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Engineering the Geological Chaos of Franciscan and Other Bimrocks

Engineering the Geological Chaos of Franciscan and Other Bimrocks. Edmund Medley, PhD, PE, CEG, F.ASCE Session 12: Melanges, Mixed Materials and Chaotic Rocks ARMA Conference 2008 San Francisco, CA Tuesday, July 1 2008.

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Engineering the Geological Chaos of Franciscan and Other Bimrocks

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  1. Engineering the Geological Chaos of Franciscan and Other Bimrocks Edmund Medley,PhD, PE, CEG, F.ASCE Session 12: Melanges, Mixed Materials and Chaotic Rocks ARMA Conference 2008 San Francisco, CA Tuesday, July 1 2008 This version of the presentation is provided on condition be it be used only for individual and non-commercial purposes E. Medley, 2008 www.bimrocks.geoengineer.org

  2. Franciscan melanges Melanges (mélanges) are mixtures of hard blocks in weaker matrix. In the Franciscan blocks are found at all scales of engineering interest. Franciscan Complex melange, Mendocino, California E. Medley, 2008 www.bimrocks.geoengineer.org

  3. blocks in Franciscan melanges • Block size distributions are scale independent and fractal (power law) • range in size between mountains and sand- blocks will always be found: • characterization must take blocks into account • Lc - characteristic dimension, scales rockmass • Lc - slope height, tunnel diameter, largest local block, etc. • What is block? What matrix? • smallest blocks: ≥0.05Lc • largest blocks: ≤0.75Lc E. Medley, 2008 www.bimrocks.geoengineer.org

  4. fundamental fabric: strong blocks, weak matrix Blocks and sheared matrix within San Andreas fault zone, northern California Corestones soil Decomposed granite, Hwy 50, Tahoe area, California E. Medley, 2008 www.bimrocks.geoengineer.org

  5. bimrocks • bimrocks:short for block-in-matrix rock: melanges, fault rocks, weathered rocks, etc. • geologically neutral term, no geological connotation. “mixtures of rocks composed of geotechnically significant blocks within a bonded matrix of finer texture” • “Geotechnically significant blocks ”: there are enough blocks to make a difference to mechanical properties of the rock mass E. Medley, 2008 www.bimrocks.geoengineer.org

  6. the major problem with bimrocks: WYTYSINWYG* *what you think you see is not what you get Matrix Matrix Blocks, inclusions, lenses, etc Matrix Matrix scale: 1:?????? E. Medley, 2008 www.bimrocks.geoengineer.org

  7. the seduction of straight lines.. Wrong!! Right! After Wakabayashi and Medley, 2004 E. Medley, 2008 www.bimrocks.geoengineer.org

  8. outcrop BH “shallow landslide in soil over bedrock” BH BH 15 m excavation looking for the “failure plane” the misery of mischaracterization E. Medley, 2008 www.bimrocks.geoengineer.org

  9. 30 Scott Dam melange 25 Physical models Irfan and Tang, 1993 20 Scott Dam melange 15 10  Incr.. Friction Angle, degrees 5 conservative trend 0 (Lindquist 1994a) -5 0 20 40 60 80 100 Volumetric Block Proportion (%) a nice and simple engineering relationship Strength of bimrock increases with volumetric block proportion E. Medley, 2008 www.bimrocks.geoengineer.org

  10. estimating block proportion from drilling Linear block proportion total length of block/boring intersections divided by total lengths of boring How accurate are linear block proportions relative to actual volumetric block proportion?? E. Medley, 2008 www.bimrocks.geoengineer.org

  11. how wrong can we be in assuming Linear = Volumetric Proportions? Actual volumetric proportion is 32% beware uncertainty in estimates of block proportion!!! E. Medley, 2008 www.bimrocks.geoengineer.org

  12. conclusions • Franciscan melanges share many characteristics with other rock/soil mixtures: hence “bimrocks” • Bimrocks are disorderly and often chaotic • We have to work with bimrocks • We can characterize chaos: simple methods are available: http://bimrocks.geoengineer.org or: type “Ed Medley” in Google Search E. Medley, 2008 www.bimrocks.geoengineer.org

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