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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
[email protected]@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.
Limestone Olistolith (Block) Embedded in an Irregularly Foliated Matrix of Shale, Siltstone and Sandstone
Egnatia Motorway, Greece Photo by Prof. Gunter Riedmueller
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
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,..
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
Motivation: analysis of slopes in Motivation: analysis of slopes in bimrocksbimrocks
Photo: Exponent
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??
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
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
Abstract failure surfaces, Abstract failure surfaces, perform slope stability perform slope stability
analysesanalyses
1.760
FS ~ 1.65
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)
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….)
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
Failure surfaces in sectioned TX Failure surfaces in sectioned TX SpecimensSpecimens
Lindquist, 1994; Medley, 2004
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
Profiles from many Profiles from many specimensspecimens
Medley, 2004
15 cm
not “joint roughness”
Calculate several parameters……Calculate several parameters……
Medley, 2004
One ME measure of tortuosity
Lengthening of tortuous failure Lengthening of tortuous failure surfacessurfaces
Lengthening apparently not much sensitive to block proportion nor block orientation
Tortuosity (as ME measure Tortuosity (as ME measure “roughness”)“roughness”)
Tortuosity ~ 5%-15% of characteristic dimension and more sensitive to block proportion than orientation??
Block contacts along tortuous Block contacts along tortuous surfacessurfaces
Conservative to assume block/shear surface (%) contact is ~ volumetric proportion (%)
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)
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
Precedent: slope stability of Precedent: slope stability of Hong Kong Boulder ColluviumHong Kong Boulder Colluvium
Hong Kong GEO TN 4/92
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
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
Step 2: Step 2: Create random arrays of Create random arrays of blocksblocks
50%
25%
13%
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
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
