Contents lists available at ScienceDirect Marine Structures journal homepage: www.elsevier.com/locate/marstruc Soil reaction curves for monopiles in clay Youhu Zhang a,* , Knut H. Andersen b a Technical Lead Offshore Geotechnics, Norwegian Geotechnical Institute, Sognsveien 72, 0855, Oslo, Norway b Norwegian Geotechnical Institute, Sognsveien 72, 0855, Oslo, Norway ARTICLE INFO Keywords: Monopile Clay Lateral loading p-y spring Base shear Stress-strain ABSTRACT Large diameter steel pipe pile foundations, typically known as monopiles, are the predominant foundation solution for offshore wind turbines. This paper deals with soil reaction curves for monopile analysis under lateral loading in clay. Developed from extensive parametric finite element analyses, models are proposed that allow for construction of site-specific soil reaction curves based on the stress-strain responses measured in laboratory tests. Assisted by failure mechanisms revealed by finite element analyses, a conceptual framework is proposed to analyse the monopile response under lateral loading using the beam-column approach. The conceptual framework makes use of the p-y model for wedge failure developed in this study along the monopile above the rotation point, existing p-y model for the flow-around failure suggested in the literature below the rotation point and base shear s-u model developed in this study at the pile tip. Validation exercises against finite element simulations and field pile testing results demon- strate excellent capability of the framework to predict monopile responses. The soil reaction models proposed in this paper provide practising engineers with a simple yet powerful approach to use site-specific soil reaction curves in monopile design based on element soil behaviour measured in laboratory, without the need for advanced numerical analyses. 1. Introduction Large diameter steel pipe pile foundations, typically known as monopiles, are the predominant foundation solution for offshore wind turbines. Today's monopiles have a diameter (D)4–8 m and a penetration length (L) 25–40 m. This leads to a penetration to diameter ratio typically around or less than 6. Monopile foundations for offshore wind turbines are predominantly loaded by hor- izontal force and over-turning moment. Design of the monopile foundation is typically performed using the beam-column approach where the pile is represented by an equivalent beam and the soil reactions are represented by a series of non-linear springs along the pile length. As per today's practice, monopiles are largely designed using methods borrowed from the offshore oil and gas industry. However, question arises whether this is appropriate as monopiles differ in a few ways from "traditional" pile foundations used in the oil and gas industry. Compared to piles used to supporting offshore jacket platforms, which have a typical pile diameter of 1.5–2.5 m and a penetration length 40–100 m, monopiles are considerably stiffer due to smaller L/D ratio. Under lateral loading, the soil around a monopile reacts predominantly in a wedge failure along the upper part of the pile and a rotational mechanism along the lower part and around the pile tip. The localised flow around failure, which can be found over a significant length of a slender pile, may be not relevant at all for a monopile. In addition, the soil resistance at the pile tip may become an important component of soil reaction to lateral and overturning moment loading due to the large pile diameter. Furthermore, the vertical component of the pile-soil skin friction mobilised along a monopile may become a meaningful component of resistance to overturning moment loading. For a https://doi.org/10.1016/j.marstruc.2018.12.009 Received 26 February 2018; Received in revised form 16 August 2018; Accepted 28 December 2018 * Corresponding author. E-mail addresses: youhu.zhang@ngi.no (Y. Zhang), knut.h.andersen@ngi.no (K.H. Andersen). Marine Structures 65 (2019) 94–113 0951-8339/ © 2019 Elsevier Ltd. All rights reserved. T