In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

Over the past 5 years, a substantial research effort aimed at optimising the design of offshore wind turbines has led to significant reductions in the projected cost of developing offshore wind. Optimising the geotechnical design of these structures, through modern analysis techniques such as 3D Finite Element Modelling (FEM), has played a key role in helping to reduce costs. This paper presents a methodology for accurately modelling monopile behaviour using Cone Penetration Test (CPT) data to calibrate the non-linear stress dependent Hardening Soil (HS) model. The methodology is validated by comparing the modelled behaviour to field tests on a range of pile geometries. The paper also demonstrates how the soil-pile reaction response curves can be extracted from the FE model by isolating the stresses on each element of the pile. The contribution of each component to the overall lateral resistance is shown to vary with the pile geometry and is examined using the extracted soil reaction curves.

This paper presents the results of field tests performed to investigate the field behaviour of winged-monopile foundations. The principle of the winged monopile is that steel plates are attached to a standard monopile (in the area near the ground or seabed surface) to increase the foundation stiffness and lateral resistance. The experimental tests described in this paper consisted of load tested driven instrumented prototype scale standard (reference) monopiles and piles with varying wing geometries at two sand sites. The overall load–displacement performance and mobilised bending moment profiles were examined to assess the potential benefits of adding wings to monopiles. Experimental p–y curves were developed for the piles to analyse how the presence of wings influenced the soil–structure interaction of the foundation system. The use of simplified p–y methods for predicting the winged-pile response was assessed. The experiments proved that the addition of wings greatly improved the lateral resistance and stiffness of the piles; however, the results suggest that conventional p–y curve methods are limited as they cannot account for the effect that the enhanced stresses mobilised by the wings have on the strength and stiffness response of the pile below the wing location.

Lateral loading is often the governing design criteria for piles supporting offshore wind turbines and with the recent growth of this sector, the reliability of traditional design approaches is receiving renewed interest. To accurately assess the behavior of a laterally loaded pile requires a detailed understanding of the soil reaction that is mobilized as the lateral deflection of the pile occurs. Currently, the p-y curve method is widely adopted to model the response of laterally loaded piles. The limitations of existing p-y formulations are widely known, and there is acceptance that load tests on large-diameter stiff monopiles are urgently required to formulate appropriate design methods. However, interpretation of the data from instrumentation placed on stiff monopiles is not straightforward. This paper proposes an optimization technique to derive the soil reaction profile along the shaft of instrumented piles, from which the correlated p-y curves can then be obtained. The method considers force equilibrium, pile deflection, and additional boundary conditions. A set of fourth-order polynomial equations are assumed to model the soil reaction profile under each load step during a monotonic load test. By minimizing the difference between the measured and calculated bending moment and considering equilibrium of the shear forces acting on the pile, the soil reaction profile and the concentrated tip resistance can be obtained simultaneously. A stiff instrumented test pile installed in over-consolidated sand was load tested and the results were used to test the performance of the proposed method. The results are compared with other methods used in literature and practice. The method provides a consistent framework to derive p-y curves from measured strain data. The results of the field test and derived p-y curves confirmed that existing design methods do not accurately capture the lateral loading response of piles in dense sand.

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