Calibration of Undrained Shear Strength Probability Density Function of an Offshore Clay Profile in the Gulf of Mexico using CPT Data and Slope Stability Analyses

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4. Calibration of Undrained Shear Strength Probability Density Function of an Offshore Clay Profile in the Gulf of Mexico using CPT Data and Slope Stability Analyses

This paper describes the implementation of a Bayesian calibration method in slope stability analysis to develop more accurate values of undrained shear strength. Typically, one-dimensional and two-dimensional limit equilibrium analysis methods are used to calculate the factor of safety of clay slopes located upslope of planned deepwater seafloor infrastructure. The two most relevant input parameters for these analyses are the unit weight and the undrained shear strength. The unit weight and undrained shear strength can be obtained from direct measurements in the laboratory. These parameters can be characterized using probability density functions that incorporate uncertainty. When these probability density functions are used to calculate factors of safety, the results can sometimes be illogical, particularly for values of shear strength lower than the mean, indicating values lower than unity for slopes that are evidently stable. This suggests that the probability density function may include unreasonable parameter values. The Bayesian method is therefore used to adjust the probability density function to yield more accurate values of the undrained shear strength. Herein, stratigraphic information from the subbottom profiler was used to identify slopes without prior indications of slides (i.e., stable slopes) and implement that information to modify the prior distribution of the undrained shear strength that was estimated from cone penetration test data. The posterior probability distribution function was then calculated using likely factors of safety for stable slopes.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

2 - Site characterization, in-situ and laboratory testing, measurement