Design of laterally-loaded monopiles in layered soils

Abstract

This paper describes an implementation of the methodology, developed in Phase 2 the recent PISA (PIle Soil Analysis) joint industry research project, for the design of laterally-loaded monopiles in layered soils. The software PLAXIS MoDeTo and PLAXIS 3D are employed to obtain the soil reaction curves that are required for the method, following the PISA ‘numerical-based’ design approach. A particular design space is selected to define the variation of the geometrical parameters assigned to the three-dimensional (3D) Finite Element (FE) calibration models. The parameters that span the design space are the embedded length (L), the outer pile diameter (D), the pile wall thickness (t) and the height above the mudline (h), where the design load is applied. The soil reaction curves are determined from the 3D FE calibration models for separate homogeneous soil conditions consisting of stiff normally consolidated clay and very dense sand. The calibration set consists of eight 3D FE models, for each homogeneous soil profile. Subsequently, the soil reaction curves are parameterised and used to calibrate a one-dimensional (1D) FE model, formulated by means of Timoshenko beam theory, which allows for fast and robust design calculations. A final design model (DM) is defined and its response is studied considering the two homogeneous profiles and four additional layered soil profiles. The results of each 1D analysis are compared with equivalent 3D FE models and a 1D FE model developed at the University of Oxford (OxPile) as part of the PISA research. The accuracy metric eta (η) is used to compare quantitatively the response among the employed models, focusing on large displacements at ground level (about D/10). The results indicate a very good match for all considered soil profiles; all computed η values exceed 90%. The research findings support the applicability of the PISA design methodology in both homogeneous and layered soil conditions.