Microstructural observations of shear zones in sensitive clay: Fid-Bau Portal

Microstructural observations of shear zones in sensitive clay

Gylland, Anders Samstad / Rueslåtten, Håkon / Jostad, Hans Petter / Nordal, Steinar

Abstract The present work assesses the structure and evolution of shear zones in sensitive ‘quick’ clays from the initiation to the well-established state. This assessment is done by utilizing laboratory and small scale field experiments combined with the analysis of thin sections in light microscopy and SEM. High resolution X-ray tomography (‘micro-CT’) is also applied. In combination, detailed 2D and 3D visualization of the shear zones is achieved. Shear zones are suggested to be formed in mm-thick continuous fields of displacement. With increasing shear displacement, sets of micron-sized minor shears develop within the main shear zone. These initiate, terminate and merge both in the plane and out-of-the-plane directions. The minor shears are characterized by distinct particle reorientation where the flaky clay particles and mica grains are aligned parallel to the direction of shear. A porosity reduction is observed in the minor shears. This is a direct observation of material contractancy and implies migration of water from the shear zone into the undisturbed matrix. The width of the main shear zone is considered to be related to the applied rate of shear while the minor shears have a width determined by the micro-texture of the sediment. The observed multi-scale structure of the shear zones, which is governed by complex inner mechanics, is challenging the ‘smooth shear zone’ concept that is commonly applied in numerical simulations. This is motivating a further improvement of such tools and the testing of other types of clay sediments.

Highlights • The structure and evolution of shear zones in sensitive ‘quick’ clays are assessed. • The study includes optical microscopy, SEM and high resolution X-ray tomography. • Shear zones are proposed to be formed in mm-thick continuous fields of displacement. • Micron-sized minor shears develop within the main shear zone. • A reduction of porosity is observed in the minor shears.