基坑工程花岗岩残积土的力学性能衰变特征与细观结构演化机制

In view of the fact that disasters occur frequently during the construction of foundation pit engineering in granite residual soil areas of South-eastern coast of China, the mainline of this project is to investigate meso-structure evolution law of granite residual soil, taking the disturbance damage, excavation unloading, wetting process and water infiltration time into consideration, so as to understand further the deterioration mechanism of mechanical performance of granite residual soil. Based on the mechanical response behaviors of granite residual soil to the self-boring pressuremeter tests and borehole shear tests by considering the effects of stress history and water infiltration，the static triaxial tests on undisturbed specimens after experiencing different cyclic loadings，unloading stress path tests on specimens experiencing different numbers of cyclic wetting-drying，triaxial tests and measurement of shear-wave velocity by bender elements under the conditions of unsaturated state, saturated state and various water infiltration time are carried out. Meanwhile, the Nuclear Magnetic Resonance (NMR) and Computerized Tomography (CT) techniques are adopted for continuously tracking and measuring the variation characteristics of pore size distribution and interior structural morphology of the tested samples under the above conditions. Through seeking the characteristic parameters to be used to describe state of soil meso-structure，the association patterns of meso-structure state parameters and mechanical properties of granite residual soil are analyzed, and the quantitative relations between meso-structure state parameters and strength, rigidity or shear wave velocity established. On the basis, the damage variables and damage evolution law of granite residual soil are proposed, and the structured constitutive model of granite residual soil established within the framework of compound damage constitutive model. Furthermore, the degradation mechanism of mechanical performance of granite residual soil is revealed by numerical simulation. The above-mentioned results will provide theoretical and technical support for promoting engineering design, construction and disaster prevention in granite residual soil areas.

针对东南沿海花岗岩残积土地区的基坑工程建设灾害频发现状，围绕花岗岩残积土细观结构随扰动损伤、开挖卸荷、湿化过程与浸水时间的演变规律这一主线，深入认识其力学性能的衰变机理；以考虑应力历史与浸水影响下花岗岩残积土自钻式旁压与孔内剪切试验的原位力学响应性状为基础，开展历经不同循环荷载振动次数的原状土静三轴试验，干湿循环土样的卸荷应力路径、非饱和-饱和-浸水历时的三轴试验及弯曲元剪切波速测试，采用核磁共振(NMR)与计算机层析识别(CT)技术，跟踪量测其孔隙大小分布与内部结构形态的变化特征，通过探求描述土体细观结构状态的特征参数，分析其与力学特性的关联模式，建立与强度、刚度指标或剪切波速的定量关系；在此基础上，提出花岗岩残积土的损伤变量及损伤演化律，以复合体损伤模型为框架，建立其结构性模型，并基于数值分析揭示其力学性能劣化机制，为推进花岗岩残积土地区工程建设的设计施工与灾害防治提供理论和技术支撑。

本项目围绕花岗岩残积土工程性能与细观结构随扰动损伤、开挖卸荷与湿化过程等环境因素的演变规律主线，深入研究了花岗岩残积土力学性能的衰变特征与内在机制。通过开展自钻式旁压试验(SBPT)与预钻式旁压试验(PMT)以及预钻成孔滞后不同时间旁压对比试验，揭示了花岗岩残积土原位力学特性的钻探扰动与卸荷滞时效应特性；基于孔内剪切试验(BST)，获得了花岗岩残积土原位强度特性与强度参数随深度变化规律，建立了其强度参数与砾粒含量的关系；提出结合地震波扁铲试验(SDMT)和自钻式旁压试验(SBPT)确定原位G-γ衰减曲线新方法，分析了花岗岩残积土小应变刚度特性。采用应力路径三轴试验，研究了应力路径对花岗岩残积土应力-应变与强度特性的影响，通过引入定量表征应力路径的变量加载角，论证了应力路径对其强度参数、变形特性、刚度性质与柔度性质的影响；利用压力板仪与核磁共振(NMR)技术，获得了花岗岩残积土在脱湿和吸湿路径下的土水特征曲线(SWCC)以及吸湿过程中的水分迁移动态过程，建立了可以模拟“墨水瓶”效应的非饱和土滞后阻塞模型。提出基于简单多阈值分割算法的计算机层析识别(CT)快捷分析处理方法，结合三轴CT试验与数字体积相关 (DVC)方法，厘清了花岗岩残积土在三轴剪切过程中的细观应变场与剪切带演化特性。借助Micro-CT与NMRI试验系统，揭示了干湿循环和冻融作用对花岗岩残积土的细观结构与孔隙分布特征的影响规律；基于考虑干湿循环次数影响下单轴压缩和三轴剪切试验的电阻率实时监测结果，探讨了花岗岩残积土性能弱化效应，建立了电阻率损伤力学模型。获取了不同含水率下花岗岩残积土崩解特征，建立了半定量崩解速率模型，基于数学形态学方法(PM)提出了18格子模型并模拟了崩解特性及机理。此外，还研究了冲击荷载作用下花岗岩残积土的力学响应特征。上述研究，为推进花岗岩残积土地区工程建设的设计施工与灾害防治提供了帮助。