The Post Pampeano Formation is composed of high plastic clays and silts which was deposited in fresh fluvial water and in marine environments. Actually this formation covers the left bank of the Rio de la Plata and the Delta of the Paraná river. At some places the thickness of the deposit reach 30 m. In this work, the Post Pampean Formation was characterized by means of in situ and laboratory tests. The results were modelled by means of the Cam-Clay constitutive model. A procedure is described for the prediction of the undrained shear strength of the soil Formation if known: the index properties mean effective pressure and water content. The reported data will be used in a large-scale projected to be constructed in this soil Formation.