--> Utilization of Factor of Safety in Geotechnical Solutions

AAPG Eastern Section Meeting

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Utilization of Factor of Safety in Geotechnical Solutions

Abstract

The factor of safety concept is indispensable to engineering practitioners. Factor of safety is simply the ratio of capacity to load. A factor of safety of 1.0 means the approximation of load is equal to the approximation of capacity. Increasing factor of safety to 2.0 means the expected capacity is twice the expected load.

The concept is frequently misunderstood, misapplied and misrepresented. This latent confusion leads to excessive confidence in a design as well as excessive cost due to overdesign and unwarranted criticism of appropriate designs. Those who make decisions based on factor of safety need to understand what it means and what it does not mean.

Using engineering analysis, this paper demonstrates that a wide range of safety factors can be calculated for a given design problem, all of which could be considered reasonable. This paper will further review the costs associated with demanding an excessive factor of safety.

Two examples will be used to illustrate the associated costs of increasing the factor of safety. Engineering analysis will illustrate the design parameter changes that will need to be employed to increase each solution's factor of safety and the associated non-linear cost increase for each design. This paper will illustrate that 1) factor of safety alone is not sufficient to select an appropriate design 2) site conditions and risk assessments must also be considered to effect an appropriate solution for the specific application 3) factor of safety mandates can increase costs but may not increase safety.