Characterisation of natural and remoulded Onsøy clay with focus on the influence of mica

Abstract

Loss of strength caused by the destructuration of sensitive soils has been identified as the trigger of numerous landslides, some of them of devastating proportions. Norwegian sensitive clay deposits are typically found with a mineralogical composition of inert rock flour, including clay-sized chlorite and dioctahedral mica. The present paper presents a cross-scale study conducted on a sensitive clay from Norway with special focus on the role of mica on the material characteristics. Block samples of Onsøy clay obtained from different depths (4–10 m) were characterised and studied in terms of their mechanical properties, fabric and mineralogy. Oedometer compression tests were carried out in a high-pressure oedometer, applying stress up to 25 MPa on samples in an undisturbed and remoulded state. Microstructural investigations were carried out on undisturbed and remoulded samples using cryo-broadionbeam scanning electron microscopy (cryo-BIB-SEM). Mineral composition was analysed using X-ray diffraction on the bulk samples and the fraction ≤2 μm confirming absence of expandable clay minerals (smectites) and kaolinite. Coarse silt sized and irregular shaped particles were dominated by Na-rich and Ca-rich feldspar, quartz, and framboidal pyrite aggregates. A comparison with data sets from several Norwegian sensitive clay deposits showed that the plasticity index of these clays showed a tendency to decrease with an increasing amount of mica when the salinity is lower than 10 g/l. In contrast, for deposits with a salt concentration of more than 10 g/l, a significant increase in the plasticity index was observed with increasing amount of mica. The formation of aggregates was discussed with relation to its impact on the soil mechanical behaviour-