Rigid pile response to cyclic lateral loading: laboratory tests

Abstract

This paper presents experimental work aimed at improving understanding of the behaviour of rigid monopiles, in cohesionless soils, subjected to lateral cyclic loading. It involves 1g laboratory model tests, scaled to represent monopile foundations for offshore wind turbines. The test programme is designed to identify the key mechanisms governing pile response, and is divided into four main parts: (a) investigation of loading rate effects; (b) hysteretic behaviour during unloading and reloading; (c) pile response due to long-term single-amplitude cyclic loading; and (d) multi-amplitude cyclic loads. The results show that the pile response conforms closely to the extended Masing rules, with additional permanent deformation accumulated during non-symmetric cyclic loads. This ratcheting behaviour is characterised by two features: first, the ratcheting rate decreases with cycle number and depends on the cyclic load magnitude, and second, the shape of the hysteresis loop tightens progressively, involving increased secant stiffness and decreased loop area. Test results involving multi-amplitude load scenarios demonstrate that the response of the pile to complex load scenarios can be analysed and understood using the conclusions from single-amplitude cyclic loading. Such test results should be sufficient for deriving the principles of new modelling approaches.

A method for calibration of the Hyperplastic Accelerated Ratcheting Model (HARM)

“…Results from laboratory tests provide fundamental insights on the foundation behaviour that determine the form of development of theoretical and numerical models. The work published by Abadie, Byrne [1,3] and Abadie [21] involves laboratory floor model tests, scaled to represent a full-scale wind-turbine monopile in drained sand, and elaborates clearly the important effects that need to be accounted for in detailed modelling:…”

“…The tests considered here were performed in dry sand, reproducing drained loading offshore. In these conditions, Abadie et al [1] showed that rate effects can be neglected, allowing a focus on the development of a modelling method for ratcheting. This is a step towards a comprehensive modelling framework for offshore monopile design.…”

“…The remaining tests, as well as the two critical tests from Abadie, Byrne [2], which involve more complicated load scenarios, are finally used for validation of the calibrated model. A summary of the tests used in this paper is provided in Table 1 and the detail of the testing set-up, as well as the limitations of the testing conditions, are discussed in [1,21].…”

“…The model is applicable to rate-independent and rate-dependent materials. However, Abadie, Byrne [1] show that there is no evidence of strain rate affecting the pile response, and therefore, the rate-independent version of the model is used in the following. Alternatively, the rate-dependent form can be employed, with use of an appropriately small value of the viscosity .…”

“…This paper describes theoretical developments for HARM that enable to (i) accommodate ratcheting behaviour accurately and (ii) calibrate the model to address the load-deflection relationship for a pile under lateral loading. The experimental test results presented in Abadie, Byrne [1,3] are used as an impetus for the work, and the model accuracy is evaluated by comparing the predictions with the test results. The modelling approach, specific to offshore wind turbine structures, is explained at the beginning of the paper (Section 2).…”