Freezing and thawing characteristics of seasonal frozen soil along Moscow-Kazan High-Speed Railway

According to the geomorphologic unit, micro-geomorphology, stratigraphic lithology and hydrogeological conditions and other factors, 14 monitoring sites were established in seasonal frozen soil areas along the Moscow-Kazan High-Speed Railway, which is 770 km long. FromOctober 1, 2016 to April 26, 2017, the lithology, density and moisture content of seasonal frozen soil, groundwater level, earth temperature, air temperature near the ground surface, snow thickness, and snow density were continuously measured once every 10 days. Based on the measuring data, the freezing and thawing characteristics of seasonal frozen soil along the railway were studied. Research result shows that in the seasonal frozen soil region along the MoscowKazan High-Speed Railway, snow cover lasts from late October to following April. Snow cover thickness varies from 20.2 to 38.2 cm and the average value is 27.3 cm. The maximum snow cover thickness varies from 25 to 60 cm, occurs in early to middle February, and the average value is 44.4 cm. The soil starts to freeze from middle November to late November, and thaws completely from early March to following middle April. The unfrozen time lasts from 100 to 165 days, and the average time is 122 days. The freezing rate of the soil changes from 0.27 to 1.20 cm·d^-1, and the average value is 0.50 cm·d^-1. The thawing rate changes from 0.27 to 2.52 cm·d^-1, and the average value is 1.14 cm·d^-1. In the freezing period of the soil, snow cover reduces the freezing rate of the soil. In thawing period of the soil, snow cover delays the thawing beginning time of the soil from the top to the bottom, also reduces the thawing thickness of the soil from the top to the bottom, and changes the double-direction thawing between the top and the bottom into the single-direction thawing from the bottom to the top. Along the Moscow-Kazan HighSpeed Railway, the maximum average freezing depth of the soil is 0.45 m within the range of 0.19-0.90 munder natural conditions, which generally occurs in early or middle February. Snow cover can reduce the seasonal freezing depth of the soil, and the reduction is 22.2%-32.6% when the thickness of snow cover is 26.1-28.6 cm. Snow cover has both cooling and insulating effect on the soil in the periods of early snow formation and late snow melting, but the cooling effect is major. However, in the stable period of snow cover, the insulating effect is major. Snow cover has a substantial insulating effect on the soil temperature, and the influencing depth and extent depend on the moisture content of the soil. The higher the moisture content is, the less the influencing depth and extent are, and vice versa.