Results of in situ tests of shear wave propagation are presented and analysed to evaluate the effects of geologic processes, stress history and ageing on the shear modulus of soils. These results show clear tendencies of soil structures to get stiffer as consequence of ageing or surcharging to high stress levels. The evolution of shear modulus, as verified by these results, is significant as it changes the response of soil deposits to seismic loadings.