Observations on Chaotic Failure Surface Trajectories in Bimrocks (Block-in-Matrix Rocks) Observations on Chaotic Failure Surface Trajectories in Bimrocks (Block-in-Matrix Rocks) Dr. Edmund Medley, PE, CEG GeoSyntec Consultants, Oakland, California AEG Annual Meeting, Las Vegas, Nevada September, 2005

Melanges and Similar Bimrocks Bimrocks: block-in-matrix rocks Bimrocks: block-in-matrix rocks – mixtures of rocks composed of geotechnically significant blocks within a bonded matrix of finer texture – Often severe spatial variability and mechanical/lithological heterogeneity: cost $$$$ – e.g.: melanges, fault rocks, weathered 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 Slope Bolu Tunnel, Turkey Shears negotiate around blocks tortuously Not smooth rotational “failure surfaces” but chaotic trajectories

Melange: shears negotiate blocks tortuous shear

shears Franciscan Complex melange

Block/matrix contacts weakest

Fault Rocks and Shear Rocks Riedmueller et al, 2001 Fault zones and Shear zones may have blocks millimeters to 100s of meters wide: BLOCK SIZE DISTRIBUTIONS TEND TO BE SCALE- INDEPENDENT 5m, 5km,..

Mixture of weaker soil and strong blocks (decomposed granite at Hwy 50, California) DG: a weathered rock bimrock soil Corestones, blocks, boulders

Motivation: analysis of slopes in bimrocks Photo: Exponent

What are the influences on 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 analysis of increases in Factors of Safety for Slope Stability in a bimrock matrix-only failure surface

Simple analysis of increases in Factors of Safety for Slope Stability matrix-only failure surface tortuous failure surfaces

Abstract failure surfaces, perform slope stability analyses FS ~ 1.65

Blocks increase slope stability 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 characterize the actual block distribution in bimrocks, so we can never predict the actual trajectories of tortuous failure surfaces in slopes! (as now shown….)

Review failed physical model melanges 150 mm diameter Tx specimens (Lindquist, 1994) failure surfaces tortuously negotiate blocks Lindquist, 1994; Medley, 2004

Failure surfaces in sectioned TX Specimens Lindquist, 1994; Medley, 2004

Measure lengths of failure surfaces at circumference of specimens Medley, 2004 trace of matrix-only failure surface actual failure surface Contact between block and failure surface

Profiles from many specimens Medley, cm not “joint roughness”

Calculate several parameters…… Medley, 2004 One ME measure of tortuosity

Lengthening of tortuous failure surfaces Lengthening apparently not much sensitive to block proportion nor block orientation

Tortuosity (as ME measure “roughness”) Tortuosity ~ 5%-15% of characteristic dimension and more sensitive to block proportion than orientation??

Block contacts along tortuous surfaces Conservative to assume block/shear surface (%) contact is ~ volumetric proportion (%)

Slope Analysis Cartoon H

Slope Analysis Cartoon continued 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)

Conclusions Increases in tortuosity are most influenced by block volumetric proportionIncreases in tortuosity are most influenced by block volumetric proportion Rather than analyzing individual tortuous failure surfaces in bimrocks, analyze trial failure zones with a width 5% to 15% of slope height or other characteristic dimension scaling the problem at handRather than analyzing individual tortuous failure surfaces in bimrocks, analyze trial failure zones with a width 5% to 15% of slope height or other characteristic dimension scaling the problem at hand Use mechanical properties based on estimates of Volumetric Block Proportion (taking into account uncertainty)Use mechanical properties based on estimates of Volumetric Block Proportion (taking into account uncertainty) Beware consistent adverse block orientations that result in adverse shear fabricBeware consistent adverse block orientations that result in adverse shear fabric

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

Precedent: slope stability of Hong Kong Boulder Colluvium Hong Kong GEO TN 4/92

Trial tortuous failure surfaces with blocks oriented 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 Step 1: matrix critical failure surface Φ = 25º c = 10 kPa FS ~ 1.26 Using Slope/W

Step 2: Create random arrays of blocks 50% 25% 13%

Step 3: Create Model Tortuous Failure Surfaces Areal block % = Vol. block % horizontal rectangular blocks BSD = partial Franciscan Random arrays 5m trial tortuous trial surface

Step 4: Perform slope stability analyses on the model tortuous failure surfaces FS ~ % block proportion