Field-testing of piles in an expansive clay is required for the validation of the design criteria for wind turbine foundations in such soil profiles. The test programme includes lateral load tests to investigate the horizontal stiffness of the soil and vertical plug pull-out tests to examine the axial capacity of pile-soil interface. These tests will be conducted in both dry and wet conditions to study the range of expected responses of the pile and evaluate how the changing soil moisture condition affects the pile behaviour. In addition, an instrumented pile socketed into bedrock will be used to determine the strain induced in the pile due to soil heave as the soil saturation levels increase. For the execution of these tests, a large open area with expansive clay of a sufficient depth is required; a stable stratum below the clay is desirable for the socketing of the long piles. An important component of the investigation is the characterisation of the expansive clay profile. A preliminary site investigation has been carried out on the field-testing site which will be supported by further laboratory testing. Details of the soil profile and site characterisation which will be used in the future analyses of the field-testing results are presented.