David Igoe

The paper describes the characterisation of a test site in an active sand quarry located near Kilmuckridge, County Wexford, Ireland. Two test areas have been selected in this 47-acre sand and gravel quarry, approximately 150 m apart. Test Area A covers an area of approximately 3100 m², and Area B is approximately 1500 m². Both test areas are over-consolidated with dense to very dense deposits; Area A has slightly gravelly sand to silty sand layers, and Area B is primarily sandy silt. A series of pile lateral and axial load tests are planned at the two sites. In Area A, CPT cone tip resistance q_c values up to 60 MPa were recorded in the top 2 m, with the values dropping to approximately 20 MPa in deeper layers below 2 m. The paper presents the results from in situ cone penetration tests, geotechnical laboratory characterisation tests on disturbed samples, and chemical characterisation analyses. Shear box tests and drained monotonic triaxial tests were performed on reconstituted sand samples to investigate the shear strength.

Monopiles are the most popular type of foundation for offshore wind turbines. Although capturing the effects of cyclic loading is critical to the design of monopiles, there is no recommended approach in the main design standards and no consensus in the literature as to how this can be achieved. Hence, this paper presents the step-by-step methodology and validation of a new cyclic model in sand and clay. The model consists of the degradation of monotonic soil reaction curves based on soil cyclic contour diagrams. The model is found to provide a very satisfactory match with the PISA field tests in Dunkirk dense marine sand and Cowden stiff glacial till.

Screw piles (often referred to as helical piles) are widely used to resist axial and lateral loads as deep foundations. Multi-helix piles experience complex interactions between the plates which depend on the soil properties, pile stiffness, helix diameter, and the number of helix plates among other factors. Design methods for these piles are typically highly empirical and there remains significant uncertainty around calculating the compression capacity. In this study, a database of 1667 3D finite element analyses was developed to better understand the effect of different inputs on the compression capacity of screw piles in clean sands. Following development of the numerical database, various machine learning methods such as linear regression, neural networks, support vector machines, and Gaussian process regression (GPR) models were trained and tested on the database in order to develop a prediction tool for the pile compression capacity. GPR models, trained on the numerical data, provided excellent predictions of the screw pile compression capacity. The test dataset root mean square error (RMSE) of 29 kN from the GPR model was almost an order of magnitude better than the RMSE of 225 kN from a traditional theoretical approach, highlighting the potential of machine learning methods for predicting the compression capacity of screw piles in homogenous sands.

In recent years, there has been a significant push to develop optimized pile designs in the offshore industry, driven by the growth of offshore wind. One of the largest remaining uncertainties governing axial pile design in sands is the influence of ageing effects and how an aged pile responds under axial cyclic loading. This paper presents results from a long-term campaign of field tests on pile ageing and axial cyclic loading in sands, undertaken at the Blessington geotechnical test site in Ireland. Five open-ended driven steel piles were subjected to a total of 33 static and cyclic tension pile load tests undertaken over a 3-year period. The tests were planned and coordinated in order to accurately quantify the effects of ageing and cyclic loading on the pile shaft capacity over time. The tests showed that significant gains in pile shaft capacity occurred over time as a result of pile ageing, in line with results presented by other researchers. Piles subjected to cyclic tension loading remained stable under cyclic loading at load levels much larger than their 1-day capacity. The 1-day capacity appears to offer a lower bound for the degraded pile capacity following tension cyclic loading to failure. The reduction in capacity caused by cyclic loading to failure was related to the pretest increase in capacity caused by ageing. The findings from these field tests in addition to reinterpretation of previous testing indicated that previously published cyclic interaction boundaries may be overly conservative.

The design of axially loaded piles has been an area of focus for the offshore industry in recent years. A number of studies report substantial increases in the shaft capacity of piles driven in sand, known as pile ageing. The offshore industry has been slow to implement ageing into practice because of uncertainty over the mechanisms controlling ageing and variability on the impact of ageing. This paper presents the results of tests using an instrumented pile with two separate installations considered, one where the pile was load tested 4 days after installation and the second where the load test was performed after an ageing period of 116 days. Data from the installation, ageing period, load testing and pile extraction provide further insights into the mechanisms governing the effect of time on the axial capacity of piles in sand.

The results obtained from a field testing campaign on laterally loaded monopiles, conducted at a dense sand site in Dunkirk, northern France are described. These tests formed part of the PISA project on the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained from monotonic loading tests on piles of three different diameters (0·273 m, 0·762 m and 2·0 m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. The tests consisted principally of the application of monotonic loads, incorporating periods of held constant load to investigate creep effects. The influence of loading rate was also investigated. Data are presented on the overall load–displacement behaviour of each of the test piles. Measured data on bending moments and inclinations induced in the piles are also provided. Inferences are made for the displacements in the embedded length of the piles. These field data will support the development of a new one-dimensional modelling approach for the design of monopile foundations for offshore wind turbines. They also form a unique database of field measurements in a dense sand, from lateral loading of piles at a vertical distance above the ground surface.

This paper describes the results obtained from a field testing campaign on laterally loaded monopiles conducted at Cowden, UK, where the soil consists principally of a heavily overconsolidated glacial till. These tests formed part of the PISA project on the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained for monotonic loading tests on piles of three different diameters (0·273 m, 0·762 m and 2·0 m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. The tests included the application of monotonic loading incorporating periods of constant load to investigate creep effects, and investigations on the influence of loading rate. Data are presented on measured bending moments and inclinations induced in the piles. Inferred data on lateral displacements of the embedded section of the piles are determined using an optimised structural model. These field data support the development of a new one-dimensional modelling approach for the design of monopile foundations for offshore wind turbines. They also form a unique database of field measurements in an overconsolidated clay, from lateral loading of piles at a vertical distance above the ground surface.

The PISA Joint Industry Research Project was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England, and in dense, normally consolidated marine sand at Dunkirk, northern France. This paper describes the experimental set-up for pile testing, with unique features of load-application mechanisms and built-in fibre optic strain gauges. New procedures are described for the interpretation of pile loading data, and specifically for providing precise interpretation of pile displacements.