Cooling effect of convection-intensifying composite embankment with air doors on permafrost

Hong Sun ,XiuRun Ge ,FuJun Niu ,Ge Liu ,JinZhao Zhang

1.School of Naval Architecture,Ocean and Civil Engineering,Shanghai Jiao Tong University,Shanghai 200240,China

2.State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

3.CCCC First Highway Consultants Co.,Ltd.,Xi’an,Shaanxi 710075,China

1 Introduction

Engineering construction activities in permafrost regions disturb the original heat balance of the underlying frozen soil.Numerous thaw settlement problems were caused by roadway construction in the Qinghai-Tibetan Plateau (QTP).Therefore,it is imperative in protecting the underlying permafrost and to provide embankment stability in roadway engineering of permafrost regions.

At present,a series of cooling techniques have been adopted to protect the underlying permafrost in railway engineering,e.g.,duct-ventilated,crushed-rock,and thermosyphon embankments (Maet al.,2002;Liet al.,2006;Niuet al.,2006;Chenget al.,2009).However,the underlying permafrost in some highway engineering projects is still in a warming or thawing trend due to high temperatures of the asphalt pavement surface,even if cooling measures have been adopted in permafrost regions (Zhanget al.,2009).Therefore,the cooling measures in railway engineering cannot be directly applied to the highway,especially to wide expressways with an asphalt-paved surface.Thus,it is necessary to study new cooling measures to construct high-grade large-width highways in permafrost regions (Laiet al.,2009;Donget al.,2012).

Both duct-ventilated and crushed-rock embankments are good cooling measures.With perforated ducts,the cooling effects can be intensified because of increased contact surface between the air and the embankment soil (Huet al.,2004;Jiang and Ge,2006;Liuet al.,2008;Donget al.,2010).Crushed-rock embankment can cool the underlying frozen soil by airflow in porous media (Wuet al.,2005).The composite embankment with ventilation ducts and crushed-rock can intensify the thermal air convection among the ducts and crushed-rock,strengthening the thermal conduction between air and porous media.

The temperature fields of the composite embankment installed with air doors at two sides of the duct and without air doors were simulated in QTP,in which the air doors were closed in warmer days to prevent embankment heat absorption from thermal winds.It is expected that the composite embankment would be highly efficient in protecting permafrost stability.

2 Numerical model of the composite embankment

A perforated ventilation duct is embedded in the middle of a large porosity crushed-rock layer so as to intensify the air flow within this layer (Figure 1).

Figure 1 Embankment structure

Air flow within a composite embankment is a typical multiphysics coupling problem of temperature and air flow fields in porous media.Air flow is divided into two zones (Figure 1):'a' is ventilation duct;'b' is porous media,i.e.,crushed-rock layer zone and perforated walls of ventilation duct.The air flow problem within this configuration is solved by the simplified Navier-Stokes equations inside ventilation ducts and the Brinkman equations in crushed-rock and perforated walls of duct region,and the continuity boundary condition of the velocity and stress in interface between ducts and crushed-rock layers.

The model consists of mass and energy conservation equations,and heat transfer of internal ventilation duct and embankment fill complies with their respective governing equations:Inside the duct,air convection and heat exchange are considered;For the porous medium region,the heat transfer governing equation should consider the influence of air in which the volumetric heat capacity of permafrost and the coefficient of thermal conductivity of crushed-rock are expressed by comprehensive influence of solid and air.For the embankment fill region,the heat transfer governing equation is expressed by volumetric heat capacity of permafrost and the thermal conductivity of permafrost without the influence of air.The phase changes occur in the process of frost heave and thawing subsidence,so the thermal physical parameters through temperature of phase change interface are adopted for the heat conduction problem.The apparent heat capacity method is used in this paper.

3 The composite embankment model

The calculating geometric model of the composite embankment is presented in figure 2.The upper surface of the composite embankment is 10 m in width and 3 m in height,with a slope of 1:1.5.The computational grid is composed of 223,406 triangular elements and 112,114 nodes in total,in which regions of perforated ventilation duct and crushed-rock layer are in a refinement mesh.

Figure 2 The calculation area for the embankment temperature field

From top to bottom,there are fill layers of 1.8 m in thickness,crushed-rock layer of 1.2 m in thickness(void ratio is 0.4,coefficient of permeability is 3.48×10–6m2),silty clay of 6.6 m in thickness,and mudstone at the bottom.Soil physical parameters are obtained by Niuet al.(2013).

Initial temperatures were obtained from measurements of the natural foundation (Sunet al.,2011).Heat flux was 0.06 W/m2at the bottom boundary,while side boundaries of AH and FG were adiabatic.Embankment boundary temperatures were sinusodial to time,embankment construction was completed in August,and initial phase of temperature was π/2.Niuet al.(2013) provided thermal boundary conditions considering the effect of asphalt albedo and emissivity obtained by Wuet al.(1988)

In this study,perforated ventilation duct was 0.4 m in diameter,0.08 m in thickness,placed at 0.25 m above the ground surface,and holes were drilled in two sides of the duct wall.To eliminate the influence of summer heat,the ducts were closed by air doors from May to October,while opened at other times.Wind velocity of the porous media system in composite embankment was set to 0 m/s when the air doors were closed.Wind velocity at the inlet of the perforated ventilation duct was 4 m/s when the air doors were opened.Pressure of the outlet boundary was atmospheric pressure in the plateau.

4 Results and analyses

4.1 Wind velocity field of the composite embankment

The wind velocity field in the porous media of the composite embankment is closely related to the cooling effect.Wind velocity of the transverse section in the embankment centerline is presented in figure 3.

