A microphysics-based understanding of mechanical and hydraulic processes in clay shales is required for developing advanced constitutive models, which can be extrapolated to long-term deformation. Although many geomechanical tests have been performed to characterise the bulk mechanical, hydro-mechanical, and failure behaviour of Opalinus Clay, important questions remain about micromechanisms: how do microstructural evolution and deformation mechanisms control the complex rheology? What is the in situ microstructural shear evolution, and can it be mimicked in the laboratory? In this contribution, scanning electron microscopy (SEM) was used to image microstructures in an Opalinus Clay sample deformed in an unconsolidated–undrained triaxial compression test at 4 MPa confining stress followed by argon broad ion beam (BIB) polishing. Axial load was applied (sub-)perpendicular to bedding until the sample failed. The test was terminated at an axial strain of 1.35 %. Volumetric strain measurements showed bulk compaction throughout the compression test. Observations on the centimetre to micrometre scale showed that the samples exhibited shear failure and that deformation localised by forming a network of micrometre-wide fractures, which are oriented with angles of 50^∘ with respect to horizontal. In BIB–SEM at the grain scale, macroscale fractures are shown to be incipient shear bands, which show dilatant intergranular and intragranular microfracturing, granular flow, bending of phyllosilicate grains, and pore collapse in fossils. Outside these zones, no deformation microstructures were observed, indicating only localised permanent deformation. Thus, micromechanisms of deformation appear to be controlled by both brittle and ductile processes along preferred deformation bands. Anastomosing networks of fractures develop into the main deformation bands with widths up to tens of micrometres along which the sample fails. Microstructural observations and the stress–strain behaviour were integrated into a deformation model with three different stages of damage accumulation representative for the deformation of the compressed Opalinus Clay sample. Results on the microscale explain how the sample locally dilates, while bulk measurement shows compaction, with an inferred major effect on permeability by an increase in hydraulic conductivity within the deformation band. Comparison with the microstructure of highly strained Opalinus Clay in fault zones shows partial similarity and suggests that during long-term deformation additional solution–precipitation processes operate.

中文翻译：

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在三轴试验中导致蛋白石粘土剪切破坏的微观机制：高分辨率 BIB-SEM 研究

开发先进的本构模型需要对粘土页岩中的机械和水力过程有基于微物理学的理解，该模型可以外推到长期变形。尽管已经进行了许多地质力学测试来表征 Opalinus 粘土的整体力学、流体力学和破坏行为，但关于微观力学的重要问题仍然存在：微观结构演化和变形机制如何控制复杂的流变学？什么是原位微观结构剪切演化，能否在实验室中模拟？在这项贡献中，扫描电子显微镜 (SEM) 用于对 Opalinus Clay 样品的微观结构进行成像，该样品在 4 MPa 围压下的松散-不排水三轴压缩试验中变形，然后进行氩宽离子束 (BIB) 抛光。轴向载荷施加（亚）垂直于垫层，直到样品失效。测试在轴向应变为 1.35% 时终止。体积应变测量结果显示在整个压缩测试过程中体积压实。在厘米到微米尺度上的观察表明，样品表现出剪切破坏，变形通过形成微米宽的裂缝网络而局部化，这些裂缝以 50 度角定向^∘相对于水平。在颗粒尺度的 BIB-SEM 中，宏观尺度裂缝显示为初期剪切带，显示出膨胀的粒间和粒内微裂缝、颗粒流、页硅酸盐颗粒弯曲和化石中的孔隙坍塌。在这些区域之外，没有观察到变形微观结构，表明只有局部永久变形。因此，变形的微观机制似乎由沿优选变形带的脆性和延性过程控制。裂缝的吻合网络发展成宽度达数十微米的主要变形带，样品沿其断裂。微观结构观察和应力应变行为被整合到一个变形模型中，该模型具有三个不同阶段的损伤累积，代表压缩的 Opalinus 粘土样品的变形。微观结果解释了样品如何局部膨胀，而体积测量显示压实，推断变形带内水力传导率增加对渗透率产生主要影响。与断层带中高应变 Opalinus 粘土的微观结构比较显示出部分相似性，并表明在长期变形过程中，额外的溶解 - 沉淀过程在起作用。通过变形带内水力传导率的增加推断出对渗透率的主要影响。与断层带中高应变 Opalinus 粘土的微观结构比较显示出部分相似性，并表明在长期变形过程中，额外的溶解 - 沉淀过程在起作用。通过变形带内水力传导率的增加推断出对渗透率的主要影响。与断层带中高应变 Opalinus 粘土的微观结构比较显示出部分相似性，并表明在长期变形过程中，额外的溶解 - 沉淀过程在起作用。