The behaviour of an unstable rock slope is largely controlled by pre-existing bedrock structure. Understanding the dominant controlling structures of a slope prior to failure increases the accuracy of volume and runout predictions of failure scenarios. This is particularly important in mountainous Norway, where impact and secondary hazards from catastrophic failure of unstable slopes pose a serious risk. Limited studies exist which link inherited weaknesses in the bedrock of the unstable slopes in northern Norway with rupture formation, deformation processes and progressive damage. We identify two rockslides in in Troms County, where the bedrock is dominated by complex ductile fabrics resultant from the Caledonian orogeny. Both slopes exhibit cataclinal foliation which displays a lower than ideal dip angle for sliding. We identify thin lenses within the rockslides' host rock which display evidence of ductile shear processes. These are weaker than the surrounding rock mass, and are preferentially weathered. Examination of the petrography of the zones reveals mineral alignment consistent with Caledonian-era shearing, suggesting the zones represent areas of strain localisation during thrusting and nappe emplacement. The fabric has allowed a present day preferential strength degradation largely a result of groundwater fluid channelisation. The zones also contain lenses of fine grained sand, silt and some clay. Residual strength testing of the soil shows a shear strength of between 17 and 18.5°, equal to or higher than the in the host rock dip angle. We propose that, if present at depth in the slope, these zones combined with the gravitational forcing effect from the active upper portion of the rockslide may enable sliding. We also observe groundwater partitioning and seepage along the zones, leading to a lower effective shear strength along the foliation during a pore pressure event which can control slope stability.
The Role of Lithological Weakness Zones in Rockslides in Northern Norway
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Vick, L. M., Bergh, S. G., Höpfl, S., Percival, J., and E. B. Daines. "The Role of Lithological Weakness Zones in Rockslides in Northern Norway." Paper presented at the ISRM International Symposium - EUROCK 2020, physical event not held, June 2020.
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