基于力场修正的深埋粘土微观物理力学特性分子模拟研究

Insight into the high stress mechanical properties of clay is of great importance in the fields of deep mine construction and nuclear waste disposal engineering, and the basic mechanisms of high stress mechanical response for clay lies in the disclosure of the micro-structure and water characteristics change under different pressure. So in this study, by employing computer simulation, a systematic studies are conducted to investigate the physical and mechanical properties of three typical hydrated clay minerals. Firstly, the mineral composition and micro-structure of samples are comprehensively ascertained by modern testing instruments. Secondly, by integrated with TIP4P/2005 water model, which is provide to be the best description of almost all properties of water, the CLAYFF force filed is improved, that is helpful for the follow-up simulation. Thirdly, the detail microstructure of clay minerals and water diffusion characteristics are investigated by equilibrium molecular dynamic simulation, where all crystallographic cell parameters and atom positions are allowing to vary during simulation, and the edge surface of clay minerals are also evaluated in this study. Finally, the nano-scale mechanical properties of hydrated clay minerals under compression and shear are simulated by means of steered molecular dynamics technology, the stress-strain curve and strength under various contacts are also studied quantitative, which is the basic of the meso-scale simulation, and this study lays the foundation of the bottom-up hierarchical multi-scale study system for clay mechanical properties research under high stress.

深部矿井建设和深地质核废料埋藏工程对粘土的高压力学特性研究提出了需求，决定粘土高压力学响应的关键在于不同压力下其微观结构和土中水特性的改变，但目前微观分子层面的研究十分欠缺，由此提出对粘土的微观物理力学特性进行分子模拟，以揭露粘土高压变形的机理所在。首先综合现代分析测试手段查明深部粘土、西部第三系泥岩和核废料缓冲膨润土的矿物组成及微观图像和结构信息；其次基于全面反映水分子特性的TIP4P/2005模型修正粘土矿物CLAYFF力场，奠定后续模拟精度。并在此基础上对不同温度压力条件下的水化粘土进行释放原子所有自由度，且矿物层面和边缘端面外水特性描述兼顾考虑的物理特性模拟，细致刻画粘土的微观结构，离子及水分子运动特征；最后基于拉伸分子模拟手段对水化粘土进行拉压和剪切力学特性模拟，获得其变形规律和强度数值，并通过原子力显微镜对模拟结果进行实测验证。本项目的实施可为深部地下工程建设提供相关基础资料。

深部地下工程建设对黏土的高压力学特性研究提出了需求，高压条件下深埋粘土的力学响应与常压条件存在显著差异，同时决定黏土高压力学响应的关键在于不同压力下其微观结构和土中水特性的改变。但目前对于深埋黏土无论是宏观高压力学特性试验还是微观分子层面模拟研究均十分欠缺，由此提出对黏土矿物的高压物理力学特性开展宏微观研究，以揭露黏土高压变形的控制机理所在。本项目研究首先通过微观测试分析的手段查明了华东深埋黏土、西部岩层中泥质胶结物及商用高岭矿土的矿物组成与各组分相对含量、微观结构电镜扫描图像和原子键长、键角等定量信息，为下一步高压试验和微观模拟提供了基础数据。其次，利用自行研制的可考虑固结过程中孔隙水压消散、侧壁摩擦力影响的高压K0固结仪和改造底座后可测量试样内部孔隙水压的GDS三轴试验机，开展从低压到高压的K0固结和三轴不排水剪切试验研究，并采用扫描电镜对三轴试样的微观结构进行了扫描分析。再次，依据建立的典型黏土矿物三维全原子结构模型和修正的CLAYFF力场，对水化蒙脱石体系进行了完全释放体系原子所有自由度的微观分子动力学模拟，获得了水化蒙脱石的完整吸水膨胀过程曲线，并基于水化能和侵入能的定义获得了不同体系的热力学稳态含水量。最后，进一步基于常应变法对热力学稳态含水量体系的微观力学特性进行了分子模拟，获得了水化蒙脱石体系的线弹性特征及刚度矩阵参数，并进一步分析了典型刚度矩阵参数随含水量的变化规律。同时基于约束剪切模拟手段对蒙脱石层间水的抗剪强度进行了定量模拟，获得了层间水的剪切粘度随含水量的变化规律。深埋粘土微观物理力学特性分子模拟研究揭示出，高应力条件下粘土变形的内在机理为控制粘土高压压缩的基本机制在于土中结合水的部分脱附，而对于高压剪切的基本机制则认为随着压力的增大存在矿物间摩擦向层间结合水抗剪转化的过程。本项目的研究可为深部矿井建设和深地质核废料埋藏工程提供基础资料。