1 INTRODUCTION A comprehensive site investigation was conducted at two sites in the Irish midlands being developed as onshore wind farms for Bord Na Mona Powergen. The test sites are located at Mount Lucas, Co. Offaly (approximately 80km west of Dublin) and at Bruckana Co. Kilkenny (approximately 120km south of Dublin) as shown in Error! Reference source not found.. The Mount Lucas wind farm comprises of 28x3MW turbines spread over an 1100ha site. The Bruckana site has an area of approximately 687ha and consists of 14x3MW turbines. Both locations are active peat production bogs and have significant depths of peat overlaying glacial tills. The initial site investigation at each location consisted of trial pits, window sampling, dynamic probing, rotary core drillholes, geophysical surveys (including multichannel analysis of waves – MASW and seismic refraction surveys), SPT and CPT field testing. The data from the initial tests was reviewed and interpreted to determine the soil profile at each turbine location and the relevant location specific engineering design parameters. The wind turbines are supported on 20m diam- eter concrete spread foundations or reinforced precast driven concrete piles depending on the suitability Determination of a dynamic Young’s modulus from in-situ tests P. Doherty, G. Murphy and D. Igoe Gavin and Doherty Geosolutions ltd., Dublin, Ireland K. Gavin School of Civil, Structural and Environmental Engineering, University College Dublin, Ireland J. Donohoe Bord na Mona Powergen ABSTRCT: The soil stiffness, and in particular the dynamic Young’s modulus (Edyn), is an important de- sign parameter for assessing the suitability of the foundation bearing strata to resist long term cyclic loads generated by wind turbines. However, there are no well-established design standards or industry guidelines to specify how the Edyn value should be determined. This paper presents a case study of an Irish wind farm where the selection of a representative stiffness was critical for the foundation design. A comprehensive site investigation was conducted that included drilling, in-situ tests and a suite of geophysical tests. The preliminary Edyn values were correlated from CPTu and Vs measurements, and were compared to the com- plete load-reload response measured in large scale plate load tests. The sites are characterised by a surface layer of organic peat that is underlain by glacial till deposits over a limestone formation. The wind turbine foundations were primarily constructed on the glacial till deposits. Accurate classification of the glacial soils is challenging due to the material heterogeneity and difficulty in sampling/testing such materials. For these wind farms, in-situ testing proved to be highly advantageous. The shear wave velocity and CPTu data allowed the formation level for the foundations to be determined and an estimate of the operational soil stiffness to be derived. The plate load tests then served as a quality control measure on site, where the Edyn values measured during a reload loop at the appropriate stress level were compared to the turbine design requirements. The plate load tests were also seen to provide reasonable agreement with a non-linear stiff- ness degradation model that utilised the site specific field test data. 3 rd International Symposium on Cone Penetration Testing, Las Vegas, Nevada, USA - 2014 645