Laboratory model tests on drag and dynamically embedded plate anchors in layered clay profiles
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Layered soil profiles can complicate the ability to install a foundation and affect the performance of a foundation and therefore increase the cost of an offshore foundation. The goal of this research is to explore concepts to improve anchor performance in layered soil profiles of clay by further understanding (1) the drag trajectory and capacity for a drag embedded plate anchor in layered soil profiles; (2) the drag trajectory and capacity of tandem drag embedded plate anchor systems; and (3) the free-fall penetration trajectory and resistance of a dynamically embedded plate anchor in layered soil profiles. The methodology is to perform drag embedment and free-fall penetration tests with model anchors in a variety of test beds containing marine clays with different profiles of undrained shear strength versus depth. For a drag embedment anchor, model tests with a single anchor in different sizes show that the ratio of normal to tangential displacement is affected by model scale, with smaller ratios applying to larger anchors. By appropriately adjusting this factor in a prediction model, the model is able to represent well the model test results and can be used rationally to predict behavior for a field-scale anchor. The drag embedment tests and analytical results in layered soil profile show that anchor can penetrate into stiff layer with shear strength 1.5 times higher than the surrounding soil and cannot penetrate into stiff layer with shear strength 5 times higher than the surrounding soil. In the layered soil profile with stiff layer of shear strength 2 to 4 times higher than the surrounding soil, the anchor may be able to penetrate the stiff layer if the pitch of the fluke as it approaches the layer is at about 30°. For a tandem system with two drag embedded anchors, the capacity of the system can be more than twice the capacity of a single anchor provided that the spacing between two anchors is at least two fluke lengths. The second anchor in the tandem system can penetrate deeper than a single anchor and the front anchor. For a dynamically embedded plate anchor, the strain rate effect from undrained shearing is higher than that from bearing as measured from pure normal and pure in-plane shearing tests. A calibrated predication model accounting for the strain-rate effects strain rate parameters produces results similar to the model test results, generally matching or slightly under-predicting the actual penetration in non-layered and layered soil profiles. A dynamically embedded plate anchor can penetrate through stiff layers that would cause difficulty for a conventional drag embedded anchor.