青藏高原封闭道碴层对下部多年冻土保护作用的现场实验研究

In-situ Experiment on the Cooling Effect of Closed Ballast Layer on Permafrost on the Tibetan Plateau

为了研究封闭道碴层对其下部多年冻土是否具有积极的保护作用,在青藏铁路北麓河试验段附近建立了封闭碎石道碴坑和卵石地表对比试验场,并对下部地温进行监测.结果发现:经过两个冻融循环后,道碴坑底部(1.3 m深度处)年平均地温为-1.11℃,比卵石地表相同深度低0.73℃;道碴坑中部(0.7m深度处)年平均地温为-1.60℃,比卵石地表相同深度地温低1.4℃.封闭碎石道碴层可以提升冻土上限,降低多年冻土温度,对下部多年冻土起到很好的保护作用.封闭道碴层的这种降温效果是由于道碴层具有可变导热系数的特点,暖季道碴层上部温度高,下部温度低,不产生对流,等效导热系数小,传入道碴层以下土体的热量较少;相反寒季道碴层上部温度低,下部温度高,产生自然对流,等效导热系数增大,有利于道碴层以下土体释放热量.

In order to study the cooling effect of the closed ballast layer on permafrost on the Tibetan Plateau, a test field with a closed ballast pit and a gravel surface was built near the Beiluhe test site of the Qinghai-Tibet Railway, and the ground temperatures beneath the test field have been monitored. The test results indicate that： 1） After two years of freezing and thawing, the annual mean ground temperature at the bottom of the closed ballast pit （at the depth of 1.30 m） is --1.11℃, which is 0.73 ℃ lower than that at the same depth beneath the gravel surface without a closed ballast layer; the annual mean ground temperature at the middle of the closed ballast pit （at the depth of 0. 70m） is --1. 60 ℃, which is 1. 4 ℃ lower than that at the same depth beneath the gravel surface. 2） The closed ballast layer can raise permafrost table, decrease annual mean ground temperature below the ballast layer, and protect the permafrost effectively. 3） The main reason that a closed ballast layer can cool the underlying soil on the Tibetan Plateau is that the equivalent conductive coefficient of the ballast layer is variable; In summer, the temperature at the top of the ballast layer is much higher than that at the bottom, so the natural convection in the ballast layer can not take place, and the heat transfer is only in the form of conduction; in winter, however, the temperature at the top of the ballast layer is much lower than that at the bottom, so that the convective flow induced by the density gradient occurs in the closed ballast layer, which can significantly increase the rate of upward heat loss.