Probabilistic Study of Offshore Monopile Foundations Considering Soil Spatial Variability

Abstract

The objective of this paper is to investigate how the spatially varied soil stiffness affects the ultimate lateral load and failure mechanism of monopiles embedded in soft clay. Responses of a laterally loaded monopile in spatially varied soils were simulated by a three-dimensional finite-element analysis combined with random field theory. The influence of the coefficient of variation (COVE) and the correlation distance (δz/lL) regarding the stiffness parameters was investigated. Results revealed that the spatial structure has an impact on stiffness distribution and leads to a significant discrepancy between the ultimate load of the spatially random soil and the uniform soil. The assumption of uniformity in stiffness yields an overestimation of the ultimate load and an excessively high cost of construction. Both the COVE and δz/lL intensify the discrepancy and increase the probability of the foundation exceeding the ultimate limit states. Due to the spatial pattern of soil, the failure plane exhibits an unsymmetrical pattern and an irregular shape in random soil instead of a symmetrical shape in uniform soil. The results provide a better explanation of the failure mechanisms of monopile foundations and reveal the importance of considering the reliability indices in the designing process.