Application study of the awning measure to obstruct solar radiation in permafrost regions on the Qinghai-Tibet Plateau

WenJie Feng , ZhiZhong Sun, Zhi Wen, GuoYu Li, Ze Zhang, WenBing Yu

State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental Engineering Research Institute, Chinese

Academy of Sciences, Lanzhou, Gansu 730000, China

Application study of the awning measure to obstruct solar radiation in permafrost regions on the Qinghai-Tibet Plateau

WenJie Feng*, ZhiZhong Sun, Zhi Wen, GuoYu Li, Ze Zhang, WenBing Yu

State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental Engineering Research Institute, Chinese

Academy of Sciences, Lanzhou, Gansu 730000, China

With globe warming, road safety will change dramatically, especially within the Qinghai-Tibet Plateau permafrost regions. Because of higher elevation and better atmospheric transparency, the Qinghai-Tibet Plateau has stronger radiation than other regions,which can change the daily variation of ground surface temperature on the Plateau. The awning measure (shading board) is one of the actively protected permafrost measures, which was adopted along the Qinghai-Tibet railway and highway and the Qing-Kang Highway in China. Field test results show that embankment surface month mean net radiation is 60-130 W/m2, but the value is below 20 W/m2under the shading board, and the reducing level of natural net radiation is 80%-90%. The shading board reduced the heat flow entering into the embankment by 80%-90% or more, with heat entering into the soil on the common embankment,but emitting from the embankment under the shading board. At the same time, ground surface temperature under the shading board is 6-8 °C lower than the exposed embankment. Test results show that the shading board measure can rapidly and effectively reduce net radiation and heat flow into the embankment, decrease embankment surface and interior temperature, effectively delay increase rate of soil temperature under globe warming, ensure stability and safety of the embankment, and guarantee unblocked road projects in cold and permafrost regions.

permafrost; sunshading (awning) measure; embankment; net radiation; heat flow

1. Introduction

The permafrost area on the Qinghai-Tibet Plateau (QTP)is estimated to be about 1.5×106km2, accounting for 70% of the total permafrost area in China (Jinet al., 2000a, b; Zhouet al., 2000). This is the third largest permafrost region worldwide and also the one with the highest altitude (Zhouet al., 2000). During the recent ten years, with the influence of global warming, climate of the QTP has obviously changed, and this fact has also affected the distribution and extent of permafrost. With this background in mind, engineering stability is more significant in cold regions, and the methods and measures to protect permafrost become more important. The QTP is regarded as the "starter" and "amplifier" of global air temperature change and its warming range exceeds the global mean value (Cheng, 2003). Thus, air temperature will rise faster and get to a higher value than in other regions of the world. At the same time, because of high altitude and better atmospheric transmittance on the QTP, all the above reasons produce a higher intensity of sun radiation on the QTP than other places in China, causing stronger daily variations of ground surface temperature (Gonget al.,1997; Chou, 2008). Because the Qinghai-Tibet railway and highway embankment trends are mainly along a northeast-southwest direction, sun radiation is not symmetrical on the two side slopes. Thus, there is a distinct shady-sunny slope effect which may cause stress in the roadbed leading to instability and destruction. Almost 70%-80% of the aforementioned problems occur at those parts of the Qinghai-Tibet railway and highway where there is an obvious shady-sunny slope direction. 85% of the destruction initiates as longitudinal cracks at the sunny side slope, which eventually extend up to the road surface, leading to large scale damage of the road (Chenet al., 2006; Chou, 2008).

The crack appearance probability increases on the south side slope. The main reason is asymmetric embankment soil temperature field, where soil temperature on the sunny side slope is higher than that on the shady side slope. This causes a stronger thawing of permafrost under the roadbed on the sunny side slope thus causing stretch cracks to appear on the sunny side slope embankment shoulder. Along the sunny side slope shoulder and below this side slope, soil temperature is higher than on the shady side slope, which sustains a longer thawing time. With other conditions being the same,the observed results show that side slope direction has more effect on embankment temperature in annual mean air temperature lower regions than in higher regions (Zhanget al.,2003). Huet al. (2002) has investigated the influence of solar radiation on the embankment surface thermal regime of the Qinghai-Tibet Railway. It is noted that when embankment direction is close to a north-south direction, the embankment side slope absorbs more solar radiation over a day, while absorbing less solar radiation when the direction is east-west; when the railway direction is 135°, the radiation difference is the maximum between the two embankment side slopes, namely the shady-sunny side slope effect reaches a maximum (Huet al., 2002, 2006). Kondratjev(1996) put forward a new method to strengthen roadbeds in ice-rich permafrost regions, and pointed out the advantage of adopting the sunshading (awning) measure to protect side slopes in permafrost regions. Because the awning prevents solar radiation from reaching the soil surface, and decreases the embankment side slope surface temperature, it will reduce the difference between the embankments of both side slopes when the awning measure is adopted on the sunny side slope. This creates a more uniform embankment temperature field, ensuring stability of the embankment and increased traffic security. The awning measure has been adopted on the Qinghai-Tibet Railway, the Qinghai-Tibet Highway and the Qing-Kang Highway, which produced an increased achievement in awning application (Fenget al.,2006, 2009a; Kömle and Feng, 2009).

