Rock-slope failures in Norway; type, geometry, deformation mechanisms and stability
pp. 67-88

Extensive studies of Norwegian rock-slope failure areas support a subdivision into three principle types: (1) Rockfall areas, (2) rockslide areas, and (3) complex fields. The classification is based on structural geometry and style of deformation, slope gradient, and the volumes involved. Rockfall areas, or toppling sites, are found in sub-vertical mountain sides. One or more unstable blocks are bound by a steep crevasse near the slope edge that is nearly cliff-parallel, whereas cliff-oblique extension fractures limit the blocks laterally. Rockslide areas are found on moderately dipping slopes, where slope-parallel, basal sliding planes, or detachments (along foliation, exfoliation surfaces), bound unstable blocks. The block size is controlled by steep fractures. Complex fields reveal complicated structural geometries and a rough morphology. Typical structures include a back-bounding graben, trenches and local depressions, fault scarps, crevasses, rotated fault blocks and deep-seated, low-angle detachments. They also involve significantly larger rock volumes (> 10 mill m3) than the other types. Complex fields can be subdivided into either listric or planar styles based on their internal fault geometry.

Several structural features are diagnostic for areas undergoing rock-slope deformation above a basal detachment. On a large scale, the areas consist of detached blocks resting on a low-angle fault (rock on rock) or fault-rock layer (membrane) above non-deformed bedrock. Fault rocks appear to be common. This is consistent with a two-layer model, with an upper layer of more or less fractured bedrock, and a lower detachment layer of noncohesive fault rocks that have mechanical properties more like soil (soft sediments). Development of non-cohesive fault breccia and gouge along a basal detachment, and especially if altered to clay, may drastically reduce the stability of a rock-failure area. Driving forces and deformation mechanisms in rock-slope failure areas can be evaluated from short-term factors, such as seismic activity, water pressure and/or frost-related processes. In addition, an important long-term factor is gradual change in mechanical properties of slide planes. High water pressure and/or frost wedging are presumably most important for present day rockfall areas. Gradual reduction in the shear resistance of a detachment layer in combination with water pressure and freeze-thaw processes are probably critical aspects of rockslide areas and complex fields. Seismic activity above a critical surface acceleration could cause the final triggering leading to an avalanche for all types of rock-slope failures.

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