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Existing guidance on the installation of screw piles suggest that they should be installed in a pitch-matched manner to avoid disturbance to the soil which may have a detrimental effect on the in-service performance of the pile. Recent insights from centrifuge modelling have shown that installing screw piles in this way requires large vertical compressive (or crowd) forces, which is inconsistent with the common assumption that screw piles pull themselves into the ground requiring minimal vertical compressive force. In this paper, through the use of the Discrete Element Method (DEM), the effects of advancement ratio, i.e. the ratio between the vertical displacement per rotation to the geometric pitch of the helix of the screw pile helix, on the installation resistance and in-service capacity of a screw pile is investigated. The findings are further used to assess the applicability of empirical torque capacity correlation factors for large diameter screw piles. The results of the investigation show that it is possible to reduce the required vertical compressive installation force by 96% by reducing the advancement ratio and that although over-flighting a screw pile can decrease the subsequent compressive capacity, it appears to increase the tensile capacity significantly.

In many fields of geotechnical engineering, the modelling of interfaces requires special numerical tools. This paper presents the formulation of a 3D fully coupled hydro-mechanical finite element of interface. The element belongs to the zero-thickness family and the contact constraint is enforced by the penalty method. Fluid flow is discretised through a three-node scheme, discretising the inner flow by additional nodes. The element is able to reproduce the contact/loss of contact between two solids as well as shearing/sliding of the interface. Fluid flow through and across the interface can be modelled. Opening of a gap within the interface influences the longitudinal transmissivity as well as the storage of water inside the interface. Moreover the computation of an effective pressure within the interface, according to the Terzaghi's principle creates an additional hydro-mechanical coupling. The uplifting simulation of a suction caisson embedded in a soil layer illustrates the main features of the element. Friction is progressively mobilised along the shaft of the caisson and sliding finally takes place. A gap is created below the top of the caisson and filled with water. It illustrates the storage capacity within the interface and the transversal flow. Longitudinal fluid flow is highlighted between the shaft of the caisson and the soil. The fluid flow depends on the opening of the gap and is related to the cubic law.

Screw piles potentially offer quieter installation and enhanced axial tensile capacity over straight-shafted driven piles. As such, they have been suggested as a possible foundation solution for offshore jacket supported wind turbines in deeper water. To investigate the feasibility of their use in this setting, centrifuge testing of six model screw piles of different designs was conducted to measure the installation requirements and ultimate axial capacity of the piles in very-dense and medium-dense sand. The screw piles were designed to sustain loads generated by an extreme design scenario using published axial capacity and torque prediction formulae. Single and double-helix designs, including an optimised design, intended to minimise installation requirements, with reduced geometry were installed and tested in-flight. Piles in the medium-dense sand for example had significant installation requirements of up to 18.4MNm (torque) and 28.8MN (vertical force) which were accurately predicted using correlations with cone resistance data (CPT). Existing axial capacity design methods did not perform well for these large-scale screw piles, overestimating compressive and tensile capacities. Revised analytical methods for installation and axial capacity estimates are proposed here based on the centrifuge test results.

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Most analytical approaches for the design of shallow plate and screw anchors in tension are based on the limit equilibrium of a rigid soil wedge for which a horizontal stress distribution acting on the failure plane is assumed. Finite element analysis for a wide range of soil properties was carried out to identify the shape of the failure mechanism and to study the stress distribution at failure. Results show that soil deformation modifies the stress field around the anchor and increases the uplift capacity. A semi-analytical approach is proposed to describe this stress distribution, based on peak friction angle.

Deep foundations maybe used in a range of soil types where significant foundation resistance is required but their installation is often associated with disturbance due to noise and vibration. Greater restrictions on use in urban and offshore environments is now commonplace. Screw piles and rotary jacked straight shafted piles are two potential methods of silent piling that could be used as alternative foundation solution, but the effects of certain geometric and installation properties such as installation pitch i.e. the ratio between vertical displacement and rotation, on the required installation torque and force in sand are not well understood. In this paper the effects of installation pitch and base geometry on the installation requirements of a straight shafted pile are simulated in 3D using the discrete element method (DEM). The installation requirements of straight shafted piles into sand have been validated against centrifuge testing, in three different relative densities. The DEM shows reductions in installation force can be achieved by increasing the installation pitch or including a conical tip. An existing cone penetration test (CPT) based prediction method for installation requirements has been improved to include the effects of installation pitch and base geometry for rotary installed piles in sand.

Screw anchors have been recognised as an innovative solution to support offshore jacket structures and floating systems, due to their low noise installation and potential enhanced uplift capacity. Results published in the literature have shown that for both fixed and floating applications, the tension capacity is critical for design but may be poorly predicted by current empirical design approaches. These methods also do not capture the load-displacement behaviour, which is critical for quantifying performance under working loads. In this paper, a Finite Element methodology has been developed to predict the full tensile load-displacement response of screw anchors installed in sand for practical use, incorporating the effects of a pitch-matched installation. The methodology is based on a two-step process. An initial simulation, based on wished-in-place conditions, enables the identification of the failure mechanism as well as the shear strain distribution at failure. A second simulation refines the anchor capacity using soil-soil interface finite elements along the failure surface previously identified and also models installation through successive loading/unloading of the screw anchor at different embedment depths. The methodology is validated against previously published centrifuge test results. A simplified numerical approach has been derived to approximate the results in a single step.