The Qinghai-Tibet Railway traverses a 550-km-long section of permafrost, from Xidatan, on northern slope of the Kunlun Mountain, to Amdo, at the southern foot of Tanggula Mountain.

Permafrost is ice-bound earth that remains in a permanent frozen state. When the temperature in the environment drops in winter, the earth freezes iron hard and expands. When the temperature rises again and melting occurs, normal flat land will become disfigured with small mounds or depressions. Such changes will transform and even distort a construction structure, and can act like a time bomb in railway transportation. Railways traversing areas of permafrost in the world have existed for over a hundred years. The rate of track damage in such places is very high and trains have only been able to travel at around 60-70 kph for safety reasons. For example, the reported rate of damage of the Trans-Siberian Railway was 45 percent in 1996, and the rate of damage on a forest railway through a section of permafrost in northeast China was 40 percent in 1990.

Generally, for every 100 meters increase in elevation, the average ground temperature will drop 0.8 to 0.9 degrees Centigrade, which can produce up to 20 m depth of permafrost. Latitude obviously makes a big difference. For every degree of southward latitude, the annual average ground temperature will rise 0.9-1.0 degrees Centigrade, and the frozen area will be correspondingly thinner. The Qinghai-Tibet Railway traverses a 550 km section defined as permafrost, although the real permafrost part is 400 km long, including 190 km considered "unstable" and "more unstable" and 100 km of "most unstable".


Half a Century of Research

The success or failure of the Qinghai-Tibet Railway lies in its track foundations, and the key to the success or failure of the track foundations lies in tackling the permafrost problem. Research on permafrost along the Qinghai-Tibet Railway has been distinguished by establishing world marks in terms of extensive content, huge material and human investment, as well as the time taken.

In August 1974, in accordance with the command of the Central Government and the requirements of quickening the work of reconnaissance and design, a leading group of experts from the Chinese Academy of Science, Ministry of Railway, the No.1 Machinery Ministry, the PLA Railway Corps, Qinghai Province and the Tibet Autonomous Region, with four branches of salt lake permafrost group, plateau electromechanical device group, communication signals group and construction group, was set up. At least 1,700 people, from 68 factories, the military, research centers, colleges and universities of 19 provinces, cities and autonomous regions, were posted to various stations for scientific research, and recorded great achievements.

In the 1960s, the First Survey & Design Institute of the Ministry of Railways (FSDI) together with the Glacier & Permafrost Research Institute of the Chinese Academy of Science and Northwest Research Institute of Academy of Science of the Ministry of Railways had already selected the area of Fenghuo Mountain as a representative site to carry out research into permafrost on the Qinghai-Tibet Plateau. With unremitting observation for nearly 40 years, the experts mastered the basic distribution features of permafrost along the proposed railway route.

At the end of 2000, so as to provide exact data of ground temperature distribution in the permafrost area for the concrete design and construction of the railway, the permafrost scientific research team, consisting of experienced geological engineers from the FSDI as well as the experts and scholars from Cold and Arid Environmental and Engineering Research Institute (originally, the Permafrost Research Institute), carried out elaborate geological and topographical surveys, using satellite remote sensing and ground investigation. This helped produce the "construction principle on permafrost and engineering measures", a special chapter in the "Suggested Outline for Construction of the Golmud-Lhasa Section of the Tibet Railway".

In 2001, the Chinese Academy of Science took pains in geological exploration on the permafrost area along the Qinghai-Tibet Railway, focusing on boring, and using ground penetrating radar, seismic reflection and electromagnetic methods as supplements, drilling a hole every 200 meters. For example, the 39 borers dispatched by the Lanzhou Branch of FSDI, drilled altogether 280 holes from Nahshankou to Nachitai. This exploration encompassed 170 km of geological drilling work, 60,000 groups of civil engineering tests, 800 ground temperature observation wells and a complex geophysical prospecting profile stretching 400 km. As a result, the permafrost distribution along the railway route was thoroughly mapped. In the meantime, 14 teams were organized to carry out cold and warm season on-the-spot investigations, which established the distribution rules and growth features of unfavorable geological phenomenon along the 550 km-long permafrost section, and the growth trends and potential damage of 192 unfavorable permafrost phenomenon related to the railway. All these efforts provided technical parameters and reliable data and the necessary measures for damage elimination.

The key of railway construction in a permafrost area is to prevent the earth under the track bed from melting so as to provide firm foundations. The traditional method was to increase the height of the embankment and lay insulation materials so as to cut off or reduce the external heat entering the foundations. However, practice proved that this method was incapable of fundamentally improving the foundation's thermo-physical state, because the insulation not only stops the heat of the warm season from entering the foundations but the cold of the cold season as well. Undertaking such a detailed series of geological drillings and exploration, the construction headquarters of the Qinghai-Tibet Railway established the design principle of "initiating temperature fall, cooling the foundations, protecting the permafrost".

New Measures for Protecting Permafrost

With the establishment of the design principle, a series of effective cooling and permafrost protection measures came into being.

Flagstone Air Cooling. The thickness of the flagstone cold road bed mat must be not less than 0.3 meter, the design thickness of flagstone layer not less than one meter, stone diameter in the range of 0.2-0.4 meter, intensity not less than 30megapascal, a macadam layer of not less than 0.3 meters laid on the flagstone layer, geo-textile laid on the macadam layer. This method was successfully applied in the 117 km-long high-temperature and unstable permafrost section, which played a role in reducing the ground temperature under the foundations and increasing the stratum's cold reserve.

