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 emedley@geosyntec.com
AEG Annual Meeting, Las Vegas, Nevada
September, 2005
Melanges and Similar Bimrocks: Melanges and Similar Bimrocks 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.
Slide3:
Limestone Olistolith (Block) Embedded in an Irregularly
Foliated Matrix of Shale, Siltstone and Sandstone Egnatia Motorway, Greece Photo by Prof. Gunter Riedmueller
Slide4: 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
Slide5: Melange: shears negotiate blocks
Franciscan Complex melange: Franciscan Complex melange
Block/matrix contacts weakest: Block/matrix contacts weakest
Fault Rocks and Shear Rocks : Fault Rocks and Shear Rocks Riedmueller et al, 2001
Fault zones and
Shear zones may have blocks millimeters to 100s of meters wide:
Slide9: Mixture of weaker soil and strong blocks (decomposed granite at Hwy 50, California) DG: a weathered rock bimrock
Motivation: analysis of slopes in bimrocks: Motivation: analysis of slopes in bimrocks Photo: Exponent
What are the influences on slope stability in bimrocks?: What are the influences on slope stability in bimrocks? Block/matrix vol. proportion; matrix c,ϕ?? 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: One Approach: Perform simple analysis of increases in Factors of Safety for Slope Stability in a bimrock
Simple analysis of increases in Factors of Safety for Slope Stability: Simple analysis of increases in Factors of Safety for Slope Stability
Abstract failure surfaces, perform slope stability analyses: Abstract failure surfaces, perform slope stability analyses
Blocks increase slope stability: 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….): 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: Review failed physical model melanges 150 mm diameter Tx specimens (Lindquist, 1994) Lindquist, 1994; Medley, 2004
Failure surfaces in sectioned TX Specimens: Failure surfaces in sectioned TX Specimens Lindquist, 1994; Medley, 2004
Measure lengths of failure surfaces at circumference of specimens: 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: Profiles from many specimens Medley, 2004 15 cm not “joint roughness”
Calculate several parameters……: Calculate several parameters…… Medley, 2004 One ME measure of tortuosity
Lengthening of tortuous failure surfaces: Lengthening of tortuous failure surfaces Lengthening apparently not much sensitive to block proportion nor block orientation
Tortuosity (as ME measure “roughness”): Tortuosity (as ME measure “roughness”) Tortuosity ~ 5%-15% of characteristic dimension and more sensitive to block proportion than orientation??
Block contacts along tortuous surfaces: Block contacts along tortuous surfaces Conservative to assume block/shear surface (%) contact is ~ volumetric proportion (%)
Slope Analysis Cartoon: Slope Analysis Cartoon H
Slope Analysis Cartoon continued: Slope Analysis Cartoon continued
H 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: Conclusions Increases 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 hand
Use mechanical properties based on estimates of Volumetric Block Proportion (taking into account uncertainty)
Beware 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: Precedent: slope stability of Hong Kong Boulder Colluvium Hong Kong GEO TN 4/92
Trial tortuous failure surfaces with blocks oriented out-of-slope: 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: 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: Step 2: Create random arrays of blocks 50% 25% 13%
Step 3: Create Model Tortuous Failure Surfaces: Step 3: Create Model Tortuous Failure Surfaces 5m
Step 4: Perform slope stability analyses on the model tortuous failure surfaces: Step 4: Perform slope stability analyses on the model tortuous failure surfaces 50% block proportion