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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