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April 2006 |
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Discussion of Slip Surface Exit Angles in a Limit Equilibrium Analysis
Ever since version 5.13 of SLOPE/W was released (December, 2002), trial slip surfaces
analyzed with exit angles greater than 45° have been considered invalid.
Since this time, some engineers have wondered why this restriction was implemented.
In response, we have written a detailed technical note to discuss the theoretical
basis behind the enhancement.
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The underlying rationale to limit the exit angle is based on earth pressure theory. In a slope stability analysis the toe (or exit area) is analogous to a zone of passive pressure conditions, which should have a maximum inclined angle of around (45°-Ø/2). If Ø=0, the maximum angle would therefore be 45°.
One of the main concerns prior to the implementation of the exit angle restriction, was that it became possible for physically inadmissible slopes to be presented as the critical slip surface with very low factors of safety. Without looking beyond the factor of safety, it became difficult to make a sound engineering decision about why the presented slip surface should be dismissed and another accepted as being more reasonable.
The technical note outlines the various theoretical reasons for restricting the maximum exit angle and encourages engineers to thoroughly examine all the slip surface information presented in a slope stability analysis and to look beyond the computed factor of safety.
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