Martin Achmus

The soil response under the inherent cyclic loading conditions when dealing with offshore foundations can be considered by using contour plots. These plots are derived from several cyclic laboratory tests and characterize the general cyclic soil behaviour. In the design process with explicit numerical methods, such plots are needed in order to assess the soil behaviour under arbitrary loading conditions and hence estimate the cyclic foundation response. In the paper, excess pore pressure contour plots for a poorly graded medium sand are derived from numerous constant volume (CV) cyclic direct simple shear (DSS) tests and a new approach for parametrization of the plots is presented. Subsequently, the data are assessed regarding scaling for other sand soils, i.e., construction of contour plots with only a small number of test results by using the general trends observed.

Piles as onshore and offshore foundation structures are often horizontally loaded. The usual calculation method for the behaviour of piles under such loads is the p‐y method. For piles in cohesive soil, several p‐y methods are proposed in literature, leading in general to differing results. A finite element model dealing with the behaviour of horizontally loaded piles in clay soils was established and validated against field tests. Utilising the results of a vast number of numerical simulations, a new p‐y method was developed, which gives reasonable results over the entire range of possible loads for piles of almost arbitrary diameters and embedded lengths in clay soils of varying consistency. The method requires in total six soil parameters and accounts for the actual deflection of the pile and also the effect of pile tip shear resistance by iteratively adapted y‐multipliers which depend on the course of the deflection line and the distance of the considered point to the pile tip. The new method can be generally used for calculation of the bearing behaviour of piles in soft clay under monotonic loading and can serve as a reference for consideration of cyclic load effects or softening effects in stiff clays.

Although suction buckets are believed to be a promising alternative for multipod foundations of offshore wind turbines, the economic and ecological advantages are still limited because of a lack of knowledge regarding the tensile bearing behavior. Major uncertainties exist in terms of the response under cyclic loading and the assessment of the partially drained loading condition, where negative differential pressure beneath the suction bucket’s lid contributes to the total tensile resistance. This article presents findings from 1-g model tests of suction buckets in sand subjected to various cyclic tensile loads. The suction bucket was installed via negative differential pressure (suction). The results show a strong effect of the applied load magnitude on the evolution of displacement, plug heave, and suction induced. Greater displacement accumulation is observed for the smaller considered frequency. Monotonic tests with varying displacement rates supplement the cyclic tests and serve for the verification of a hydraulic-mechanic coupled finite element model, which is afterward utilized for the back-calculation of cyclic model tests. Although a rather simple elasto-plastic material law was used for the sand, good agreement was found, indicating that the bearing behavior is mainly governed by the hydraulic conditions and only subordinately by the soil mechanics.

Raft rigidity is one of the governing factors that control the performance of piled rafts; it influences the interactions of the raft with the piles and the soil. By employing extensive three-dimensional Finite Element Analyses, the effects of raft stiffness on the absolute and differential settlements of piled rafts were investigated with the aim of achieving optimal design with limited number of piles. An appropriate constitutive model was applied to simulate the soil behavior, whose parameters were calibrated using pile load test measurements. While raft thickness is found to have minor effects on the settlements of piled rafts with closely spaced piles, a range of raft-soil stiffness ratios between 0.03 and 0.08 are found to minimize settlements for the case of widely spaced piles. Rigid rafts with raft-soil-stiffness ratios higher than 0.1 are recommended to be avoided for not enhancing the performance of the foundation.