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Holistic approach for the design of single piles and pile groups under cyclic loading

Abstract

Problem description
The long-term load-displacement behaviour of piles under cyclic axial and lateral loading is essential for the design of a series of pile foundations in geotechnical engineering. Accurate prediction of this behaviour is, especially within the offshore sector, challenging due to the distinct type of cyclic loading as well as the extremely high numbers of load cycles compared to conventional engineering structures. Owing to the rapidly developing on- and offshore wind industries, interest in precise predictions is, however, greater than ever, and in practice also a wide range of other engineering pile structures are subjected to such considerable cyclic loads. As the imposed static and cyclic loads as well as the size of all these structures itself increase, the design requirements become more challenging. Hence, a design approach is needed which enables more accurate means of predictions and thereby facilitating more cost-effective pile design for offshore wind turbines, based on laboratory tests, centrifuge models and numerical simulations.

Objective
The objective of this research project is the development of a novel holistic design approach for the prediction of the load-displacement behavior of piles under cyclic lateral and axial loading in granular and cohesive soils, for low and high numbers of load cycles.

Methodology
Monotonic and cyclic triaxial tests on two model soils used in the entire research project (fine uniform silica sand and plastic kaolin clay) are performed at Ruhr-Universität Bochum. In these tests the amplitude, initial stress, overconsolidation ratio, loading frequency, cut-out direction (vertical/horizontal) and the direction of the cycles including polarization changes are varied. Based on the test results and additional data from DSS tests performed in Hamburg, an anisotropic visco-hypoplastic model for clay is extended to include small-strain and cyclic accumulation effects. A high-cycle accumulation model for clay is improved including a simplified calibration procedure similar to sand. In collaboration with the group in Hamburg these constitutive models are used in numerical simulations of centrifuge models of single piles under axial and lateral cyclic loading with the purpose of validation. The constitutive models are further simplified to be included into an engineer-oriented model for monopiles foundations of offshore wind turbines developed by the group in Hamburg.