Deep mixing columns commonly are used to reduce settlement under road and railway embankments on soft soils. However, the structural behavior of the soil volume improved with columns is difficult to predict, due to the existence of considerable uncertainties in the mixing process and the structural interaction between the columns and the untreated soil. This paper probabilistically investigated two serviceability limit states of deep mixing columns from a system reliability perspective. A design framework employing the observational method is proposed that considers allowable residual settlements, excessive settlement from column yielding, and the curing time of the columns. The design framework facilitates an effective reduction of the geotechnical uncertainty during construction and promotes risk-aware decision-making during both design and construction of the embankment.