正冻粉质黏土冷生构造发育过程试验及数值模拟研究

Cryostructure in freezing soil is a dynamic interactional process of hydrothermal migration and structure degradation. It also an important cause of complex periglacial landforms and inhomogeneous frost damages. The development process and mechanism of cryostructure needs to be improved. This project takes visual dynamic test and numerical simulation as the research method, selects the density and temperature gradient as the influencing factors, and carries out the one-dimensional freezing test of saturated silty clay under closed system. The Charge Coupled Device (CCD) image acquisition and computed tomography scan technology to dynamically collect the freezing shrinkage crack morphological data, which is used to invert the three-dimensional cryostructure evolution process and analyzed its influencing factors. By analyzing the distribution and variation of temperature field and water field during the formation of freezing shrinkage crack, the hydrothermal criterion for it is proposed. By comprehensively considering the heat conduction process, the water migration driving force, the water migration process, the formation and development process of freezing shrinkage crack in the samples and finally establishing the finite element model for solving the cryostructure development process of the freezing silt clay. The experiment result is used to verify debug the model. The research results of this project can improve the dynamic meso-experimental research methods and theories of freezing soil, realize the organic unification of cryostructure test, mechanism and prediction, and optimize the existing frost heaving model, which can contribute to the genetic analysis of the various periglacial landforms and forecast of frost damage in engineering.

正冻土中冷生构造发育是水热运移和结构劣化相互作用的动态过程，是复杂冰缘地貌和冻土病害的重要诱因，冷生构造发育过程及机理研究亟待完善。本项目以可视化动态试验和数值模拟为研究手段，选取密度和温度梯度作为影响因素，开展封闭系统下饱和粉质黏土单向冻结试验，运用CCD图像采集和CT扫描技术，动态采集试样冷生裂隙形态要素数据，反演三维冷生构造演化过程，分析其影响因素。在此基础上，通过分析冷生裂隙形成时温度场与水分场分布及变化规律，提出冷生裂隙形成的水热判据。最后，系统考虑试样中热传导过程、水分迁移驱动力、水分迁移过程、冷生裂隙形成与发展过程，建立求解正冻粉质黏土单向冻结冷生构造发育过程的有限元模型，对比试验结果进行模型验证与调试。本项目的研究成果可以完善正冻土动态细观试验研究方法和理论，实现冷生构造试验、机理和预测的有机统一，优化已有冻胀模型，为冰缘地貌成因分析和工程冻害问题预测提供理论和技术支持。

正冻土中冷生构造发育是水热运移和结构劣化相互作用的动态过程，是复杂冰缘地貌和冻土病害的重要诱因。本项目首先在东北岛状多年冻土区开展了野外冰缘地貌调研工作，发现自冻结剧烈冻结作用下，野外地表形成变成约2m，深度约15cm的多边形构造形态，且地层呈现出分凝冰层与土层互层的冷生现象。接着通过技术创新与集成，研发了基于数字图像技术的正冻土冻结过程动态图像采集试验系统，开发了试样纵剖面采集的CCD图像和横剖面采集的CT图像的处理软件，对粉质黏土在封闭系统冻结过程中的温度、纵剖面冻结形态、冻胀位移数据以及冻结前后试样含水率、横剖面CT扫描图像进行了采集处理，得到主要结论包括：粉质黏土冻结过程中热量运移在24h内基本达到稳定，试样纵剖面冷生构造发育形成微薄层状构造带、薄层状构造带和最暖端厚层状构造带，横剖面形成多边形构造形态，试样在顶板温度较高时仍有明显的冻胀发展，试样冻结过程中水分从未冻区向已冻区发生迁移等。接着采用黏聚力理论建立了冷生构造数值模型，并采用三点弯曲断裂试验对模型进行了校正，通过给实体土体单元边界嵌入cohesive黏聚力单元，采用双线性牵引分离准则实现了实体单元之间的断裂模拟，数值模拟结果发现黏聚力模型在模拟正冻土冷生构造方面具有很好的适应性，基于此分析了冷生裂隙发育机理，清楚认识到土冻结过程中冷生裂隙先于冰分凝形成的事实。本项目的研究成果为构建新的土冻胀数值模型提供了理论铺垫，为冰缘地貌成因分析和工程冻害问题预测提供理论和技术支持。