High-speed railways on deep seasonally frozen ground in China
State Key Laboratory of Frozen Soils Engineering (SKLFSE), Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences (CAREERI), Chinese Academy of Sciences (CAS), Lanzhou 730000, China
China has built numerous high-speed railways(HSR) in the past decade (Figure 1). High-speed is defined here as a minimum travel speed of 250km/h. According to a report from the China Railway Co., Ltd., the total length of operating HSR will reach 18,000 km by the end of 2015. Among these HSR lines, the Harbin-Dalian High-Speed Railway, or Hada Railway (HDR), connects Harbin, the capital of Heilongjiang Province and Dalian, the southernmost seaport in Liaoning Province in northeast China. Construction work began on August 23, 2007 and the first commercial services began operating on December 1, 2012.The HDR is the first HSR in China to traverse deep seasonally frozen (>1.5 m) ground (Figure 2).
Figure 1. Railway networks and zones of frozen ground in China. Note: yellow lines are the recently built high-speed railways in regions affected by deep seasonally frozen ground. Red lines are conventional railways and HSR in regions affected by shallow seasonally frozen ground, or short-lived frozen ground, except for the railway crossing the Tibet Plateau which runs across permafrost.
Figure 2. A view of the HDR in winter (Photo provided by Shenku Liu)
The HDR winds across 909 km terrain affected by seasonally frozen ground, and about 25% of the line (231 km) is built on an embankment. The design speed is 350 km/hand the current operating speed is 300 km/h in all seasons. The line crosses terrain with a maximum depth of frost penetration ranging from 2.4 m in the north to 0.9 m in the south. The main challenge in the construction of the railway was how to mitigate frost heave of the railbed so that it stays within the acceptable vertical limit of±15 mm within a longitudinal distance of 200 m. This had to be achieved in spite of frequent and heavy dynamic loading due to high-speed trains, the huge range of ambient air temperatures (-40°C to+40°C; mean annual air temperature range from 3.5°C to 10°C), extensive frost susceptible soils and shallow ground water tables (1-3 m in many flat areas).
A typical embankment comprises three layers: an upper layer (0-0.4m from the roadbed surface) of well-graded gravels with cement; a middle layer (0.4-1.4m) of A and B group fills with cement, and; a lower layer (1.4-2.7m) of A and B group fills or mixed crushed gravels and sand. According to the China Railway Subgrade Construction Standard, A group is the highest quality fill, including block stone, well-graded soil, gravel with a fines content less than 15%; B group is high quality fill, including gravelly soils, gravels, coarse sand, and a fines content of15% to 30%). Underlying frost-susceptible clayey loess is treated by 20-meter long Cement Fly-ash Gravel piles placed at intervals of 1.5 m. The surface of the embankment is covered by China Railway Travel Service I track plate for supporting the tracks.
During construction, field tests were carried out by the research group from the State Key Laboratory of Frozen Soils Engineering (SKLFSE),to evaluate the effectiveness of mitigative measures for frost heave of foundation soils. These measures included refilling of the railbed with non-frost-susceptible soils, application of thermal insulation as a construction material, surface waterproofing and drainage control. Since the HDR was put to service at the end of 2012, the deformation of foundation soils along the whole line has been measured manually once a year. In addition, automatic monitoring devices were installed at 50 sections with complex geological conditions and prone to significant frost heave of the railbed (Figure 3). Three years of monitoring shows that deformation resulting from frost heave of foundation soils is less than 12mm at 97% of the 50 measured sections, i.e. less than the specified limit of 15mm.
Figure 3. A ground temperature and deformation monitoring site along the HDR. The yellow tube with temperature probes inside is for ground temperature monitoring; the concrete plates with white tubes are for deformation monitoring. The data are collected by datalogger and accessed remotely in Lanzhou.
During the first two years of operation (2013 to 2014), the HDR had a running speed of 200 km/h in winter and up to 300 km/h in summer. The running speed increased to 300 km/h year-round in 2015.
In the official design codes of China relating to frost heaving of foundation soils, they are considered to be free of frost action when the frost heave coefficient is less than 1%. This works well with traditional geotechnical engineering projects. However, for certain high-impact infrastructure such as airport runways and high-speed railbeds, this standard is not acceptable. For example, given that the HDR foundation soils are generally 2.7 m in thickness, deformation permitted by the current design code would exceed the allowable heave of the railbed. Consequently, the research team at SKLFSE, led by Professor Fujun Niu, proposed that the silt and clay contents in the A and B group fills must be less 5% in order to ensure the railbed would not experience excessive heave-induced deformation. This was based on intensive studies along the HDR embankment on processes and features of micro-frost-heaving (<1%), a new category of frost heaving proposed by Professor Niu, laboratory experiments and prototype testing, as well as from taking into account the thickness of railbed fills and maximum depths of frost penetration. The research results on the silt and clay contents suitable for high-speed railbeds will help modify future codes for designing high-speed railway and road foundations in cold regions in China and beyond.
Following the success of the HDR, another high-speed railway was built on seasonally frozen ground and began operating on August 17, 2015, running from Harbin west to Qigi‘har (HQR) on the Songhua-Nen Rivers Plain (Figure 1). This 282-km-long railway traverses still colder terrain than the HDR, and the design speed is currently 250 km/h. In Northwest China, a new high-speed railway connecting Lanzhou (the Capital of Gansu Province) and Urumqi (the Capital of Xinjiang Uygur Autonomous Region), with a length of 1,776km and a design speed of 250 km/h (155 mph), was opened for service on December 26, 2014. Currently it is being monitored by the SKLFSE.
China is building and is planning to build many more high-speed railways. Given that more than 70% of the area of the Chinese land territory is influenced by ground freezing and thawing (Figure 1), it is anticipated that more extensive and in-depth research will be conducted and more practical engineering experience will be obtained regarding high-speed railways in the near future. Such works will be beneficial to the construction of Moscow-Kazan High-speed Railway, which will be jointly designed by China and Russia, and the SKLFSE will be involved in that undertaking.
Report prepared by Huijun Jin and Fujun Niu, CAREERI, Lanzhou, CAS ( email@example.com)