Dike ruptures may lead to disastrous consequences such as loss of lives and economic disasters. To prevent the risk of breakage, special supervision of the hydraulic structure is required. Usual methodologies for the reconnaissance of earthen dikes include complementary geophysical and geotechnical investigation methods. This paper introduces a new methodology to combine data from these two types of information sources, taking into consideration their specificities (respective level of imperfections and spatial distribution). This innovative procedure considers the mathematical belief masses theory and improves the characterization of lithological materials within a real case study. It provides information on the level of conflict between information sources as well as displays a confidence index associated with the results. The considered investigation campaign involves electrical resistivity tomography, multi-channel analysis of surface waves as well as core drilling with particle-size analysis from laboratory testing. The fusion results highlight the capacity of this combination methodology to characterize the lithological sets and to indicate the interfaces’ positions as well as associated levels of confidence.