GLE/CEE 330: Soil Mechanics

Infinite Slope Analysis

Geological Engineering

University of Wisconsin-Madison

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

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

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

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

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

Example 1

Example 2

Water has two effects:1) increases driving forces because unit weight increases2) decreases resisting forces because effective stress decreases