Graduate Thesis Or Dissertation
 

Uplift capacity and displacement of helical anchors in cohesive soil

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kh04dt979

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  • Helical anchors are a type of deep foundation element that can be installed quickly in almost any location and can accept the immediate application of operational loads. The use of helical anchors has expanded in recent decades from its established application in the power transmission industry to more traditional civil engineering applications such as residential construction, communication tower installations, and static and seismic structural retrofitting and reconstruction. Despite the wide range of helical anchor applications, few advances have been made in improving the understanding of their behavior. For example, existing helical anchor design methods, for cases where the anchors are loaded in uplift in cohesive soils, are based on the assumption that the soil above the helical plate is mobilized in a manner analogous to that beneath a deep foundation in bearing. An appropriate design method would acknowledge the effect of load directionality on the assumed failure mechanism. This thesis evaluates the existing cylindrical shear and individual plate bearing design methods for helical anchor capacity in uplift. Additionally, new capacity models are proposed to improve prediction accuracy and reduce prediction variability. A load test database of helical anchors loaded in tension is established from tests reported in the literature. The existing and proposed capacity models are compared to the capacities observed during loading tests using the statistical bias and its distribution. Load and Resistance Factor Design (LRFD) resistance factors are derived from closed-form solutions using First Order Second Moment (FOSM) reliability procedures. Finally, load-displacement models are developed through the evaluation of observed individual anchor plate breakout behavior and back-calculation of side shear capacity from load tests on multi-plate anchors. The new displacement models are compared to the load-displacement tests in the database. In general the comparisons indicate that the displacement-based models developed in this thesis provide a reasonable estimate of load-displacement behavior of helical anchors for service-level displacements. These findings provide engineers with new tools for design of helical anchor foundations.
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