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
This version of the presentation is provided on condition be it be used only for individual and non-commercial purposes
E. Medley,

2. E. Medley, 2008 www.bimrocks.geoengineer.org
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,

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,

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,

5. E. Medley, 2008 www.bimrocks.geoengineer.org
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
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6. the major problem with bimrocks: WYTYSINWYG*
*what you think you see is not what you get
Matrix
Blocks, inclusions, lenses, etc
scale: 1:??????
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7. the seduction of straight lines..
Wrong!!
Right!
After Wakabayashi and Medley, 2004
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8. the misery of mischaracterizationBH
outcrop
“shallow landslide in soil over bedrock”
15 m excavation looking for the “failure plane”
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9. a nice and simple engineering relationship-5 5 10 15 20 25 30 40 60 80
100
Volumetric Block Proportion (%)
 Incr.. Friction Angle, degrees
Scott Dam melange
Physical models
Irfan and Tang, 1993
conservative trend
(Lindquist 1994a)
Strength of bimrock increases with volumetric block proportion
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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??
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11. how wrong can we be in assuming Linear = Volumetric Proportions?
Actual volumetric proportion is 32%
beware uncertainty in estimates of block proportion!!!
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12. or: type “Ed Medley” in Google Search
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:
or: type “Ed Medley” in Google Search
E. Medley,