The paper presents an analysis of the qualitative impact of soil properties of varied origin on the rigidity index variability. This coefficient is defined as the ratio between the shear modulus G and undrained shear strength su. In order to determine values of this coefficient in four genetically varied soil groups, CPTU and SDMT tests were performed. The article discusses the concepts of evaluating the G modulus based on the values measured with the SDMT test, as well as the coefficient IR ’ = G0 · su −1. The results have revealed that the significant influence on the IR variation, apart from the characteristic soil properties, such as type of soils or plasticity index, is displayed by the cementation and over consolidation effects, and the anisotropy of macrostructure of certain sediments.