青藏铁路高路堤下多年冻土热状态分析

ANALYSIS OF THERMAL STATE OF PERMAFROST UNDER HIGH EMBANKMENT ALONG QINGHAI—TIBET RAILWAY

对青藏铁路高路堤下多年冻土热状态监测结果进行分析研究。选取北麓河试验段3个不同高度(3.0,4.2,5.0m)的路基监测断面,对其多年冻土的地温特性进行研究。结果显示,稍高路堤的修筑有助于多年冻土上限的抬升,在经过2个冻融循环以后,试验路基下多年冻土上限抬升了0.3～0.7m,说明高路堤对多年冻土的保护起到积极作用。但是路堤的高度也不是越高越好,过高的路堤反而会造成工程效果下降,因此需要选择合理的路基高度。从分析结果也可以看出,高路堤的修筑也存在着潜在的不利方面。由于高路基的表面年平均气温要高于天然地表,路基表面的融化期较天然地表长而冻结期较天然地表短,同时路基表面的地温在融化期较天然地表高而在冻结期则与天然地表基本相当,造成路基表面融化指数大于天然地表而冻结指数要小于天然地表。高路基的另一个不利因素是路基填土在阻止暖季热量向下传导的同时也阻碍了寒季冷量向多年冻土的补给,导致多年冻土上限的抬升主要靠下部冻土的冷量消耗来维持。监测结果显示,高路基铺设后,上限以下多年冻土有明显的升温。这些将为路基的长期稳定性带来潜在的不利影响。通过对原天然地表下冻土温度变化过程的分析,得出路基对下伏冻土温度的影响范围在原天然地表下10.0m以上。

Thermal state of permafrost under high embankment along Qinghai—Tibet Railway is monitored and studied. Three monitoring profiles with different embankment heights(3.0,4.2 and 5.0 m) in the Beiluhe test site are chosen to analyze the characters of ground temperature under the embankment. The in-situ monitoring results show that the reasonable embankment height has positive effects on frozen soil protection and can promote the rise of permafrost table under the embankment. The permafrost tables under the three embankments have 0.30.7 m rise after two freezing-thawing circles. But the engineering effect does not increase linearly with the embankment height. The embankment with a height of 3.0 m has better engineering effects that those with heights of 4.2 m and 5.0 m according to the analysis. To ensure the thermal stability of the roadbed,the reasonable embankment height must be met. The analytical results also indicate that the high embankment has potential disadvantages. The thawing process of the embankment surface is longer while the freezing process is shorter than that of the natural surface. The embankment surface temperature,which is similar with that of natural surface in freezing process,is higher in thawing process. The consequence is that the thawing exponential of embankment surface is greater while the freezing exponential is smaller than that of the natural surface. At the same time,the conduction of cold energy in cold semi-year as well as heat energy in warm semi-year is held back because of temperature resistance of embankment material,which causes the ground temperature below the permafrost table to increase for sustaining the rise of the permafrost table. These disadvantages may affect the long-term stability of the permafrost under the embankment. Analysis of the observation data indicates that the engineering thermal disturbance depth of high embankment to the permafrost is about 10.0 m.