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GLE/CEE 330: Soil Mechanics
Infinite Slope Analysis
Geological Engineering
University of Wisconsin-Madison
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Learning Objectives
• Learn basic analysis approach
• Describe example problems for total stress analysis (undrained) and effective stress analysis (drained)
• Discuss when infinite slope analysis may be appropriate
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Common Features of Slope Analysis Methods
• Shape and location of failure surface is not known a priori but assumed (trial and error to find minimum FS)
• Limit equilibrium analysis (static equilibrium)
• Safety factor analysis:
available shear strength of soil
equilibrium shear stress
• Two-dimensional analysis
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Infinite Slope Analysis• Translational failure along single failure plane with failure surface
parallel to slope surface.
• Ground water table parallel to slope surface
• Ratio of depth to failure surface to length of failure zone is small (<10%)
• Applies to surface raveling in granular materials or slab slides in cohesive materials (Translational Slides)
Assumed Failure Surface
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a= angle of failure (and slope) surfaceD = depth of failure surfacezw = height of GWT above failure surfaceW/b = Weight of element (per unit width)N/b = Normal force on failure surfaceT/b = Shear force on failure surfaceSL/b and EL/b = Shear and normal forces on leftSR/b and ER/b = Shear and normal forces on right (same as left)Can also account for seepage force
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Effective Stress Analysis (ESA)
For dry granular soil (zw = 0, c’ = 0):
For saturated granular soil (zw = D, c’ = 0):
Total Stress Analysis (TSA)
FS decreases with depth
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Example 1
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Example 2
Water has two effects:1) increases driving forces because unit weight increases2) decreases resisting forces because effective stress decreases