The awning measure on the embankment side slope is an effective engineering measure to protect permafrost in cold regions, but there are numerous gaps in mechanism research.Although we have performed some studies on awning structure mechanics (Shiet al., 2007) and wind speed below the awning (Fenget al., 2009b, 2011), we do not know the capability of awning elimination of solar radiation and are unaware of how high solar net radiation and heat flow into embankment soil can go. Thus, we would like to clarify these questions by a field test, and discuss the awning measure advantage in terms of quantitative values.

2. Field test design

The field test lies in the southern part of the Beiluhe Basin between the Kekexili and Fenghuoshan basins on the QTP which belongs to the Beiluhe alluvial-proluvial high plain landform. The terrain is open and slightly undulating,and the surface is covered by vegetation to an extent of 10%-50%. The test section is located 500 m west of the Qinghai-Tibet Highway (QTH) which lies in the QTP dry regions where is cold and arid, and the four seasons are not clearly separated there. The air is rarefied, the air pressure is low, the frozen period is 7-8 months long, and evaporation is higher than rainfall. Based on data from the Beiluhe automatic weather station, annual mean air temperature is -3.8°C at the Beiluhe region.

Based on the test section permafrost type and distribution characteristic, the permafrost table judge method is used to analyze the drilling core, thick-layer subsurface ice and frozen soil structure. According to the aforementioned method, the test section natural permafrost table is 1.6-2.4 m and annual mean ground temperature is from -1.41 °C to-1.68 °C at the 15-m depth, which belong to the lower temperature basic stable permafrost regions.

The embankment trend is 230°, nearly a northeast-southwest direction, and the embankment side slope gradient is 31.5°. The test section embankment side slope shading board (awning) measure adopts two high quality block board combination. We applied three layers of green paint on the board surface to prevent water uptake and production of moisture. The block board was fixed to a steel tube frame, and the board is parallel with the side slope from the embankment surface 40-cm away. The awning measure was finished at the end of July 2010, after which we began the monitor system and data retrieval.

The position of the different sensors on the embankment side slope is illustrated in Figure 1. Two net radiometers are installed side by side: one is below the awning board and the second is at the same height above the common embankment. They measure both long-wave and short-wave radiation in both directions, from which net radiation entering the soil can be derived.

In addition, air temperature near the soil surface, heat flow entering the soil, and superficial soil temperature are measured. The temperature probes interval is 5 cm under the side slope surface, and the air temperature probes lie in 20-cm and 40-cm distance to the surface. The net radiometer sensor is 20-cm high above the side slope ground surface, and heat flow plates lie at 5 cm and 10 cm under the soil surface, as shown in Figure 1. All data were obtained by the DT500 data-taker, and were automatically taken once per 20 min.

Both the awning measure test section and the common embankment test section are composed of general sand-gravel soil. The temperature probe is a thermistor manufactured by the State Key Laboratory of Frozen Soil Engineering, CAS, China. The thermistor precision is ±0.05°C, and when data is automatically taken, the temperature precision is ±0.01 °C by calculation. The net radiometer is a CNR4 four components radiometer manufactured by Kipp& Zonen Company, Netherlands, its short wave detector sensitivity is 7-20 (µV·m2)/W, the long wave sensitivity is 5-10 (µV·m2)/W, its operating temperature range is from-40 to 80 °C, and the temperature sensitivity is less than 5%.The heat flow monitor contains the HFP01 thermal sensors manufactured by Hukseflux Company, Netherlands, its sensitivity is ±0.02 W/m2and the operating temperature range is from -30 to 70 °C, with temperature dependence less than 0.1%/degree. The heat flow monitor range is ±2,000 W/m2.In this paper, the analysis data time range is from July 27,2010 to April 4, 2011, including altogether eight months of complete data.

Figure 1 Embankment side slope monitoring schemes

3. Data analysis

3.1. Air temperature analysis

Because air temperature is monitored 20-cm above the ground, air ventilation is unobstructed, despite the awning shadow effect. We found that air temperature difference is not large between the awning’s inner and outer sides, and there is only a slight difference at the 40-cm awning height.Figure 2 shows the air temperature curves.

Figure 2 Air temperature curves at 40 cm above the embankment side slope surface

From Figure 2 we find that the awning inner air temperature is lower than the outer and that in the region without an awning, the difference is more evident in the higher temperature season. In July and August it can be as large as 2-4 °C. With the approach of winter, air temperature decreases day by day, and the temperature difference among the three regions reduces gradually, reaching about 1.0-1.5°C in November. This shows that the awning measure significantly reduces air temperature during the summer, but the effect is distinctly weaker in winter.

