The main factors affecting the mechanical behaviour of the different types of volcanic material of the Canary Islands can be found through classifying them. It is proposed that the materials be classified according to the extent to which the particles are imbricated and welded. A Hoek and Brown non-linear strength law is used when endeavouring to interpret the triaxial tests that have been conducted. The strength parameters are adjusted on the basis of the laboratory results. These parameters are reduced taking into account the scale effect, with a view to applying them to the rock mass as a whole. The strength parameters are shown graphically on the basis of the rock density and Hoek's GSI index.
The aim of this work is to establish the geomechanical behaviour of some of the granular, loose or cemented materials of volcanic origin, and to do so in a way that is consistent with a non-linear failure criterion. The Hoek and Brown failure criterion (1980) has been selected. The materials on the Canary Islands can be classified into two types (Bascones, 1996); I. Lava flow: two types of materials could be formed from an eruption that has reached the exterior when the mass is flowing on the surface, associated with nonviolent eruptions. One is formed as soon as the mass comes into contact with the surface, cooling quickly and giving rise to a chaotic scoria base that leads to a granular soil or a crushed or extremely disturbed rock. The end product of the other type of material, formed as a result of a much slower cooling, is a rock type material. These two types of materials, whose geotechnical behaviour is different (soil and rock), give way to each other vertically and, in many cases, they "overlap" on a horizontal direction. There is another type of lava flow known as "pahoehoe", which is extremely vesicle. It is not generally very thick. This type behaves as a stratified rock mass, that either has a very low level of heterogeneousness or none at all. II. Pyroclasts; it is formed when gases are violently released and when the melted or solid material is projected onto the surface. They are deposited close to the source or at great distances from it. They are classified on the basis of their origins, their size and their composition. All these deposits can be found connected to each other. This is mainly due to two processes: a) Welding; when the material is exposed to certain temperatures; b) Cementing; when fluids flow over or through them and secondary deposits build up on them.
The non-linear failure behaviour of all these materials is typical of rock (Serrano, 1976 and 1997 and Hoek and Brown, 1997). The first three factors are usually associated with each other. The more imbricated and welded the particles are, the greater their degree of compaction generally is. However, this is not always the case.