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The Qinghai–Tibet Railway is a 1,956 km long railway network located between Golmud and Lhasa in China. It is also known as the “heavenly road” as it is the longest and highest highland rail route in the world.
The construction of the railway line began on 29th June 2001, and it became operational on 2nd July 2006. The main challenge faced during the construction of the Qinghai–Tibet Railway network was the widespread permafrost and fragile environmental conditions.
A great number of innovative and technological advancements were observed during this project because of which it was awarded the most prestigious award of China, the National Science and Technology Progress Award, in 2008.
Approximately 960 km of the high-elevation railway network is located 4000 m above mean sea level, and its highest point is on the Tanggula Mountains at a height of 5072 m above mean sea level.
A total length of 632 km is located in the permafrost terrain region, out of which 275 km is categorized under the warm permafrost region (mean yearly ground temperature somewhere in the range of 0 and ?1°C) and 221 km is categorized under the ice-rich permafrost region (ice content > 20% by volume). Also, there is a section of 134 km that is categorized under both warm and ice-rich permafrost regions.
At the commencement of the Qinghai-Tibet Railway project, Chinese researchers and scientists were faced with two adverse variables: the warm condition of the permafrost plateau region and an increase in global temperatures.
After constructing the Qinghai-Tibet Highway, the road surface developed 60% talik pockets due to increased thawing, which occurred in the warm permafrost region. Roughly 85% of the damages to roadway banks were brought about by the settlement due to thawing in the ice-rich permafrost region.
These methods include adjusting and controlling the amount of solar radiation, heat convection, and heat conduction, as well as combinations of the above-mentioned measurements by using different embankment configurations and fill materials. Researchers had developed various measures to reduce the thawing effect and cool down the roadbed of the Qinghai-Tibet Railway.
1. Solution for Solar Radiation
Qinghai-Tibet Plateau is subjected to severe solar radiation due to the combination of high altitudes and low latitudes. Therefore, it becomes important to reduce the ground temperature for the effective working of the railway track. By shading the surface from sunlight, the ground temperature can be brought down adequately.
Scientists had researched the impact of awnings and estimated the ground surface temperature at 2 PM throughout the span of one year in the Fenghuoshan area. It was observed that the ground surface temperature was 8°C-15°C lower inside the shades than outside. Also, by shading the surface, the maximum temperature difference was 25°C.
In any case, awnings were not suitable in the plateau region because of the strong winds. Thus, on the embankment side slopes at Beiluhe, the shading boards were installed to study the effect of temperature differences. Results indicated that a temperature difference of 3.2°C was observed between the inside and outside of the shading board. Also, a temperature difference of 1.5°C was observed between the inside of the shading board and the natural ground surface.
The embankment of the railway track may also lose its strength after going through repeated freezing and thawing cycles. A shading board can decrease the impact of freezing and thawing cycles. It can likewise shield the embankment from erosion caused by winds and rains.
2. Solution for Convection Patterns
To reduce and control the effect of convection patterns within embankments, ventilation ducts, crushed rocks, and thermal tubes were installed on the Qinghai-Tibet Railway network.
2.1 Ventilation Ducts
Researchers conducted field experiments at Beiluhe on ventilation ducts. The materials used for the construction of the ventilation ducts were PVC and concrete. The diameter of the ducts was 35 cm and the spacing between the two ducts was two times the duct diameter. The ventilation ducts were constructed at a depth of 0.5-0.7 m from the ground level into the embankment.
On average, the annual temperature of the surrounding air on the Tibet Plateau is about 3°C colder than the annual temperature of the underlying ground surface. Therefore, constructing the ventilation ducts in the embankment can viably bring down the temperature of the underlying ground surface.
Researchers demonstrated that ventilation ducts do bring down ground temperatures. Ventilation ducts constructed beneath or close to the ground surface have a more noticeable cooling impact than those constructed at higher depths into the embankment. After three years of construction of the ventilation ducts, the permafrost table significantly moved up to the same level as the ground surface.
