高放废物地质处置工程围岩双重损伤演化模型研究

For the High-Level radioactive Waste (HLW) disposal project, the initiation and propagation of the microcracks (damage) in the surrounding rock may lead to degradation of the mechanical behavior of the rock mass, and provide potential paths for the nuclide migration. Hence, the damage evolution of the surrounding rock is critical to the stability and safety of the project. Different damage evolution kinetics under compressive and tensile stresses conditions have been revealed and confirmed by numerous experimental studies. However, the current damage criteria are generally formulated based on the damage evolution kinetics under a single stress condition (compressive or tensile), therefore the limitation is evident in this regard. This work focuses on the development of a coupled tensile-compressive damage criterion, which can reasonably reproduce the damage evolution kinetics under different stress conditions. In the present study, the mechanical behavior and damage evolution kinetics of intact granite under different stress conditions are first investigated in the laboratory. The granite is taken from the Beishan site, where is considered as the most potential site for China's HLW Disposal. Different measurement techniques (acoustic emission, elastic modulus and permeability) are used to characterize the induced damage of the rock. Different loading paths (tensile, uniaxial and triaxial compression with different confining pressures) are considered. Based on the experimental investigations, the coupled tensile-compressive damage criterion, which considers both compressive and tensile damage evolution mechanisms, will be established. A series of damage variables are proposed and appropriately determined with micromechanical consideration in this criterion. In the framework of thermodynamics, a coupled elastoplastic damage model is then developed to represent the fundamental mechanical performance of Beishan Granite. The relationship between damage evolution and permeability is also studied. Then,the proposed model is implemented in the numerical tool to simulate the mechanical behaviors of the Beishan granite. At last, on the basis of the preliminary concept of China's HLW disposal project and the geological environment in the Beishan site, the numerical simulations of HLW disposal project excavation will be performed to investigate the distribution of excavation damage zone (EDZ), and its influence on the project safety. The research work can provide an alternative and effective method for the stability and safety analysis of the HLW disposal project. The achievements can also be extended to other geomaterials and engineering fields.

高放射性废物深地质处置围岩损伤扩展是影响工程长期稳定性和安全性的关键因素。鉴于现有损伤模型多是基于拉伸或压缩某一特定应力状态建立的，忽略了不同应力状态下损伤扩展机理的显著差异，其相关成果具有一定局限性。为了突破这一瓶颈，本课题以北山深部花岗岩为研究对象，拟通过不同加载方案的力学和渗透试验，对拉、压应力状态下岩石损伤演化规律及其相互耦合机理进行研究，提出综合考虑拉、压及复杂应力状态的各向异性双重损伤演化方程，并基于热动力学理论构建岩石弹塑性损伤力学模型，获得损伤演化对渗透特性的影响规律；把建立的模型嵌入到数值模拟工具中，基于室内试验结果对模型进行验证，并结合北山工程地质条件和中国高放废物地质处置概念模型，模拟处置库开挖过程，剖析工程开挖损伤区分布特征，揭示损伤分布对围岩渗透特性和工程安全性的影响。研究成果不仅可为高放废物处置工程稳定性和安全性评价提供理论基础，还可丰富和完善岩石损伤力学理论。

对于高放废物地质处置工程，工程开挖损伤区的产生和扩展会降低围岩力学性能而影响工程稳定性，同时还会为放射性核素提供潜在的迁移通道，进而降低工程的长期安全性。因此，处置库围岩力学特性研究，特别是围岩损伤(裂隙)演化机制和损伤模型研究，是高放废物地质处置工程技术研发的重要前提和基础。本课题以北山深部花岗岩为研究对象，开展不同加载方案的力学和渗透试验，对拉、压应力状态下的损伤演化规律及其损伤变量的组合方法进行研究。试验结果表明，在低围压条件下岩石主要发生的是脆性破坏；随围压增大，岩石力学行为逐渐向延性转化，表现出剪胀、塑性变形等非线性行为。结合微裂隙产生和扩展规律，对岩石在外力作用下的损伤演化过程和破坏机制进行分析，认为北山花岗岩的破坏及非线性行为是损伤和塑性变形共同作用的结果。基于岩石三轴压缩应力–应变全过程渗透特性试验，结合三维声发射监测信息，研究花岗岩在不同围压条件下力学损伤演化机制及其对岩石渗透特性影响规律。试验结果表明，在压缩应力作用下，花岗岩的损伤演化始于微裂隙的产生和扩展，并在岩石破坏时和峰后阶段发展迅速。该损伤演化的阶段性特征与声发射监测数据一致，进一步说明了裂隙扩展是导致花岗岩力学特性劣化的根本原因。随着微裂隙的扩展，岩石渗透性不断增强，但在峰前加载阶段渗透性变化明显滞后于损伤演化过程。该结果表明，在裂隙贯通并产生宏观破坏面之前，裂隙扩展对花岗岩渗透性影响非常有限。结合声发射监测数据，对不同应力条件下损伤演化与渗透特性的相互关系进行分析，并提出花岗岩渗透率与损伤和围压的相关经验公式。基于这一认识，提出考虑拉、压及复杂应力状态的各项异性双重损伤演化方程，并基于热动力学理论构建岩石弹塑性损伤力学模型，获得损伤演化对渗透特性的影响规律；将该模型嵌入法国THMPASA多场耦合数值分析软件，开展了不同地应力分布条件下硐室开挖过程数值模拟分析。数值结果表明，该模型不仅可以描述开挖过程岩石力学特性，还可以直接用以评价微裂隙扩展对岩石力学特性和渗透特性的影响。