Existing guidance on the installation of screw piles suggest that they should be installed in a pitch-matched manner to avoid disturbance to the soil which may have a detrimental effect on the in-service performance of the pile. Recent insights from centrifuge modelling have shown that installing screw piles in this way requires large vertical compressive (or crowd) forces, which is inconsistent with the common assumption that screw piles pull themselves into the ground requiring minimal vertical compressive force. In this paper, through the use of the Discrete Element Method (DEM), the effects of advancement ratio, i.e. the ratio between the vertical displacement per rotation to the geometric pitch of the helix of the screw pile helix, on the installation resistance and in-service capacity of a screw pile is investigated. The findings are further used to assess the applicability of empirical torque capacity correlation factors for large diameter screw piles. The results of the investigation show that it is possible to reduce the required vertical compressive installation force by 96% by reducing the advancement ratio and that although over-flighting a screw pile can decrease the subsequent compressive capacity, it appears to increase the tensile capacity significantly.

Screw piles potentially offer quieter installation and enhanced axial tensile capacity over straight-shafted driven piles. As such, they have been suggested as a possible foundation solution for offshore jacket supported wind turbines in deeper water. To investigate the feasibility of their use in this setting, centrifuge testing of six model screw piles of different designs was conducted to measure the installation requirements and ultimate axial capacity of the piles in very-dense and medium-dense sand. The screw piles were designed to sustain loads generated by an extreme design scenario using published axial capacity and torque prediction formulae. Single and double-helix designs, including an optimised design, intended to minimise installation requirements, with reduced geometry were installed and tested in-flight. Piles in the medium-dense sand for example had significant installation requirements of up to 18.4MNm (torque) and 28.8MN (vertical force) which were accurately predicted using correlations with cone resistance data (CPT). Existing axial capacity design methods did not perform well for these large-scale screw piles, overestimating compressive and tensile capacities. Revised analytical methods for installation and axial capacity estimates are proposed here based on the centrifuge test results.

This volume forms the proceedings of the 1st International Symposium on Screw Piles for Energy Applications (ISSPEA), held at the University of Dundee, 27-28 th May, 2019. This conference is the first such event organised at the University of Dundee and was originally designed to be a small event to disseminate the findings of the EPSRC sponsored Supergen WindHub Grand Challenges project: Screw piles for wind energy foundation systems. The impetus to expand the guest list and scope of this event came after discussion with Dr Alan Lutenegger of the University of Massachusetts Amherst who organised the successful 1st International Geotechnical Symposium on Helical Foundations. Unfortunately, the eagerly anticipated 2 nd symposium in this series did not occur as planned so it was decided to partially plug this gap in screw pile innovation reporting by expanding the scope and invitees of ISSPEA. This conference has been organised by the Geotechnical Engineering Research Group at the University of Dundee representing the Screw piles for wind energy foundation systems project partners with academic teams at Durham University and the University of Southampton. The first ISSPEA provides an excellent opportunity for academics, engineers, scientists, practitioners and students to present and exchange the latest developments, experience and findings in screw pile engineering for renewable energy applications. The proceedings contains 12 papers and 9 extended abstracts with the latter representing the presentations made at the event that were not supported by a full paper. The proceedings contain one invited keynote paper from Alan Lutenegger on the current state-of-understanding of the engineering behavior of screw piles and helical anchors. This paper presents an overview of historical applications of screw piles, with discussions on aspects of their design and behaviour which are both understood and in need of further research, using case studies as examples. Other papers in the proceedings look at a variety of topics including: installation requirements and effects; cyclic behaviour; advanced numerical modelling of screw piles, including the use of DEM and MPM to incorporate installation effects into the models; and screw piles used in industrial applications. It is hoped that this proceedings and symposium will lead to similar future meetings and serve as a useful indicator of the current state of innovation and deployment. It is also hoped the event and proceedings will act as the springboard for new lines of research and development and increased use of screw piles for a variety of applications. We are grateful to all the authors and reviewers for their efforts in the preparation of the papers. Finally, the Organisers would like to acknowledge the support and efforts of the Local Organising Committee, paper reviewers and the support of our industrial partners.

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