HR: 0800h
AN: G11A-1180    [Abstracts]
TI: Grain characteristics of landslide breccias: the influence of grain size distribution, aspect ratio, grain size and composition on potential rockslope failure.
AU: * Henderson, I
EM: iain.henderson@ngu.no
AF: Geological Survey of Norway, Leif Eirikssons veg 39, Trondheim, 7491 Norway
AU: Venvik-Ganerod, G
EM: guri.venvik.ganerod@ngu.no
AF: Geological Survey of Norway, Leif Eirikssons veg 39, Trondheim, 7491 Norway
AB: This study presents the preliminary microstructural analysis of a nationwide study of potential rockslope failures in Norway. All of the potential rockslope failures examined have a similar geometry. These include i) a steep extensional fracture detaching the back edge of the failure block ii) a transfer fault parallel to the movement direction of the block down the fall line and iii) a shallow-angle detachment surface, or Basal Shear Zone (BSZ) along the base of the potential failure block. 15 sites have been examined in Norway as part of this study and all of these features are present. Slow-creep shearing on the BSZ has resulted in the formation of a fault breccia, or gouge, in all cases. 20 breccia and gouge samples have been collected from these BSZ's throughout Norway. The undeformed wall-rock to all of these breccias is granitic to granodioritic gneiss and consists of quartz, feldspar and variable amounts of white mica and biotite with grain sizes of 1-2mm. However, the result of mechanical breakdown along the BSZ has created a non-cohesive breccia averaging 50-80æm in grain size consisting of angular fragments of quartz and feldspar, crushed and milled biotite and white mica grains and the formation of clays from the chemical breakdown of feldspars. SEM paragenesis also demonstrates that these fault rocks have been brecciated several times and cemented by low temperature zeolites. Grain size distribution curves for these samples show a remarkable variation from extremely grain supported to extremely matrix supported. Both Ring Shear and Direct Simple Shear Strength tests have been carried out on the sample material. Comparison of the ring shear tests for the most matrix supported and most grain supported samples as two end members clearly shows that the grain supported samples have the highest residual strength whereas the most matrix supported samples have the lowest residual shear strength. Comparison of the residual shear strength with the amount of fine material in the sample (<64μm) demonstrates and inverse power law. As the amount of fine material increases, so reduces residual shear strength of the fault material. Residual shear strength varies from 450-700kPa, corresponding to 40-70m overburden on top og the BSZ. In most of the field cases this is a critical residual stregnth for potential failure. SEM image analysis has been carried out on all of the samples to determine average aspect ratio of the loose material. Aspect ratio, like grain size, displays a remarkable variation between samples and also between samples taken from different locations on the same fault zone. This suggests two things; that the different breccias have recorded a remarkably varied movement history during their development and that the variable development of these breccias has major consequences for the further stability of the rock slope. There is also an interesting relationship between grain size and grain aspect ratio across the whole of the sample series. As grain size decreases below 10-20μm, grain aspect ratio is reduced close to 1:1, suggesting that, through time, as the fault rock evolves, the grains become rounder. This study demonstrates that a wide variation in fault rock properties is developing on potential landslide breccias throughout Norway. The strength of these loose fault rocks underlying potential rockslope failures is, in part, dependent on the microscopic properties of the fault rock.
DE: 8175 Tectonics and landscape evolution
SC: Geodesy [G]
MN: Fall Meeting 2005