Two main types of "macroporosity" in pyroclastic volcanic rocks can be distinguished: "reticular" and "vacuolar". The first type is produced when the large pores are located between grain particles. The second type is produced when large pores are located inside a vitreous rock mass. However, a mixed type can be defined in most cases, when both kinds of void are present. At other times, the pyro-clasts do not exhibit any kind of "macroporosity". An extensive study of how the type of porosity may affect the strength of the material is being carried out at CEDEX geotechnical laboratory. Samples with different types of "macroporosity" have been tested under isotropic loads. As a result of a theoretical study, an expression of the isotropic collapse load has been obtained, for any type of "macroporosity", and compared with test results.

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