failures, especially at the fixed end of the cantilevered part. Damages of these concrete ducts are attributed to cyclic freezing-thawing attack, thermally-induced stresses, moisture-induced stresses, and concrete swelling. These physical attacks are caused by the harsh weather in the Qinghai-Tibet plateau. It is recommended to insulate the exposed part of the ducts and to fabricate durable and dense concrete ducts.
Highlights
Air ducts have been applied to cool foundations built in cold regions since 1970s [1,2,3,4,5,6]
Air inflow into the duct leads to the heat intake in the duct, in the Qinghai-Tibet plateau the duct tends to introduce a negative heat budget to the ground due to the windy weather in winter but the clam weather in summer
To diminish heat intake, temperaturecontrolled shutters may be installed in wind inlet and outlet to prevent air inflow during summertime [8, 9]
Summary
Air ducts have been applied to cool foundations built in cold regions since 1970s [1,2,3,4,5,6]. Ventilation duct has been applied to cool roadbeds built in cold regions, such as the Qinghai-Tibet Railway (Figure 1) [1,2,3]. Duct-ventilated embankments have been widely reported as powerful foundation to cool the permafrost in the sub-grade [11,12,13]. In the last decade, there are extensive studies on the techniques of keeping roadbeds in permafrost regions by reducing the solar absorption [17,18,19], enhancing the air convection [20, 21], and their combinations [22, 23], the long-term structural stability of air ventilation ducts remains unknown and has not been reported yet. It is recommended to cast durable concrete ducts against potential damages and to prevent cantilevered parts of the ducts being exposed to harsh environment
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