3.2. Embankment temperature analysis

Ground temperature difference can directly reflect the cooling effect of the shading measure. In particular, surface temperature difference is the best evidence for awning affect. Figure 3 shows a comparison of ground surface temperature curves under the awning and outside the shadowed region.

During the high temperature season, because the sun’s radiation is relatively intense, and the awning blocks direct solar radiation from reaching the ground surface, a remarkable temperature difference can be observed. From Figure 3 we can see that the awning inner ground surface temperature is lowered by 6-8 °C compared to the natural ground temperature in the high temperature season. With the change of seasons, when entering autumn and winter seasons, the sun’s radiation declines gradually and the awning effect becomes less important. The ground surface temperature difference between the shadowed and open regions decreases gradually and both regions reach almost the same temperature during the winter season. As shown in Figure 3, in November the temperature curves begin to overlap gradually, which shows that the embankment side slope shading board (awning)measure is most effective during the high summer season.When spring arrives, the air temperature diversity begins to change day by day, just like in February and March, 2011 as shown in Figure 3.

Figure 3 The ground surface temperature curves

3.3. Heat flux and radiation influence analysis

The main reason for the observed ground surface temperature difference between the shadowed and open region is the awning blocks solar radiation from reaching the ground surface. This leads to different heat flows entering into the embankment, creating different soil temperatures.Figures 4 and 5 show the month mean net heat flux under the side slope surface 5 cm and 10 cm on the shadowed section and open embankment, respectively. Figure 6 shows the entire embankment side slope ground surface month mean net radiation (positive value means heat entering the embankment soil, negative value means heat diffusing from the embankment soil towards the surface).

Figure 4 The month mean net heat flux under the side slope surface 5 cm

From Figures 4 and 5, we can obviously find that under the protection of the side slope awning measures, the month mean net heat flux values in a depth of 5 cm and 10 cm below the surface are by far smaller under the awning embankment than under the natural embankment. On the common embankment, the net heat flux mainly enters into the embankment, at the two different depths the month mean net heat flux is higher than the awning section, respectively. As for the awning measure embankment, the net heat flux is much smaller than the natural one. The shading board reduces the heat flow entering into the embankment by 80%-90% or more. The results show that the awning measure can effectively reduce embankment surface and soil temperature and ensure the stability of the embankment.

From Figure 6, we can find that the embankment soil month mean net radiation at the natural embankment is more than the awning section. At the natural section, the month mean net radiation is from 60 to 130 W/m2, but at the awning section, because of the awning shelter, a good portion of solar radiation cannot reach the embankment surface and enter into the soil. Thus, the month mean net radiation is below 20 W/m2, the embankment emits heat flow in November and December 2010 and January 2011 (as shown in Figure 6), and the reducing level of natural net radiation reached 80%-90%.

Figure 5 The month mean met heat flux under the side slope surface 10 cm

Figure 6 The month mean net radiation of the side slope surface

4. Conclusions

The observation results show that the shading board(awning) measure is very effective for blocking solar radiation energy from the embankment side slope surface. It can significantly reduce the month mean net radiation entering into the embankment soil. The month mean heat flux enters into the natural section of the embankment soil, but is emitted from the embankment soil at the awning section embankment, and the month mean net radiation at the natural embankment is more than at the awning section embankment. At the same time, ground surface temperature under the shading board is lowered by 6-8 °C compared to the natural embankment.

Field test results show that the common embankment surface month mean net radiation is 60-130 W/m2, but the value is below 20 W/m2under the shading board, and the reducing level of natural net radiation is 80%-90%. The shading board reduces heat flow entering into the embankment by 80%-90% or more. Heat enters into the soil on the common embankment, but is emitted from the embankment under the shading board. These test results show that the awning measure can rapidly and effectively reduce net radiation and heat flow into the embankment, decrease embankment surface and interior temperature, effectively delay the rise in soil temperature under globe warming, ensure stability and safety of the embankment, and guarantee unblocked road projects in cold and permafrost regions.

The currently available data record is only restricted to a period of eight months. A more detailed analysis of the awning measure effect can only be given after a longer series of data (at least over one freezing-thawing cycle, best over several years) have been collected.

This project was supported by the Funds of the State Key Laboratory of Frozen Soils Engineering, CAS (Grant No.SKLFSE-ZY-03), the National Key Natural Science Foundation of China (Grant No. 50534040), the National Natural Science Foundation of China (Grant Nos. 40821001,40801022, 40801024, 40801026, 50976120, 41001041),and the Western Project Program of the Chinese Academy of Sciences (Grant No. KZCX2-XB2-10).

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10.3724/SP.J.1226.2012.00121

*Correspondence to: Dr. WenJie Feng, Associate Professor of Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. No. 326, West Donggang Road, Lanzhou, Gansu 730000, China. Tel:+86-931-4967460; Email: wenjief@lzb.ac.cn

August 12, 2011 Accepted: November 23, 2011