The application of deep soil mix (DSM) technology in Belgium is sharply increasing. Next to soil improvement applications, DSM walls are extensively used for excavation support. Even permanent retaining and foundation applications with soil mix are increasingly applied. This paper gives an overview of the DSM procedures carried out in Belgium and deals with methodologies applied to determine the amount of soil inclusions, the elastic modulus and the compressive strength of DSM material. The influence of soil inclusions on the stiffness of DSM material is investigated with the help of numerical simulations. The methodologies are validated on a large population of laboratory tests on in situ DSM material, executed in Belgian soils. These research activities have been performed in the framework of the research program “Soil Mix” that the Belgian Building Research Institute (BBRI) carries out in collaboration with the Catholic University of Leuven and the Belgian Association of Foundation Contractors (ABEF). This research program is financially supported by the Agency for Innovation by Science and Technology of the Flemish Region IWT (BBRI, 2009-2013).