Plastbloc is a new process. Its principle is to use compressed and ligatured blocks made of plastic materials. These blocks could be used to build lightweight embankments, to minimize loads on underground pipes and behind retaining walls. Because the compressing during the making of blocks is unidirectional, this material is made up of stake of layers. In this case, it has an axial orthotropy. With the purpose to study the rheological behavior of this material, a large number of simple compression tests were carried out. But the identification of all parameters is possible by realization of biaxial tests. A biaxial apparatus conceived specially for this type of material was built. Preliminary studies showed the good function of this apparatus. Then, biaxial tests were carried out and the characteristic functions were identified.