The deep soil mixing technique consists of an in situ mechanical mixing of the soil with an injected binder (e.g. cement). However, the presence of soil inclusions (poorly or even unmixed soil) in the artificial material is unavoidable. This heterogeneous character of soil mix material makes it different from traditional building materials. The presented research investigates the influence of the volume percentage of inclusions on the strength, stiffness, stress-strain behaviour and fracture pattern of soil mix material. 2D numerical simulations are conducted using a Discrete Element Program (UDEC) and the results are compared with experimental data. It is observed that the reduction of the strength and stiffness of a sample is significantly larger than the weighted average of the UCS and Young's modulus, taking into account the volumes of the well mixed material and the softer inclusions. However, the strength is remarkably more affected by the volume percentage of inclusions than the stiffness. Moreover, other parameters than the percentage of weak material are also important (e.g. shape, size and relative position of the inclusions) and result in wide ranges of resulting strength and stiffness.