1 Shear Resistance Of Biotite-Schist: Peak And Post- Peak Behaviour L.G. F. S. de Mello –MSc, Assistant Professor, University of São Paulo / Vecttor Projetos S/C Ltda, Brazil G.R. Sadowski –MSc, DSc, Professor, Institute of Geosciences of the University of São Paulo, Brazil C. M. Nieble – MSc, Matra Engenharia S/C Ltda,São Paulo, Brazil Abstract The knowledge of shear behavior of biotite-shist is of crucial importance for engineering purposes. There is few and sometimes controversial information published about this issue. A special investigative campaign was performed on biotite shist samples. The samples were extracted from quartz-biotite-shist bands, layered in-between fresh granite-gneiss, in Eastern Brazil. Shear resistance of this biotite-shist showed to be surprisingly low. After several special tests and investigations, it was concluded that this low shear resistance was related to the following causes: a) a very strong schistosity or textural anisotropy, due to intense quartz grain flattening around at least X/Y >=2:1 and strong laminar orientation of the micas; b) selective dissolution of interstitial quartz grains from the matrix due to differential weathering. The geomechanical consequence was an extremely low residual friction angle (< 15 o ) during solicitation. This may be explained as associated to an immediately previous negative dilation effect by collapse of the porous structure and local absence of ploughing resistance of the zones where quartz has been selectively removed... Introduction It is of common knowledge in geotechnical engineering that biotite seams in granites, gneisses, migmatites, schists and their saprolites have shown peculiar behaviour in some heavy construction sites (Horn and Deere, 1962; Knill and Jones, 1965; Jaeger, 1973a and b; Sjoeberg, 1993 etc.) . These seams with strong concentrations of biotite may be structures related directly to the original rock fabric or due to residual concentrations in saprolites of biotite bearing rocks through the selective leaching of other rock minerals such as plagioclase or K-feldspars. Primary or original concentrations may be found in biotite schists, banded biotite gneisses or even as “schlieren” (seams of concentrated dark minerals) in granites, showing different