基于“最大球”识别技术的冻土三维结构演化表征及损伤机理研究

The change of meso-structure in frozen soil determines its thermal and mechanical properties under different temperature and load, and controls its macroscopic mechanical response and damage failure mechanism to a large extent. In order to study the essential relationship between deformation and damage of frozen soil, its internal structure is the starting point of the study. But at present, it is difficult to identify and characterize the meso-structure of frozen soil, which also increases the difficulty of studying the space-time evolution law of the three-dimensional pore and skeleton structure. Based on the study of frozen soil three-dimensional meso-structure changes in the process of stress, this project identifies and describes the internal structure and morphological representation of the soil particles, ice crystals and pores under load, and studies law of evolution. On this basis, the three-dimensional pore structure model under different stress states is established, and the "maximal ball" identification technique is used to quantitatively characterize the process of three-dimensional pore structure change, to clarify the space-time evolution law of three-dimensional pore and skeleton structure, and to introduce three-dimensional meso structural damage variables reflecting the damage characteristics of frozen soil. By combining the change of meso-structure with the response of macro mechanics, the method of macro-meso scale damage identification and mechanism evaluation is established, which reveals the nature of frozen soil deformation and damage from meso-scale.

冻土内部细观结构变化某种程度上决定了它在不同温度和荷载作用下的热、力学特性，很大程度上控制了其宏观力学响应和损伤破坏机理，要研究冻土的变形与损伤的本质关系，其内部结构是研究的出发点，但是目前对于冻土细观三维结构的识别及表征还存在困难，这也增加了对其三维孔隙、骨架结构的时空演化规律研究的难度。本项目以受力过程中的冻土内部三维细观结构变化为研究基础，将宏观力学试验和细观试验相结合，对载荷作用下的土颗粒、冰晶、孔隙的内部结构、形态表征进行识别和描述，并对其演化规律进行研究。在此基础上，建立不同应力状态下的三维孔隙结构模型，应用“最大球”识别技术对三维孔隙结构变化过程进行定量表征，阐明三维孔隙、骨架结构的时空演化规律，引入反映冻土损伤特性的三维细观结构性损伤变量，将细观结构变化与宏观力学响应相结合，建立宏～细观的跨尺度损伤识别及机制评价方法，从细观尺度真正揭示冻土变形及损伤本质。

冻土的变形和损伤主要是由土体内部结构的相对变化引起的，其内部细观结构的变化决定了其在不同温度和荷载作用下的热、力学特性，在很大程度上控制着其宏观力学响应和损伤破坏过程。本课题运用冻土力学、损伤力学、体视学等学科的研究方法，从宏观和细观两个方面入手，首先解决了力学试验与CT试验的同步扫描、温度差控制、CT图像伪影等问题。其次开展了不同温度、围压、加载速率等边界条件影响下的宏观三轴剪切实验和加卸载回弹试验，利用多能量螺旋CT机，对其整个试验过程进行CT动态跟踪扫描。在此基础上，依据CT图像，应用“最大球”法建立了不同应力状态下的冻土三维细观孔隙网络模型，对三维孔隙结构演化过程进行定量表征，并对不同状态下的内部细观结构性参数（如孔隙率、等效孔隙半径、等效喉道半径、喉道长度等）及形态特征（球形度、形状因子等）进行分析，最后依据土体受力条件下的三维孔隙的时空演变特征，参照宏观变形及破坏特点，引入反映冻土变形及损伤特性的三维细观结构性损伤变量，建立一种宏～细观的跨尺度损伤演化方程，并对不同温度、围压、加载速率、压实度、冻融循环等因素影响下的冻土力学特性及细观损伤演化机理进行了研究。该项成果从细观尺度揭示了寒区工程变形破坏发生的内在机理，对于寒区地基土的稳定性研究具有重要的研究价值。