Section 2: Analysis and Design

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Global Stability Analysis

Use the following data to analyze global stability of a slope:

For global stability of a slope, a minimum factor of safety of FS ≥ 1.3 is required for both the long-term drained condition and the short term undrained condition, except make the factor of safety 1.5 for slope or walls that support abutment, buildings, critical utilities, or for other installations with a low tolerance for failure.

Experience has shown that most exposed side slopes failures begin as shallow slides and then deepen with time. With this as a guide, the following table was developed to determine the recommended upper limit on the Plasticity Index for various slope conditions to maintain a factor of safety of 1.3 for the long term or drained soil conditions using an infinite slope analysis accounting for seepage of water parallel to face of slope.

Anchor: #i1015377Table 7-1: Plasticity Index Range for Exposed Side Slopes Required for FS =1.3 for the Long Term or Drained Condition



Plasticity Index

(PI) (%)

2.5 to 1

< 5

3.0 to 1

< 20

3.5 to 1

< 35

4.0 to 1

< 55

4.5 to 1

< 85

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