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Factor of Safety for Geotechnical Engineering

Safety Factor

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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.

Failure Mode

Foundation Type



Earthwork for Dams, Fills, etc.

1.2 - 1.6


Retaining Walls

1.5 - 2.0


Sheetpiling, Cofferdams

1.2 - 1.6


Braced Excavations (Temporary)

1.2 - 1.5


Spread Footings

2 - 3


Mat Footings

1.7 - 2.5


Uplift for Footings

1.7 - 2.5


Uplift, heaving

1.5 - 2.5



3 - 5


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
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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