Slopes - Geotechnical Investigation and Design Tables - page 172

significant volume of soil mass can be mobilised in surficial failures, and surficial
does not necessarily mean a small slide.

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Deep seated failures use both translational and rotational slope stability analysis.

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Water is involved in most of the above factors that cause instability.

14.4 Factors of safety for slopes
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The factor of safety is the ratio of the restoring over the activating condition.

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The condition may be forces or moments being analysed.

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Moment equilibrium is generally used for the analysis of rotational slides. Circular
slip surfaces are analysed.

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Force equilibrium is generally used for rotational or translational slides. Circular,
plane, wedge or polygonal slip surfaces may be analysed.

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The requirement for different factors of safety depending on the facility and its
affect on the environment.

Table 14.4 Factor of safety dependency.

Variable Effect on Factor of safety Comment

Strength Lower quartile should be typically Mean values should not be

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Lowest value used. Higher or lower should have used due to the non

Lower quartile corresponding changes on acceptable normality of soil and rock •

Median factor of safety. strength parameters. •

Geometry Higher slopes at a given angle would be Benching also useful to reduce

Height more unstable than a low height slope. erosion, provides rock trap area, •

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Slope Dip of weakness plane towards and as a maintenance platform.

Benching slope face influences result.

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Stratification/
Discontinuities
Load

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Water is the most significant variable The weight acts both as an

Weight in design. Buoyant unit weight then applies activating and restoring force. •

Surcharge at critical lower stabilizing part of slope, •

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Water Conditions i.e. soil above is heavier than soil below.

Analytical methods Different methods (and some software Probability of failures/

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Method of slices programs) give different outputs for displacement criteria should

Wedge methods the same data input. Moment equilibrium also be considered in critical

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and force equilibrium methods can cases. Factor of safety for 3 -

sometimes produce different results, dimensional effect

∼

15%

especially with externally applied loads. greater than 2-D analysis.

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Choice of factor of safety also depends on quality of available geotechnical infor-
mation and choice of parameters, i.e. worst credible to probabilistic mean, or
conservative best estimate.

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Temporary works may use reduced factors of safety.

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Critical areas projects would use higher factors of safety.

14.5 Factors of safety for new slopes
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New slopes have a higher factor of safety applied as compared with existing slopes.