No abstract

The paper presents the results of field experiments performed using the Imperial College instrumented displacement pile in a soft, sensitive marine clay at Bothkennar, Scotland. These results are compared with data from similar programmes of experiments performed in other clay types with this pile so that some of the major factors controlling displacement-pile performance may be identified. Key words : displacement pile, instrumentation, sensitive clay, effective stress design.

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.

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-todiameter 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 paper describes a programme of field experiments with heavily instrumented displacement piles installed in a stiff, lodgement till. Measurements of the effective stresses recorded at various levels along the pile shaft during installation, equalization, and load testing are presented. Shaft capacity is shown to be governed by an effective stress failure criterion, with the mobilized stresses depending on the clay consistency, distance from the pile tip, shaft surface texture, type of loading, installation rate, and degree of equalization. Key words : displacement pile, instrumentation, glacial till.