The presence of hard concretions and lithified beds in the subsoil can cause serious problems during the realisation of infrastructure projects. As these concretions do most often not occur as continuous layers, they also are a real challenge for geotechnical investigations. This paper gives an overview of geological features and general characteristics of typical hard soil concretions in Flanders. Special attentions is paid to the important interaction between geology and geotechnics. Some typical examples are given of successful and unsuccessful geotechnical investigation programs and their effects on the realisation of infrastructure projects.