Engineering the Geological Chaos of Franciscan and Other Bimrocks
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Transcript of 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
This version of the presentation is provided on condition be it be used only for individual and non-commercial purposes
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Franciscan melanges
Franciscan Complex melange, Mendocino, California
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Melanges (mélanges) are mixtures of hard blocks in weaker matrix.
In the Franciscan blocks are found at all scales of engineering interest.
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• 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
blocks in Franciscan melanges
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fundamental fabric: strong blocks, weak matrix
Blocks and sheared matrix within San Andreas fault zone, northern California
soil
Corestones
Decomposed granite, Hwy 50, Tahoe area, California
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
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the major problem with bimrocks: WYTYSINWYG*
Blocks, inclusions, lenses, etc
Matrix
Matrix
Matrix
Matrix
scale: 1:??????
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*what you think you see is not what you get
the seduction of straight lines..
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Wrong!!
Right!
After Wakabayashi and Medley, 2004
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the misery of mischaracterization
BH
BH
BH
outcrop“shallow landslide in soil
over bedrock”
15 m excavation looking for the “failure plane”
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a nice and simple engineering relationship
-5
0
5
10
15
20
25
30
0 20 40 60 80 100
Volumetric Block Proportion (%)
Incr
.. F
ric
tio
n A
ng
le, d
egre
es Scott Dam melange
Physical modelsIrfan and Tang, 1993
conservative trend (Lindquist 1994a)
Scott Dam melange
Strength of bimrock increases with volumetric block proportion
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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??
estimating block proportion from drilling
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how wrong can we be in assuming
Linear = Volumetric Proportions?Actual volumetric proportion is 32%
beware uncertainty in estimates of block proportion!!!
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
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