青藏铁路工程防沙积沙后冻土地温演变规律及热交换机理研究

The sand-prevention engineering of Qinghai-Tibet Railway would result in sand deposition inevitably, how does the thermal regime of underlying permafrost change after the ground surface were covered with sandy sediments? No significant conclusions have been drawn currently. And the heat transfer mechanism that sand layer effect on thermal regime of permafrost is currently unclear. This project plans to research the effect of sandy sediments produced by the sand-prevention engineering on the thermal regime of underlying permafrost along the Qinghai-Tibet Railway and its heat transfer mechanism by adopting interdisciplinary methods such as cryopedology and desert science. The thermal regime change of underlying permafrost after the sand-prevention engineering of Qinghai-Tibet Railway were covered with sandy sediments will be explored by setting up in-situ test field and conducting continuuous, synchronous and comparative observation; The reason that caused thermal regime change of permafrost under sand layer will be analized by laboratory testing, numerical simulation combined with field observational data. Based on the above-mentioned research results, definite answer to the question that how is the thermal regime of underlying permafrost changed after the sand-prevention engineering of Qinghai-Tibet Railway were covered with sandy sediments will be given, and the mechanism on sand layer effecting thermal regime of underlying permafrost will be scientifically revealed. The research of this project can not only broaden the study area and promote mutual penetration and blending between cryopedology and desert science, enriching the theory system of the two disciplines, but also provide theoretical basis for sand-prevention and permafrost protection of the Qinghai-Tibet Railway, and provide scientific basis for security operations of the Qinghai-Tibet Railway and road engineering in other similar areas. The research of this project has both theoretical significance and practical value.

青藏铁路工程防沙会不可避免地造成积沙，地表积沙后下伏冻土的热状况如何变化？至今尚无定论。沙层影响冻土的热交换机理如何？目前仍不清楚。本项目拟采用冻土学、沙漠学等交叉学科方法，研究青藏铁路工程防沙产生的积沙对下伏冻土的热影响及机理。通过在野外建立实验场进行连续同步对比观测，探索青藏铁路工程防沙产生积沙后下伏冻土热状况的变化；通过室内分析、数值模拟并结合野外观测，分析沙层下冻土热状况改变的原因。在此基础上，明确回答青藏铁路工程防沙产生积沙后下伏冻土究竟是是如何变化的，科学揭示沙层影响冻土的热交换机理。本项目的开展不仅能拓宽学科的研究领域，促进冻土学和沙漠学的相互渗透与交融，丰富两大学科理论体系，而且可为青藏铁路的冻土保护和沙害防治提供理论基础，为青藏铁路及其它类似地区道路工程的安全运营提供科学依据。研究具有理论意义和实践价值。

作为青藏铁路沙害防治的主要方式，工程防沙不可避免地会造成积沙，多年冻土区沙漠化后，地表水热及下伏冻土地温变化目前仍不清楚。为此，在青藏铁路红梁河沙害路段建立实验场，通过野外同步对比观测，室内分析、测试、计算与数值模拟等方法，对工程防沙积沙后的下伏冻土地温、地表反照率、辐射、热流、水分演变进行了研究。发现阻沙栅栏1.2 m积沙地表下冻土地温在20 m深度内均低于无积沙地表，地温的年平均值普遍降低0.1-0.2 ℃，最大达0.3 ℃；格状沙障0.1 m积沙地表下冻土地温在上限附近（活动层向多年冻土层过渡的边界层）明显低于无积沙地表，地温的年平均值降低0.5 ℃，而以上的近地表层和以下的多年冻土层差别不明显。地表积沙影响下伏冻土地温的热交换机理为：青藏铁路工程防沙产生积沙后，由于积沙地表的反照率高于无积沙地表，积沙地表向上的短波辐射增大，在其它分量辐射接近的情况下，积沙地表反射到大气中的热量增加，因此，积沙地表的净辐射降低，导致积沙地表的热量来源减少。由于沙层是一种多孔介质、是透气的不良导体；同时，由于积沙地表土壤粗化，蓄水保水能力变弱，含水量降低，热传导性能变差。因此，沙层表面的热通量降低，通过积沙地表传导到地下的热量减少，从而引起积沙地表下伏冻土地温降低。在青藏铁路多年冻土路段，工程防沙积沙后，地表反照率、辐射、热流、水分的这些演变能防止下伏冻土升温融化，从而有利于延缓冻土退化，这对增强路基稳定性，保障铁路安全运营产生了积极影响。因此，青藏铁路目前的工程防沙措施在取得防护效益的同时，对延缓沙害防治路段的冻土退化也具有重要意义。