Figure 3 Wind velocity in the middle section of the composite embankment

The air flow velocity field in the composite embankment is divided into ventilation duct and porous media zones.The highest wind velocity was along the axis of the perforated ventilation duct at 4.069 m/s.The velocities decreased quickly from the duct wall,and wind velocity in the crushed-rock zone decreased to 0.051 m/s.Due to the perforated ventilation duct,wind velocity in the crushed-rock zone increased more than that of the crushed-rock zone without a duct in which the air velocity was 10-3–10-4order of magnitude.Jianget al.(2004) reported that air velocity in a crushed-rock layer of a railway embankment was 0.000277–0.00553 m/s obtained by a porous model.Therefore,the perforated duct wall allowed more air flow into the crushed-rock zone,and the natural convection in the crushed-rock zone was intensified.

4.2 Temperature field of the composite embankment with and without air doors

Figure 4 presents the relationship between temperature and time at depths of-1 m and-2 m under the composite embankment centerline with and without air doors.The embankment foundation temperature was sinusodial to time,and continuously decreased.The highest temperature occured basicly from August to October.The temperature obviously decreased at the first years,and fell to below 0 °C at 12 months after construction.The composite embankment temperature with air doors decreased continuously with an increase of time after 24 months.However,the composite embankment temperature without air doors basicly decreased,although slightly increased due to heat absorption from summer winds.In a word,the composite embankment temperature with air doors continously decreased,more than that without air doors.Thus,the cooling effect is more distinctive for the composite embankment with air doors.

Figure 4 Temperature vs.time at different depths under the composite embankment centerline with and without air doors

The embankment is most dangerous with highest temperature at the warmest time of the year.Figures 5 and 6 present composite embankment temperature fields with and without air doors in August.This shows that the temperature field is basicly symmetric and obviously changes with time.The permafrost temperature under the embankment with air doors was lower than that without air doors at the same depth.Moreover,a low-temperature frozen-soil core existed under the embankment and enlarged with time.In the fourth year,the scope of frozen-soil core below-2 °C with air doors was larger than that without air doors,and its depth was nearer to the ground surface.

Figure 5 Composite embankment temperature fields with air doors at the highest temperature stage at different times after construction (unit:°C)

Figure 6 Composite embankment temperature fields without air doors at the highest temperature stage at different times after construction (unit:°C)

Figures 7 and 8 present the relationships between temperature and depth under the composite embankment centerline with and without air doors in the first and fourth years after construction.The composite embankment worked during the first year,the temperature under the embankment with air doors decreased to below 0 °C except in August and October.However,temperatures were higher than 0 °C in June,August and October for the embankment without air doors.In the fourth year,the former temperature decreased to below 0 °C in all months,while the latter temperature was higher than 0 °C in June and August.Therefore,the composite embankment with air doors works better than that without air doors.

4.3 0 °C isotherm depth variations due to composite embankment with and without air doors

Table 1 presents 0 °C isotherm depths under the composite embankment for six years after construction.This shows that the 0 °C isotherm depth in the left embankment is similar to the right embankment with a difference of 1–2 cm,which indicates that the temperature field is symmetric again.The 0 °C isotherm depth obviously rises,especially under the embankment centerline and shoulders.In the first year after construction,the 0 °C isotherm rose slightly;in the second year,the 0 °C isotherm obviously rose above the ground surface;in the third–fifth year,the 0 °C isotherm was located near the low wall of the perforated ventilation duct.The 0 °C isotherm depth under the embankment shoulder and centerline with airs doors rose with similar degrees.This was obviously higher than that without air doors,which was up to about 0.40 m in the fifth year.Compared with that of the first year,the 0 °C isotherm position under the embankment centerline and shoulders with air doors rose over 2.6 m in the fifth year.

Figure 7 Temperature vs.depth under the composite embankment centerline with air doors at different months in the first and fourth years after construction (unit:°C)

Figure 8 Temperature vs.depth under the composite embankment centerline without air doors at different months in the first and fourth years after construction (unit:°C)

Table 1 The 0 °C isotherm depths under the composite embankment for six years (unit:m)

Therefore,the composite embankment is advantageous for release of heat energy and protecting the underlying permafrost,and the composite embankment with air doors can be a more effective measure to ensure permafrost stability.In the future,it should be applied to practical engineering and studied much further.

5 Concluding remarks

For construction requirements in permafrost regions,the temperature field and cooling effect of the convection-intensified composite embankment with perforated ventilation duct and crushed-rock were studied by numerical simulation.The main research results are as follows:

1) In permafrost regions,a composite embankment with perforated ventilation ducts and crushed-rock are required,in which the perforated ventilation ducts are embedded in the middle of the crushed-rock layer,and the holes are drilled in two sides of the duct wall.

2) Due to the perforated ventilation duct,wind velocity in the crushed-rock zone increases more than that outside the crushed-rock zone,and the natural convection is intensified within the crushed-rock zone.

3) The composite embankment is highly efficient in cooling and protecting the underlying permafrost in permafrost regions.The temperature of the underlying permafrost obviously reduces,and the 0 °C isotherm position rises significantly,especially under the embankment centerline and shoulders.

4) The composite embankment with air doors is more effective than that without air doors due to heat prevention by closed doors in warmer seasons.

5) The convection-intensifying composite embankment is a potential cooling measure for high-grade large-width highway construction in permafrost regions,which should be studied much further in practical engineering.

The authors greatly appreciate the financial support of the National Natural Science Foundation of China(No.41121061),the National Basic Research Program (973) of China (Nos.2012CB026101 and 2011CB013505),the Western Project Program of the Chinese Academy of Sciences (No.KZCX2-XB3-19),and the Open Fund of State Key Laboratory of Frozen Soil Engineering (No.SKLFSE201209).The authors are very thankful to two reviewers for proposing good suggestions.

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