降雨变化对多年冻土区公路路基表面能水平衡过程和稳定性影响研究

Engineering practice of highway in permafrost regions indicates that hydrothermal boundary condition is the key problem for the freezing-thawing disaster of embankment. The climate warming and rainfall increasing will aggravate the disaster. However, previous studies mainly focuse on the influence of climate warming on the thermal stability of the embankment but the influences of rainfall increasing on the hydrothermal boundary condition and thermal-moisture dynamics of embankment have not been fully considered. Therefore, the embankments under rainfall condition in the permafrost regions are taken as research object. Firstly, the energy and water balance responses of embankment surface to different rainfall intensity are monitored to reveal the energy and water balance process. In order to improve the surface hydrothermal boundary, a pavement albedo experiment under different water content is carried out to formulate prediction model of pavement albedo under rainfall. Then, combining the heat transfer theory with the saturated-unsaturated soil seepage theory, a coupled water-vapor-dry air-heat transport model of coupled water and heat transport in saturated-unsaturated partially frozen porous media are developed. In this model, the characters of rainfall infiltration and surface evaporation process are considered. Moreover, the near-surface atmospheric dynamics theory will be used to calculate the upper boundary conditions. Finally, the established model will be used to predict the thermal-moisture response of the embankment with different structure type to increasing rainfall scenarios in the next fifty years. The research achievements can provide the new ideas for understanding the inner mechanism of rainfall on the thermal-moisture dynamics of the embankment, the new methods to calculation the boundary conditions of frozen soil engineering, theoretical guidance and scientific basis for the embankment design and disaster control in permafrost regions.

多年冻土区路基工程实践表明，水热条件是冻土路基冻融问题的关键，气候变暖和降雨增加都将加剧路基冻融病害。以往研究主要关注气温升高的作用，降雨增加引起的路面水热边界条件改变和路基内部水热响应定量分析尚未引起足够重视。本项目以多年冻土区公路路基为研究对象，开展不同降雨强度下的路基表面能量-水分监测，揭示降雨作用下的沥青路面能水平衡过程；通过室内含水量-反照率试验，构建降雨作用下的路面反照率预测模型，完善路面水-能平衡边界条件；考虑降雨入渗-蒸发过程和冻土路基水热输运特点，根据多孔介质传热传质理论和近地表空气动力学理论，构建基于地表水热通量边界条件的水-汽-热-空气耦合模型；预测青藏高原暖湿化气候模式下未来50年不同结构型式公路路基的长期水热稳定性。研究成果为降雨作用下的路基水热响应机理认识和冻土工程热工计算边界条件的确定提供新思路，同时为多年冻土区公路路基的设计和病害防治提供理论支持。

既有研究主要关注气温升高对多年冻土及路基稳定性影响，而降雨引起的水热边界条件改变和路基内部水热响应定量分析尚未引起足够重视。本项目首先对青藏高原多年冻土区北麓河冻土监测站天然场地及沥青路面两个水热监测断面水热和辐射数据进行分析，结果表明：夏季降雨事件导致天然场地以及沥青路面地表辐射减小、活动层内部含水量增加以及浅层热通量、温度下降，尤其以持续降雨及强降雨事件影响较为明显，其中夏季降雨事件对沥青路面的扰动大于天然地表。秋季降雨减小事件对天然场地和沥青路面地表辐射以及浅层内部水热变化的影响趋势与夏季降雨事件的影响结果相反。为了进一步探明降雨对活动层内部水热的影响机理，通过室外试验确定出青藏粉质黏土地表含水量与反照率的数学定量表达式，完善水-汽-热耦合模型的水热边界条件。通过验证的模型研究表明：夏季降雨有助于缓解多年冻土退化，通过进一步模拟全年降雨增加工况下多年冻土区的水热响应，结果发现全年降雨增加使得活动层冻土上限整体抬升，维持了冻土稳定性。并且通过不同下垫面水热差异的模型研究发现，沥青路面覆盖层造成大量热量通过沥青路面传导至冻土中破坏冻土的稳定性。虽然降雨降低了砂砾路面地表土壤热通量，但土壤热通量降低幅度小于导热系数增大幅度，使得砂砾路面以不同于沥青路面的水热影响方式加速了冻土的退化。最后，通过室内实体路基模型试验进一步验证了有无降雨以及夏季降雨增加对路基面层、路肩、斜坡以及天然场地下部等不同部位的水热影响，结果表明：降雨会显著改变路基内部的水热变化，且夏季降雨增加会导致路基浅层含水量增加，路基浅层热通量以及温度降低，缓解了路基内部吸收的热量，保持了冻土路基稳定性。研究成果为气候变化情境下，降雨作用下多年冻土区冻土环境和路基工程水热响应机理认识和冻土水热耦合模型与边界条件确定提供新思路，同时为寒区工程设计和病害防治提供理论支持。