Observations on Chaotic Observations on Chaotic Failure Surface Failure Surface Trajectories in Bimrocks Trajectories in Bimrocks (Block-in-Matrix Rocks) (Block-in-Matrix Rocks) Dr. Edmund Medley, Dr. Edmund Medley, PE, CEG PE, CEG GeoSyntec Consultants, Oakland, California GeoSyntec Consultants, Oakland, California [email protected][email protected]AEG Annual Meeting, Las Vegas, Nevada AEG Annual Meeting, Las Vegas, Nevada September, 2005 September, 2005
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Observations on Chaotic Failure Surface Trajectories in Bimrocks (Block-in-Matrix Rocks) Observations on Chaotic Failure Surface Trajectories in Bimrocks.
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Observations on Chaotic Observations on Chaotic
Failure Surface Trajectories Failure Surface Trajectories in Bimrocks (Block-in-Matrix in Bimrocks (Block-in-Matrix
Rocks)Rocks)
Dr. Edmund Medley,Dr. Edmund Medley, PE, CEGPE, CEG
GeoSyntec Consultants, Oakland, California GeoSyntec Consultants, Oakland, California
AEG Annual Meeting, Las Vegas, NevadaAEG Annual Meeting, Las Vegas, Nevada
September, 2005September, 2005
Melanges and Similar Melanges and Similar BimrocksBimrocks• Bimrocks:Bimrocks: block-in-matrix rocks block-in-matrix rocks
– mixtures of rocks composed of mixtures of rocks composed of geotechnically significant blocks within geotechnically significant blocks within a bonded matrix of finer texturea bonded matrix of finer texture
– Often severe spatial Often severe spatial variabilityvariability and and mechanical/lithological heterogeneity: mechanical/lithological heterogeneity: cost cost $$$$$$$$
One Approach: Perform simple One Approach: Perform simple analysis of increases in Factors of analysis of increases in Factors of Safety for Slope Stability in a Safety for Slope Stability in a bimrockbimrock
BUT: We cannot even BUT: We cannot even characterize the actual block characterize the actual block distribution in bimrocks, so distribution in bimrocks, so
we can we can nevernever predict the predict the actual trajectories of actual trajectories of
tortuous failure surfaces in tortuous failure surfaces in slopes!slopes!
ConclusionsConclusions•Increases in tortuosity are most influenced by Increases in tortuosity are most influenced by block volumetric proportionblock volumetric proportion
•Rather than analyzing individual tortuous failure Rather than analyzing individual tortuous failure surfaces in bimrocks, analyze trial surfaces in bimrocks, analyze trial failure zonesfailure zoneswith a width with a width 5% to 15%5% to 15% of slope height or other of slope height or other characteristic dimension scaling the problem at handcharacteristic dimension scaling the problem at hand
•Use mechanical properties based on estimates of Use mechanical properties based on estimates of Volumetric Block Proportion Volumetric Block Proportion (taking into account (taking into account uncertainty)uncertainty)
•Beware Beware consistent adverse block orientations consistent adverse block orientations that result in adverse shear fabricthat result in adverse shear fabric
Determinaistic Investigation Determinaistic Investigation of the Mechanical Effect of of the Mechanical Effect of Tortuosity at Slope ScaleTortuosity at Slope Scale