Observations on Chaotic Failure Surface Trajectories in Bimrocks (Block-in-Matrix Rocks)...

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

emedley@geosyntec.comemedley@geosyntec.com

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 $$$$$$$$

– e.g.: melangese.g.: melanges, fault rocks , weathered , fault rocks , weathered rocks, etc.rocks, etc.

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Limestone Olistolith (Block) Embedded in an Irregularly Foliated Matrix of Shale, Siltstone and Sandstone

Egnatia Motorway, Greece Photo by Prof. Gunter Riedmueller

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Slope debris

Sandstone

Marble / Shale

Tectonic Melange

Typical Melange Showing Diverse Elongate Blocks and Irregular Foliated Matrix (S-M-C-Cataclasites)

(Photo C)

Melange Fabric in a SlopeMelange Fabric in a Slope

Bolu Tunnel, Turkey

•Shears negotiate around blocks tortuously •Not smooth rotational “failure surfaces” but chaotic trajectories

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Melange: shears negotiate blocks

tortuous shear

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shears

Franciscan Complex Franciscan Complex melangemelange

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Block/matrix contacts Block/matrix contacts weakestweakest

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Fault RocksFault Rocksand Shear Rocksand Shear Rocks

Riedmueller et al, 2001

Fault zones andShear zones may have blocks millimeters to 100s of meters wide:

BLOCK SIZE DISTRIBUTIONS TEND TO BE SCALE-INDEPENDENT5m, 5km,..

5m, 5km,..

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Mixture of weaker soil and strong blocks (decomposed granite at Hwy 50, California)

DG: a weathered rock DG: a weathered rock bimrockbimrock

soil

Corestones, blocks, boulders

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Motivation: analysis of slopes in Motivation: analysis of slopes in bimrocksbimrocks

Photo: Exponent

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What are the influences on What are the influences on slope stability in bimrocks?slope stability in bimrocks?

Block/matrix vol. proportion; matrix c,ϕ??

Block shape, block&shear orientation??

Block size, location, orientation?? Bimrock weak zones: width? variability??

Medley & Sanz, 2004

i.e: really complex problem – how then should we analyze slope stability in

bimrocks??

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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

matrix-only failure surface

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Simple analysis of increases in Simple analysis of increases in Factors of Safety for Slope Factors of Safety for Slope StabilityStability

matrix-only failure surface

tortuous failure surfacestortuous failure surfaces

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Abstract failure surfaces, Abstract failure surfaces, perform slope stability perform slope stability

analysesanalyses

1.760

FS ~ 1.65

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Blocks increase slope Blocks increase slope stabilitystability

Q: How do blocks add to slope stability?

A: Tortuosity of failure surfaces negotiating blocks (little to do with block strength)

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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!

(as now shown….)(as now shown….)

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Review failed physical model Review failed physical model melangesmelanges

150 mm diameter Tx specimens 150 mm diameter Tx specimens (Lindquist, 1994)(Lindquist, 1994)

failure surfaces tortuously negotiate blocksfailure surfaces tortuously negotiate blocks

Lindquist, 1994; Medley, 2004

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Failure surfaces in sectioned TX Failure surfaces in sectioned TX SpecimensSpecimens

Lindquist, 1994; Medley, 2004

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Measure lengths of failure Measure lengths of failure surfaces at circumference of surfaces at circumference of specimensspecimens

Medley, 2004

trace of matrix-only failure surface

actual failure surface

Contact between Contact between block and failure block and failure surfacesurface

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Profiles from many Profiles from many specimensspecimens

Medley, 2004

15 cm

not “joint roughness”

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Calculate several parameters……Calculate several parameters……

Medley, 2004

One ME measure of tortuosity

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Lengthening of tortuous failure Lengthening of tortuous failure surfacessurfaces

Lengthening apparently not much sensitive to block proportion nor block orientation

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Tortuosity (as ME measure Tortuosity (as ME measure “roughness”)“roughness”)

Tortuosity ~ 5%-15% of characteristic dimension and more sensitive to block proportion than orientation??

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Block contacts along tortuous Block contacts along tortuous surfacessurfaces

Conservative to assume block/shear surface (%) contact is ~ volumetric proportion (%)

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Slope Analysis CartoonSlope Analysis Cartoon

H

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Slope Analysis Cartoon Slope Analysis Cartoon continuedcontinued

H

a trial failure zone 0.05H to 0.15H thick

c, γ, φ of bimrock (α Vol. Prop. Blocks)

c, γ, φ of failure zone dependent on block/shear contact ratio (α Vol. Prop

Blocks)

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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

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Determinaistic Investigation Determinaistic Investigation of the Mechanical Effect of of the Mechanical Effect of Tortuosity at Slope ScaleTortuosity at Slope Scale

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Precedent: slope stability of Precedent: slope stability of Hong Kong Boulder ColluviumHong Kong Boulder Colluvium

Hong Kong GEO TN 4/92

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Trial tortuous failure Trial tortuous failure surfaces with blocks surfaces with blocks oriented out-of-slopeoriented out-of-slope

After Irfan & Tang, 1993

Inclined blocks, all same size,

regular array, spacing, etc.

Critical failure surface without blocks and model failure surface with blocks

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Try Improving Hong Kong Approach Try Improving Hong Kong Approach Step 1: matrix critical failure surfaceStep 1: matrix critical failure surface

Φ = 25º c = 10 kPa

FS ~ 1.26

1.263

Using Slope/W

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Step 2: Step 2: Create random arrays of Create random arrays of blocksblocks

50%

25%

13%

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Step 3: Create Model Step 3: Create Model Tortuous Failure SurfacesTortuous Failure Surfaces

Areal block % = Vol. block %

horizontal rectangular blocks

BSD = partial Franciscan

Random arrays5m

trial tortuous trial surface

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Step 4: Perform slope stability Step 4: Perform slope stability analyses on the model tortuous analyses on the model tortuous

failure surfacesfailure surfaces1.760

FS ~ 1.65

50% block proportion50% block proportion