Grain-scale deformation mechanisms and evolution of porosity in experimentally deformed Boom Clay

Boom Clay is a soft, slightly overconsolidated, uncemented claystone considered as potential host material for a radioactive waste repository in Belgium. We studied the evolution of microfabrics in samples which were shortened to 20% bulk strain in consolidated-undrained (CU) triaxial experiments at effective confining pressures of 0.375, 0.750 and 1.5 MPa, respectively. Results show a geomechanical behavior in agreement with previous studies, with total strain partly localized in shear zones and partly diffuse outside the shear zones. The diffuse strain is accommodated by pore compaction without any discernible microstructural changes compared to the starting material. In the shear zones, pore collapse reduces SEM-visible porosity and further deformation mechanisms within the shear zones are particulate flow (grain boundary sliding), particle rotation and the formation of microcracks. There is no evidence for comminution of quartz and feldspar grains. Strain localization on macro- and mesoscale is governed by viscosity contrasts between harder clasts (e.g. quartz and feldspar) embedded in a soft, porous, phyllosilicate-rich matrix. Our microstructures are comparable to those observed in Boom Clay deformed naturally and in the excavation damaged zone of the underground research facility. This suggests that our results are representative of Boom Clay's geomechanical behavior at the microscale.