The report presents the results of a comprehensive study on the small strain stiffness of several Swedish clays (high to low plasticity) and its tentative effect on the design of retaining structures. This research, which is part of a larger study, is commissioned by Trafikverket. A combination of field measurement techniques, seismic dilatometer and surface seismics, are complemented with bender element testing of the retrieved samples. The overall trend in the data is that the values found for the small strain shear modulus in this research are larger than currently recommended when following the TKGEO (2013) guidelines. Furthermore, the results indicate that multichannel analysis of surface waves (MASW) works best for acquiring a 2D profile of the small strain shear modulus inferred from shear wave velocities in the top 10 meters of the subsoil with reasonable accuracy. The seismic dilatometer is more appropriate to measure the small strain stiffness up to larger depths and with higher accuracy at the expense of losing the 2D spatial information. Additionally, the best laboratory based procedure that compares well to the benchmark, i.e. the in-situ dilatometer measurements, is to measure the shear wave velocity on samples that have been stored < 2 days after being brought back to the in-situ anisotropic stress level. In the current research the horizontal stress component is obtained from the in-situ dilatometer measurement. Most empirical relations for small strain stiffness that were tested are generally not recommended, however in case a first estimate is required the underlying laboratory/field data should be of similar quality (or lack of) as originally used by the proposers of the empirical relation. Stiffness degradation with strain is significant for the soils tested. Finally, the influence of the generally larger small strain shear moduli obtained in this research on the design of excavations with embedded retaining walls is elaborated by means of two case studies. Not surprisingly the higher stiffness lead to large differences in structural response. It would seem prudent for Trafikverket to insist that for critical deep excavation projects where Finite Element Analysis is performed that the consistency of the model parameters is demonstrated with element test simulations where the suitability of the chosen parameter set can easily be demonstrated.