Conventional and Probabilistic Embankment Design

The safety of a proposed highway embankment is evaluated both conventionally and probabilistically. Results from tests on undisturbed soil samples, secured during an extensive exploration program, are statistically analyzed and presented. The factor of safety is introduced as a random variable whose variability is due exclusively to the variability of the shear strength of the foundation material. Failure is defined as the event whereby the factor of safety (computer at end-of-construction) receives values smaller than or, at most, equal to unity. The probability of failure is defined as the probability of the occurrence of this event. From the three models (normal, beta, and lognormal) considered for the distribution of the factor of safety, the normal always provides the largest, and the lognormal the smallest values for the probability of failure of the embankment. This difference in the results decreases as the height or angle of the embankment increases. Results obtained using the beta model are presented in a nomography relating embankment height and slope angle to probability of failure.