23 Dec 2021
23 Dec 2021
Status: this preprint is currently under review for the journal WES.

Cyclic overlay model of p-y curves for laterally loaded monopiles in cohesionless soil

Junnan Song and Martin Achmus Junnan Song and Martin Achmus
  • Institute for Geotechnical Engineering, Leibniz University Hannover, Hannover, 30167, Germany

Abstract. The bearing behaviour of large-diameter monopile foundations for offshore wind turbines under lateral cyclic loads in cohesionless soil is an issue of ongoing research. In practice, mostly the p-y approach is applied in the design of monopiles. Recently, modifications of the original p-y approach for monotonic loading stated in the API regulations (API 2014) have been proposed to account for the special bearing behaviour of large-diameter piles with small length-to-diameter ratios (e.g. Thieken et al. 2015, Byrne et al. 2015). However, cyclic loading for horizontally loaded piles predominates the serviceability of the offshore wind converters, and the actual number of load cycles cannot be considered by the cyclic p-y approach of the API regulations. This research is therefore focusing on the effects of cyclic loading on the p-y curves along the pile shaft and aiming to develop a cyclic overlay model to determine the cyclic p-y curves valid for a lateral load with a given number of load cycles. The “Stiffness Degradation Method (SDM)” (Achmus et al. 2009) is applied in a three-dimensional finite element model to determine the effect of the cyclic loading by degrading the secant soil stiffness according to the magnitude of cyclic loading and number of load cycles based on the results of cyclic triaxial tests. Thereby, the numerical simulation results are used to develop a “cyclic overlay model”, i.e. an analytical approach to adapt the monotonic (or static) p-y curve to the number of load cycles. The new model is applied to a reference system and compared to the API approach for cyclic loads.

Junnan Song and Martin Achmus

Status: open (extended)

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Junnan Song and Martin Achmus

Junnan Song and Martin Achmus


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Short summary
In this research, a cyclic p-y overlay model for monopiles in sand soils could be developed by comparison of numerical simulations of monopile behaviour once under monotonic and once under cyclic loading with a defined number of load cycles (utilizing the stiffness degradation method). The new cyclic overlay model gives plausible results and can be applied to any monotonic p-y approach for monopiles in sand.