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Factor of Safety
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Foundatation Analysis by Bowels has good recommendations for safety factors. He evaluates
uncertainties and assigns a factor of safety by taking into account the following:
1. Magnitude of damages (loss of life and property damage)
2. Relative cost of increasing or decreasing the factor of safety
3. Relative change in probability of failure by changing the factor of safety
4. Reliability of soil data
5. Construction tolerances
6. Changes in soil properties due to construction operations
7. Accuracy (or approximations used) in developing design/ analysis methods
Typical values of customary safety factors, F.S., as presented by Bowels.
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Failure Mode
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Foundation Type |
F.S. |
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Shear
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Earthwork for Dams, Fills, etc.
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1.2 - 1.6
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Shear
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Retaining Walls
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1.5 - 2.0
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Shear
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Sheetpiling, Cofferdams
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1.2 - 1.6
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Shear
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Braced Excavations (Temporary)
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1.2 - 1.5
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Shear
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Spread Footings
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2 - 3
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Shear
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Mat Footings
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1.7 - 2.5
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Shear
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Uplift for Footings
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1.7 - 2.5
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Seepage
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Uplift, heaving
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1.5 - 2.5
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Seepage
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Piping
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3 - 5
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Other customary factors of safety, F.S., used are:
1.5 for retaining walls overturning with granular backfill
2.0 for retaining walls overturning with cohesive backfill
1.5 for retaining walls sliding with active earth pressures
2.0 for retaining walls sliding with passive earth pressures
Other soil and soil related properties are listed below:
Angle of Internal Friction
Bearing Capacity Factors
Cohesion
External Friction Angle
Factor of Safety
Lateral Earth Pressure Coefficients
Modulus of Vertical Subgrade Reaction
Soil Unit Weights
Young's Modulus or modulus of elasticity
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