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

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

Design of plate and screw anchors in dense sand:failure mechanism, capacity and deformation

Plate and screw anchors provide a significant uplift capacity and have multiple applications in both onshore and offshore geotechnical engineering. Uplift design methods are mostly based on semi-empirical approaches assuming a failure mechanism, a normal and a shear stress distribution at failure and empirical factors back-calculated against experimental data. However, these design methods are shown to under- or overpredict most of the existing larger scale experimental tests. Numerical FE simulations are undertaken to provide new insight into the failure mechanism and stress distribution which should be considered in anchor design in dense sand. Results show that a conical shallow wedge whose inclination to the vertical direction is equal to the dilation angle is a good approximation of the failure mechanism in sand. This shallow mechanism has been observed in each case for relative embedment ratios (depth/diameter) ranging from 1 to 9. However, the stress distribution varies non-linearly with depth, due to the soil deformability and progressive failure. A sharp peak of normal and shear stress can be identified close to the anchor edge, before a gradual decrease with increasing distance along the shear plane. The peak stress magnitude increases almost linearly with embedment depth at larger relative embedment ratios. Although further research is necessary, these results lay the basis for the development of a new generation of design criteria for determining anchor capacity at the ultimate limiting state