Preloading with a surcharge is today commonly used together with prefabricated vertical drains forembankment construction on clayey soil to accelerate primary consolidation and increase strength.Because of considerable uncertainty related mainly to the rate of consolidation, there is a need toaccount for this in the vertical drain and surcharge design to ensure quality in the embankmentconstruction. Addressing this issue, the paper presents a novel probabilistic design methodology that iscompatible with the observational method. The procedure evaluates the suitable surcharge load to beused in combination with a vertical drain design in order to ensure that the established design criteriaare satisfied with acceptable probability.