Since several decennia, the deep soil mix technique is used for ground improvement applications and for the realisation of retaining walls. In recent years, soil mix walls have become an economical alternative to traditional excavation support systems. However, as a natural material (i.e. soil) is being mixed, it is to be expected that the entire wall is not perfectly mixed and homogeneous. In other words, inclusions of poorly mixed or even unmixed (i.e. soil) material are present. The amount of such inclusions can be less than 1% of the total volume at some sites, but 10% or more is also observed. Some of these inclusions are very small, while in other cases they can have dimensions of several centimetres. It is generally assumed that below a certain volume percentage and/or for small dimensions of the individual inclusions, these inclusions have no negative impact on the behaviour and on the strength of the soil mix material. To quantify the maximum acceptable volume percentages of inclusions, an experimental, as well as a numerical simulation research programme has been initiated in the framework of a Flemish regional research project (IWT 080736) that is carried out in collaboration with the BBRI, the Belgian federation of foundation contractors (ABEF) and KU Leuven. This research aims also to better understand the behaviour of this material and the failure of it.