Since several decennia, the deep mixing method has been used to improve the strength and deformation characteristics of soft soils. A more recent trend is the use of this technique for the construction of soil mix walls designed as soil and water retaining structures. Within the framework of the BBRI Soil Mix project (2009-2013), seventeen large-scale bending tests have been performed on excavated soil mix elements previously executed in real construction sites. The purpose of this study was to question the contribution of the soil mix material to the bending resistance of the soil mix walls, up to now designed only on the basis of the steel (H- or I-) beam resistance. Various soil conditions and execution processes were considered in the course of this study. The present paper discusses the test results in terms of deflection, bending moment, steel-soil mix adherence and stresses in the steel reinforcement. The 'real-scale' bending stiffness of the reinforced soil mix walls is then computed by back analysis. The measurement of the stresses in the steel (HEA or IPE) beams shows an efficient interaction between the soil mix material and the steel reinforcement: the yield strength was reached in the steel beam at bending moments 20 to 70 % higher than without any contribution of the soil mix.