# Do Simplistic Methods of Foundation Design Produce Reliable Foundations?

*by*Senro Kuraoka, Univ of Wisconsin-Madison, Madison, United States,

Peter J. Bosscher, Univ of Wisconsin-Madison, Madison, United States,

**Abstract:**Many foundations today are designed using deterministic (single value) analysis techniques which do not reflect the natural variability of soil properties. If the standard deviation and mean of the soil properties (internal friction angle and cohesion) are available in designing a foundation with a classical bearing capacity equation, statistical analyses may be used to obtain standard deviation, mean, and the probability density distribution (PDD) of the bearing capacity. This quantitative information may be useful in determining an appropriate safety factor. The PDD is especially helpful to describe the potential error in the allowed bearing capacity. This paper shows typical PDD's of the Meyerhof bearing capacity equation. Cases are noted where the PDD significantly deviates from the normal distribution. The allowable bearing capacity, q

_{99.5}, is computed such that the probability of random bearing capacity being less than q

_{99.5}is 0.5%. Safety factors(SF) are back calculated using q

_{ult}/q

_{99.5}, where q

_{ult}is computed with the mean soil properties. Although the analyses do not account for the stochastic variability and the errors due to the bearing capacity equation itself, these q

_{99.5}and the back calculated SF are useful information in minimizing risk in foundation design.

**Subject Headings:**Load bearing capacity | Soil analysis | Probability | Computer aided design | Safety | Density (material)

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