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Micro-cracking and incipient shear microstructures at low-strain deformation of Opalinus Clay: Insights from triaxial testing and broad-ion-beam scanning-electron-microscopy (BIB-SEM)
Solid Earth ( IF 3.4 ) Pub Date : 2021-04-21 , DOI: 10.5194/se-2021-39
Lisa Winhausen , Jop Klaver , Joyce Schmatz , Guillaume Desbois , Janos L. Urai , Florian Amann , Christophe Nussbaum

Abstract. 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 laboratory tests have been performed to characterize the bulk mechanical, hydro-mechanical and failure behaviour of Opalinus Clay, important questions remain about microphysics: How do microstructural evolution and deformation mechanisms control the 15 complex rheology over time scales not accessible 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 specimen 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 cm- to μm-scale showed that deformation 20 localized by forming a network of μm-thick fractures. In BIB-SEM at the grain scale, incipient deformation zones show dilatant inter- and intragranular micro-cracking, granular flow, plastic deformation and bending of phyllosilicate grains, and pore collapse in fossils. Outside these zones, no deformation microstructures were observed indicating localized damage. Thus, microphysics of deformation appear to be controlled by both brittle and ductile processes along preferred orientations. Anastomosing networks of deformation bands develop into the main deformation bands along which the sample fails. 25 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 evolution. Comparison with the microstructure of highly strained Opalinus Clay in fault zones shows minor similarity and suggest that during long-term deformation additional solution-precipitation processes operate.

中文翻译:

蛋白石粘土低应变变形下的微裂纹和初期剪切微观结构:三轴测试和宽束电子扫描电镜(BIB-SEM)的见解

摘要。开发先进的本构模型需要基于微观物理学的对泥页岩力学和水力过程的理解,该模型可以推断为长期变形。尽管已经进行了许多地质力学实验室测试来表征Opalinus Clay的整体力学,流体力学和破坏行为,但有关微观物理学的重要问题仍然存在:微观结构的演化和变形机制如何在实验室无法达到的时间范围内控制15种复杂的流变学。在这项贡献中,使用了扫描电子显微镜(SEM)对在4 MPa围压下的非固结不排水三轴压缩试验中变形的蛋白石粘土样品中的微观结构成像,然后进行了氩离子束(BIB)抛光。垂直于铺垫施加(负)轴向载荷,直到试样破坏。测试在1.35%的轴向应变下终止。体积应变测量显示在整个压缩测试中整体压实。从厘米级到微米级的观察表明,形变20通过形成一个微米级的裂缝网络而局部化。在BIB-SEM的晶粒尺度上,初期变形区显示出扩张的晶间和晶内微裂纹,颗粒流动,页硅酸盐晶粒的塑性变形和弯曲以及化石中的孔隙塌陷。在这些区域之外,未观察到变形微结构,表明局部损坏。因此,变形的微观物理学似乎受到沿优选取向的脆性和延性过程的控制。形变带的吻合网络发展成为样品破坏的主要形变带。25个微结构观察和应力-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,代表着压缩的蛋白石粘土样品的变形。微观结果解释了样品如何局部膨胀,而体积测量显示压实,推测对渗透率演化有重大影响。在断层带中与高应变蛋白石粘土的微观结构进行比较,显示出较小的相似性,这表明在长期变形过程中,还会进行其他的固溶过程。25个微结构观察和应力-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,代表着压缩的蛋白石粘土样品的变形。微观结果解释了样品如何局部膨胀,而体积测量显示压实,推测对渗透率演化有重大影响。在断层带中与高应变蛋白石粘土的微观结构进行比较,显示出较小的相似性,这表明在长期变形过程中,还会进行其他的固溶过程。25个微结构观察和应力-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,代表着压缩的蛋白石粘土样品的变形。微观结果解释了样品如何局部膨胀,而体积测量显示压实,推测对渗透率演化有重大影响。在断层带中与高应变蛋白石粘土的微观结构进行比较,显示出较小的相似性,这表明在长期变形过程中,还会进行其他的固溶过程。推测对渗透率演化的主要影响。在断层带中与高应变蛋白石粘土的微观结构进行比较显示出较小的相似性,这表明在长期变形过程中,还会进行其他的固溶过程。推测对渗透率演化有重大影响。在断层带中与高应变蛋白石粘土的微观结构进行比较,显示出较小的相似性,这表明在长期变形过程中,还会进行其他的固溶过程。
更新日期:2021-04-21
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