Screw pile design optimisation under tension in sand

Many applications in offshore engineering, such as floating or jacket-founded wind turbines or wave energy converters, require a significant uplift capacity of their foundations to be kept in place. Straight-shafted or suction piles in sands have a limited uplift capacity as they resist by friction only. In contrast, screw piles or screw anchors are a promising solution which provides a similar capacity to plate anchors and does not generate disturbance for marine mammals (e.g. from pile driving operations). The optimisation of the screw pile design does not rely only on the geotechnical assessment of the uplift capacity based on soil strength, but also on operational (installation requirements) and structural (helix bending, core section stress, limiting steel plate thick-ness) constraints. This paper develops a methodology for the design optimisation of screw piles under pure ten-sion in sand, incorporating all of these constraints, based on simplified analytical or semi-analytical approaches. The results show that the uplift capacity provided by an optimised screw pile is able to meet the needs of the offshore industry, across a range of soil densities and different applications (jacket foundation pile or tension leg platform anchor), providing that adequate installation plant could be dev

Evaluation of continuum modelling approaches for reinforced concrete in geotechnical applications

Modelling the structural response of reinforced concrete (RC) elements in geotechnical applications has been implemented using various numerical approaches with different levels of confidence; ranging from simple linear elastic approximations to non-linear section behaviour using embedded beams with moment-curvature (M-κ) relationships within dummy elements. However, the non-linear structural response of continuum RC approaches has not been widely employed in the geotechnical analysis of soil-structure interaction problems. This paper evaluates and compares different combinations of modelling approaches for the concrete and reinforcement, as implemented within the FE code PLAXIS 2D, to simulate the structural response of RC beams using the continuum approach for the concrete with discretely modelled reinforcement. The Concrete Model ‘CM’ and an equivalent Mohr-Coulomb ‘MC’ approach are compared for the concrete alongside the use of either embedded plates (with interfaces) or embedded beam rows to efficiently simulate the reinforcement. These approaches are validated against well-documented experimental data of singly and doubly reinforced concrete beams obtained from the literature. The results can be utilised to improve structural precision in Finite Element models in various soil-structure interaction problems (e.g., piles, shallow foundations, retaining walls, tunnel linings) within an integrated geotechnical environment

Simulation of pile installation in chalk:Discrete and continuum approaches

Chalk is a type of porous rock formed from cemented calcite grains. It is widely found in areas across the UK, and is present beneath the North Sea where offshore wind turbines are being installed. Large piles are often driven into chalk to support these turbines. However, the installation process can cause the intact rock below the pile to crush, creating a putty-like material with different mechanical properties from the original rock. This unpredictability has made it difficult to design piles that are appropriate for use in chalk. This paper presents two approaches to modelling open-ended pile installation in chalk. The first approach is based on the Discrete Element Method (DEM), which represents the rock as separate particles bonded together. A new contact model is proposed for highly porous rocks. The second approach uses the Geotechnical Particle Finite Element Method (GPFEM), which has been adapted to account for the large displacements and nonlinearities of the problem. With GPFEM the coupled hydromechanical effects developing during pile installation are investigated using a robust and mesh independent implementation of an elasto-plastic constitutive model at large strains. With DEM the micromechanical features of pile plugging are explored and the mechanisms behind radial stress distributions inside and outside the plug are unveiled. Although both approaches have their challenges, they have been successful in modelling pile installation experiments at model scale. This offers the potential for a closer examination and improved understanding of the mechanisms underlying open-ended pile installation in chal

A coupled damage-plasticity DEM bond contact model for highly porous rocks

In view of the significant stress loss induced by structural collapse when simulating high-porous soft rocks using traditional damage bond models in DEM ( discrete element method) modelling, a novel damage bond contact model is proposed to capture the ductile failure of high-porous cemented soft rocks. To address the unrealistic physical contact distribution resulting from the use of spherical particles in DEM modelling and consider the physical presence of broken bonds, far-field interaction is introduced between grains when two untouched particles reach a specific activation gap, enabling the generation of stable, highly porous open structure samples while using spherical DEM particles. The final results demonstrate that this newly developed model facilitates the transition from the purely elastic rock-like behaviour stage to the transitional ductile failure stage of porous soft rocks, as well as reproduces the softening/hardening response of soft rocks under different confinements

A coupled damage-plasticity DEM bond contact model for highly porous rocks

In view of the significant stress loss induced by structural collapse when simulating high-porous soft rocks using traditional damage bond models in DEM ( discrete element method) modelling, a novel damage bond contact model is proposed to capture the ductile failure of high-porous cemented soft rocks. To address the unrealistic physical contact distribution resulting from the use of spherical particles in DEM modelling and consider the physical presence of broken bonds, far-field interaction is introduced between grains when two untouched particles reach a specific activation gap, enabling the generation of stable, highly porous open structure samples while using spherical DEM particles. The final results demonstrate that this newly developed model facilitates the transition from the purely elastic rock-like behaviour stage to the transitional ductile failure stage of porous soft rocks, as well as reproduces the softening/hardening response of soft rocks under different confinements