2.2 Crushed Rocks
Crushed rocks used on the Qinghai-Tibet Plateau acted as thermal semi-conductor to reduce the effect of permafrost. Generally, the air during the winters is colder than permafrost. This induces R-B convection (Rayleigh-Benard convection, it occurs when a lower layer is heated from below and the upper layer develops the pattern of convection cell) inside the crushed rocks. Thus, in winters, the temperature increases because the permafrost discharges heat into the surrounding air.
However, in the summer, the effect of crushed rocks on permafrost is reversed because the air is hotter than permafrost. No convective heat transfer takes place as the cold air sinks to the bottom due to a higher density than hot air. Therefore, heat transfer occurs only in the form of conduction. Due to the little contact area between the crushed rocks and the air, the crushed rocks act as an insulator rather than a semi-conductor. This effect reduces the heat gain in the permafrost layer from the surrounding air.
The difference between reduced heat gains in summer and increased heat releases in winter creates a net heat release throughout the year and, in this way, brings down the temperature of the underlying ground surface. Also, on the sloping surfaces, the chimney effect develops when the crushed rocks are placed at a higher slope, and it reduces the temperature of the underlying ground surface.
Several different forms and configurations were used during the construction of the Qinghai-Tibet Railway network using the crushed rocks in embankments. These forms and configurations were: crushed rock revetments embankments, crushed rock-based embankments, U-shaped crushed rock embankments, and crushed rock embankments.
2.3 Thermal Tubes
Thermal tubes were installed in more than 35 km of the Qinghai-Tibet Railway embankment. Different lengths (8, 10, and 12 m) of the thermal tubes were used based on the height of the embankment. The tubes were installed either vertically or at an angle into the side slopes of the embankment. Also, the tubes are installed in such a way that the lower end of the tube reaches at least 2-3 m below the permafrost table.
After conducting the experiment at Qingshuihe, researchers suggested that thermal tubes do bring down the ground temperature and make the permafrost table move upwards. In addition, numerical models were created to check the effectiveness of the thermal tubes. As per the results, the radius of influence and the spacing between the thermal tubes were suggested to be about 1.8 m and 3 m, respectively. The numerical model demonstrated that the cooling impact both in the embankment and at the foot of the side slope was the best when the inclination angle with respect to the side slope was between 25° and 30°.
3. Solution for Heat Conduction
At the point when water is frozen, its thermal conductivity increases multiple times, from 0.38 to 3.2 W/m·K. This characteristic of water can be utilized to produce a material with increased thermal conductivity in the frozen state than in the defrosted state. Such a kind of thermal semiconductor material will bring down the ground temperature by expanding the heat loss in winter and diminishing the heat in summer.
Researchers had conducted a laboratory experiment on the water-absorbing material. These materials were placed in two layers separated by the layers of air in a sealed container, and after that, the container was filled with water. The results of this experiment showed that the thermal conductivity of the material changed from 0.11 to 1.2 W/m·K when the material was frozen. This induced an increase of about 10 times in thermal conductivity. Thus, water-absorbing layers were used in the embankments to reduce the heat in summer and compensate for the heat loss in winters.
The Qinghai–Tibet Railway project is famous because it was constructed in widespread permafrost and fragile environmental conditions. A great number of innovative and technological advancements were witnessed during this project because of which it was awarded the most prestigious award of China, the National Science and Technology Progress Award, in 2008.
The length of the Qinghai–Tibet Railway track is 1956 km.
The project was completed with a total investment of ¥34 billion.
Qinghai–Tibet Railway is the highest railway track in the world. The highest point of the railway network is on the Tanggula Mountains, at 5072 m above mean sea level.
The construction of the Qinghai–Tibet Railway project started on 29th June, 2001.
To mitigate the problems of the permafrost region in the Qinghai–Tibet Railway project, the following solutions were proposed:
1. Solution for heat convection
2. Solution for heat conduction
3. Solution for solar radiation
The operating speed on the Qinghai–Tibet Railway track is 160 km/h.