12. The Evolution of Material Properties Within an In Situ Shear Zone in Sensitive Clay

In this paper, we present the geotechnical properties and microstructure of a clay from Mosjøen, Northern Norway, where a landslide occurred in June 2011. The evolution of the clay properties at the depth of failure (i.e. shear zone) are studied based on an integrated set of samples, laboratory tests, CPTU data and piezometer readings. Results from SEM analyses show a general lowering of the macro porosity of the clay due to shearing during the landslide. As a consequence, the lower permeability within this zone slowed down the consolidation process when compared to the surrounding intact material. Despite of this, as pore pressure decreased from a situation with nearly zero effective stress to a situation close to the hydrostatic pore pressure, the shear resistance of the clay slowly increased; from its remoulded strength and towards values slightly higher than typically found for normally consolidated clays. It was also found that the peak strength envelope of the destructured clay was above the envelope of the intact material. The reasons could be attributed to the microstructure of the remoulded material and to its silty nature.