Flagstone Slope Protection or Berm. With a view to solving the problem of the asymmetric transformation of the foundations on the permafrost section, a method of slope protection was taken by piling up and filling macadam up to 1.0-1.5 meters on one or both sides, which effectively cools the foundation. As a result, the imbalance of the ground temperature can be adjusted by changing the thickness of the berm on the tail and back of the foundations.

Ventilated Pipe. A horizontal ventilated pipe with a diameter of 0.3 meters, was buried no more than one meter long under the railway embankment, in which the convection of winter cold air reinforces the heat dissipation of the embankment filling, reduces the ground temperature of the permafrost under the embankment and enhance its stability. In order to eliminate the negative effects of air convection in summer, an auto-control air door at the mouth of the pipe opens when the outside temperature falls and closes with a temperature rise.

Hot Pin Technique. The hot pin system can realize heat conduction through convectional circulation by using the liquid-vapor phase transition of the media in the pipe and the differences in temperature between the condenser and evaporator. This technique has been applied in a 32 km section, which effectively realized the practice of a fall in the ground temperature under the embankment but a rise in the upper limit of the permafrost.

Awning. An awning over the sub-grade or side slope provides protection from solar radiation and keeps the external heat from the permafrost. This method proved effective by using a steel structure awning on a ridge-crossing section of Tanggula Mountain.

Bridge Concrete Pile Foundations. In order to avoid perturbation motion to the permafrost during bridge construction, an on-the-spot comparative trial was undertaken of three methods of bridge boring, namely cast-in-place pile, driven pile and stab-in pile. The boring driven pile was in vain because it was difficult to drive into the permafrost layer, and the boring stab-in pile was also a failure because it was impossible to control the backfill quality surrounding the boring stab-in pile.

However, the boring cast-in-place pile was distinguished by stronger bearing capacity and anti-frost-heaving capacity.

Besides, this method also had a fast speed, fine quality and less perturbation motion to the permafrost if drilled by rotary drilling rig. Therefore, the cast-in-place pile drilled by rotary drilling rig was adopted for the majority of bridges with non-rigid-rock foundations.

Open Cut Foundation Pit for Culvert in Cold Season. Through research and comparisons, a rectangular reinforced concrete structure was designated for culverts. This kind of culvert was built in the cold season with open cut foundation pit assembling or concrete foundation so as to lessen the heat perturbation motion to the permafrost and shorten the time of underlying permafrost refreezing and control the quality of construction.

Tunnel Structure. The freeze-thaw influence over tunnel structure was fully taken into account during the construction of tunnels in the permafrost section of the Kunlun Mountain and Fenghuo Mountain so as to control the environmental temperature of tunneling and lessen the scope of a surrounding rock freeze-thaw cycle. Installing an insulation layer to the tunnel lining cross section and a reinforced concrete tunnel lining structure proved capable of cutting down the heat exchange of the surrounding rock. The waterproofing and drainage system adapted to the unique features of tunnels in the cold area protected the tunnels from underground water damage. At present, after two years, the tunnels in the Kunlun Mountain and Fenghuo Mountain areas have proved to be safe. The tunnel of Fenghuo Mountain traverses permafrost, so the construction units developed a large-scale air-conditioner system, and, in this way, the tunneling temperature was controlled in the range of minus 5 degrees Centigrade to 5 degrees Centigrade, with the mould loading temperature for the tunnel lining concrete set at 5 degrees Centigrade so that the refreezing speed of the permafrost was promoted and the perturbation motion lessened. In the cutting section with permafrost of high ice content, the method of mechanical or blasting tunneling was performed step-by-step to shorten the time of exposure of the foundations to the air; the tunneled foundation pit was covered with sunshade material in the daytime and uncovered to allow refreezing at night. Through careful preparations to avoid any delays, the engineers cut down the time of each operation to lessen thermal perturbation motion.

Railway Bridges

In early morning of June 12, 2003, the task of tracking the extra-large Qingshuihe River Bridge at an elevation over 4,600 meters had been accomplished and the track laying machines marched to the southern edge of the "no-man's land" of Hoh Xil.

Qingshuihe River Bridge, stretching 11.7 km, was under the control of China Railway 12th Bureau whose task was to build it in a way that ensured the stability of the sub-grade on the permafrost section. There also had to be some 1,300 openings between the piers to ensure free passage of wild animals such as Tibetan antelopes. Thanks to the available technology and elaborate organization, the 12th Bureau only spent 205 days in boring and pouring 2,878 bridge piles, 69,744 meters of extended parts and 1,367 piers.

A great many rotary drilling rigs were used in the construction of bridge piles and the foundations, because they were able to lessen the perturbation motion to the permafrost and were thus environmentally friendly; they were also used for their advantages of free movement, exact orientation, high boring speed, fine quality, excellent pore-creating verticality free from slurry holes and small melting.

The bridge spanned a complicated permafrost section. Trains on the permafrost section of the Qinghai-Tibet Railway are expected to run at100 kph, requiring high track bed standards. After repeated tests, it was agreed to follow the design of bridging to traverse the thick ground ice section, the unfavorable permafrost phenomenon growth section and the permafrost section with high ice content. From various designs, one was chosen offering a reasonable span, and built on boring cast-in-place pile foundations and two-columned piers. The bridge, with a length of 120 km, proved effective through three years of freeze-thaw cycle trials, bringing praise from domestic and foreign permafrost experts.

China's Tibet
Jiang Shijie