ABSTRACT

Geotechnical considerations are important in identifying the conditions leading to instability of seabed-offshore-structures under waves. It is necessary to model such soil behavior via theoretical frameworks. This paper summarizes the fundamentals of constitutive modeling of cyclic seabed behavior. Two main theories namely, bounding surface plasticity and generalized plasticity theory are introduced for both granular and cohesive soils. Modeling considerations to evaluate the elasto-plastic soil response in terms of governing equations of elemental behavior are presented. Finally, verification of theoretical results through classical tests is presented to demonstrate the accuracy of constitutive modeling of cyclic behavior of seabed around coastal structures.

INTRODUCTION

Cyclic loading in terms of waves or seismic shaking constitute a significant part of the design of coastal and offshore engineering systems. Geotechnical considerations are particularly important in identifying the conditions leading to instability of such seabed-structure systems under cyclic excitations. Among those, liquefaction, shear failure and excessive deformations are the most common causes of failure in marine environments. Here, rigidity as well as dynamic strength are among the most important soil properties to be determined to decide whether there will be permanent damage in the upper structures or in the surrounding seabed. Therefore, it is of utmost significance that cyclic loading-induced soil behavior is both measured in laboratory experiments and modeled through theoretical models. While the former have been carried out since 1960's, it had to take about two more decades for latter works to emerge. Since then, theoretical studies have focused on sands and sand-like granular soils as well as clays and clay-like cohesive soils. That is, the laboratory tests along with field observations concluded that these two ends of the spectrum of soil types behave quite differently under cyclic loadings and require specific theories for understanding the underlying mechanics leading to their failures.

Constitutive modeling of soil behavior around coastal and offshore structures is essentially not that different from the analysis of the theoretical modeling of soil under earthquake loading. While obviously the period ranges of excitation as well as the response of the soilstructure system is different in its core for both problems, as far as soil behavior is concerned, both require similar aspects of soil constitutive features. That is, cyclic soil behavior is the main element to be incorporated into the analyses with the simplifying assumption that both wave and seismic loading can be idealized as harmonic-time histories.

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