冻土路基波动型变形的形成机理和演变过程研究

A “fluctuate” deformation pattern is very common for the road embankments in permafrost regions. The deformation pattern is characterized by heave during freezing periods and settlement during thawing periods, and an overall settlement trend. Little effort has been put on the interaction between the thaw consolidation process in permafrost layers and the freeze-thaw cycling in active layers. It is an obstacle to unveil the formation mechanism and evolution process of the “fluctuate” deformation pattern. In this research program, three main aspects are planned: 1) the dissipation process in the thawed permafrost layers as well as the cryosuction triggered by the freezing process in active layers are investigated by in-door experiments with an intention to reveal the interaction mechanism; 2) in combination with the indoor experimental results and an outdoor monitoring program on road sections with “fluctuate” deformation pattern, the formation mechanism of the deformation pattern is revealed from the viewpoints of both engineering geology and mechanical process; 3) the in- and out-door experimental results are employed to modify and validate the hydro-thermo-mechanical frost heave model and thaw consolidation model. The modified models are then used to simulate the evolution process of the “fluctuate” deformation pattern in the process of permafrost degradation, based on which the relationship among “geological condition-permafrost degradation- embankment deformation” is discussed. The results will be helpful to improve the applicability of the two models in permafrost regions, and to provide a more accurate deformation prediction and theoretical reference of hazard control for the road embankments in permafrost regions.

波动型变形是多年冻土地区常见的路基变形形式之一，其表现为冻结期隆起融化期沉降，且总体趋势为沉降变形。活动层冻融过程和多年冻土层融化固结过程之间的相互影响机制，目前仍缺乏系统的研究，这是揭示波动型变形形成机理和演变过程的关键。本项目拟开展：1）通过室内土柱试验研究多年冻土融化过程中孔隙水压力的消散规律和活动层冻胀过程中产生的吸力，揭示其相互影响机制；2）结合室内试验和野外路基分层变形试验结果，从物理力学过程和工程地质条件两个角度揭示波动型变形的形成机理；3）建立水热力耦合冻胀模型和冻土融化固结模型，并基于试验结果对其进行修正和验证，利用修正模型对冻土退化过程中青藏公路波动型变形的演变过程进行数值模拟，进而探讨“工程地质条件—冻土退化—路基变形”三者之间的关系。本项目将进一步提高冻胀和融化固结模型在多年冻土地区的适用性，同时为冻土路基更为准确的变形预测和科学维护提供理论依据。

冻胀和融化固结变形是路基波动型变形形成的两个最主要的物理力学过程。本项目针对这两个物理力学过程开展了较为系统的研究，包括1）将冻胀来源划分为水分迁移、迁移水相变和原位水相变，并将一维冻胀模型（水流方程）扩展到了三维；2）通过引入无量纲的温度状态变量，将未冻土的临界状态模型扩展到了冻土，进而建立了冻土与未冻土的统一弹塑性本构模型，这是冻土融化固结计算的关键；3）在前两点的基础之上，将一维冻胀与融化固结统一计算框架扩展到了三维；4）基于室内试验对本构模型和三维计算框架进行了验证，同时利用该三维计算框架对青藏公路开心岭路段进行了数值模拟。研究结果显示，基于水流方程的三维冻胀模型能够模拟土体冻胀，但是分配系数需要进一步的确定；本构模型能够很好的模拟冻土剪胀和剪缩，以及冻融循环导致的应力状态和孔隙率的变化；三维冻胀和融化固结统一计算框架能够很好的模拟冻土路基的变形过程。研究结果有望服务于我国寒区工程建设。