This paper evaluates the mechanical properties of jet-grouting columns used for soil improvement. This technique was used to create a slab for the excavation of the one subway station in Barcelona, Spain. Before field treatment of the very soft clayey soil, a field trial was built with some jet-grouting columns in order to define the correct grouting parameters and to determine the mechanical properties of the soil-cement mixture by collecting some samples from the columns. These samples were obtained with two different techniques: (1) as core samples, the traditional method in which the samples are collected in situ at a certain age and then tested in the laboratory; and (2) as fresh jet material, a new technique developed by the contractor for collecting the material in a fresh state, right after the grouting application, and then curing the sample in a controlled laboratory environment. At the same time, a laboratory-based formulation study of soil-cement was developed using samples of the clayey soil and the same kind of cement employed in the field trial. Afterwards, the mechanical properties, strength and modulus of the soil-cement mixture made in laboratory were compared with those of the field samples, for the same of specimen ages. An attempt was made to establish correlations between the mechanical properties obtained by the different test methods, for the different types of soil-cement mixtures.