Mannen unstable rock slope (Møre og Romsdal): Logging of borehole and drill core KH-01-10, geomorphologic digital elevation model interpretation and displacement analysis by terrestrial laser scanning

Abstract

Mannen is an unstable rock slope located in Romsdalen in Møre og Romsdal county. The most active part is located in the uppermost part of the slope and has an estimated volume of 2.5-3 Mm3 and moves with a velocity of approximately 4-5 cm\/year. Since 2009, the unstable rock slope at Mannen is under continuous monitoring by the Åknes\/Tafjord Beredskapssenter IKS.\rA 138 m deep, vertical cored borehole has been drilled in 2010 in the uppermost part of the Mannen rock slope. The aim of this coring was to characterize weakness zones that may explain the active deformation. This report presents the geological logging of the drill core and the geophysical logging of the borehole by an optical televiewer, which gives orientation of fractures and structures, as well as fracture frequency. Both the geological core logging and geophysical borehole logging resulted in the observation of many highly deformed and even crushed levels at several depths. Among them, the core interval between 57 and 81 m depth shows the highest density of severely crushed zones associated with fine-grained products, such as breccias or even clay-rich gouges. This interval is interpreted to be the subsurface expression of the main basal sliding surface.