Increases in Slope Stability Increases in Slope Stability of Rock/Soil Mixtures Due to of Rock/Soil Mixtures Due to Tortuosity of Failure Tortuosity of Failure Surfaces Around Rock Blocks Surfaces Around Rock Blocks Ed Medley and Pablo Sanz Rehermann Ed Medley and Pablo Sanz Rehermann Exponent Failure Analysis Associates, Menlo Park, CA Exponent Failure Analysis Associates, Menlo Park, CA GSA Conference Seattle WA November 2, 2003 GSA Conference Seattle WA November 2, 2003 VER 00G64000 0000 1201 EM63
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Increases in Slope Stability of Rock/Soil Mixtures Due to Tortuosity of Failure Surfaces Around Rock Blocks Increases in Slope Stability of Rock/Soil Mixtures.
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Increases in Slope Stability of Increases in Slope Stability of
Rock/Soil Mixtures Due to Rock/Soil Mixtures Due to Tortuosity of Failure Surfaces Tortuosity of Failure Surfaces
Around Rock BlocksAround Rock BlocksEd Medley and Pablo Sanz RehermannEd Medley and Pablo Sanz Rehermann
Exponent Failure Analysis Associates, Menlo Park, CAExponent Failure Analysis Associates, Menlo Park, CA
GSA Conference Seattle WA November 2, 2003GSA Conference Seattle WA November 2, 2003
Geological engineering Geological engineering problems of block-in-matrix problems of block-in-matrix
rocks (bimrocks)rocks (bimrocks)• Often severe spatial variabilityOften severe spatial variability
• Heterogeneity in propertiesHeterogeneity in properties
• Design and construction shortcomings of Design and construction shortcomings of geo-characterizations cost $$$$geo-characterizations cost $$$$
• Not readily analyzed by current rock Not readily analyzed by current rock mechanics OR soil mechanics techniquesmechanics OR soil mechanics techniques
• Not a hot research topicNot a hot research topic
Landslide in Franciscan Landslide in Franciscan melange…melange…
block-rich melange at toe resisted block-rich melange at toe resisted movementmovement
..leading to back-facing ..leading to back-facing scarpsscarps
Blocks added strength to Blocks added strength to matrix: but matrix: but HOW MUCH??HOW MUCH??
(a motivation for this (a motivation for this analysis)analysis)
Failed physical model melangesFailed physical model melanges150 mm diameter Tx specimens 150 mm diameter Tx specimens (Lindquist, 1994)(Lindquist, 1994)
Tortuosity of failure surfaces Tortuosity of failure surfaces influenced by low block influenced by low block proportion and vertical proportion and vertical
orientationorientation
Tortuosity influenced by high Tortuosity influenced by high proportion but horizontal proportion but horizontal
orientationorientation
Tortuosity influenced by Tortuosity influenced by Block Size Distribution Block Size Distribution (BSD) and block shapes(BSD) and block shapes
>120 multi-stage >120 multi-stage Triaxial tests Triaxial tests performed on 150mm performed on 150mm diameter specimens diameter specimens of model melangesof model melanges
-5
0
5
10
15
20
25
30
0 20 40 60 80 100
Volumetric Block Proportion (%)
Incr.. Friction Angle, degrees
Scott Dam melange
Physical modelsIrfan and Tang, 1993
conservative trend (Lindquist 1994a)
Scott Dam melange
Bimrock strength increases with Bimrock strength increases with block proportion (>~15%-25% block proportion (>~15%-25%
VVvv))
BUT……BUT……
•Q: How does one estimate Block Proportions and their uncertainty based on the usual tools of mapping and drilling???
•A: That is stereological story (already partly told …)
Complex Geologic Mixture Complex Geologic Mixture – – As Mapped, or as drilled As Mapped, or as drilled and coredand cored
Matrix
Matrix
Willis, 2000
ConclusionsConclusions
• Slope stability of a bimrock (as Slope stability of a bimrock (as indicated by FS) depends on block indicated by FS) depends on block proportionproportion
• Increases in FS are due to Increases in FS are due to increased tortuosity related to increased tortuosity related to orientation, shape, BSD, block orientation, shape, BSD, block proportion, etc.proportion, etc.
• Current 2D analytical tools are very Current 2D analytical tools are very limited for investigationlimited for investigation
• Need Monte Carlo-type analyses Need Monte Carlo-type analyses with 3D numerical modelingwith 3D numerical modeling