Volcanic rocks, and particularly those of common occurrence in Canary Islands, do exhibit a special mechanic behaviour that makes them mechanically collapsible. Under a low stress level they behave as rocks, with high deformability modulus. However, when the stress level is high its internal structure may be destroyed and its deformability may increase greatly, thus causing it to behave as a soil. A theoretical model of collapse based on energetic criteria is presented. An "energy expand law" is proposed for these mechanically collapsible rocks as well as the parametric equations for the collapse lines, all for a non associative flow rule.


Low-density volcanic agglomerates can be regarded as a typical example of macroporous rocks. Their mechanical behaviour is very special. When stress levels are low they behave like real rock with very high deformability moduli. By contrast, when stress levels are high their structure is destroyed and their deformability increases greatly, causing them to behave like a soil (Uriel and Serrano, 1973 and 1976). This phenomenon is referred to as mechanical collapse, and the materials that are subjected to this process are known as mechanically collapsible rocks. Two types of theoretical model's have been proposed: structural models (Uriel & Bravo, 1970; Uriel & Serrano, 1973) and energetic models (Serrano, 1976; Serrano, 1996; Aversa & Evangelista, 1998). This second kind of model is the one analysed here.

2.1. Behaviour domains

The behaviour pattern shown in Fig.l could be established in the stress space (q, p). There is a domain in that space that contains the origin, within which the material behaves elastically in a normal load process. For stresses within this domain the agglomerate behaves as a rock. The boundary for the elastic domain is what is referred to as the lower collapse line. When the stresses reach this line, destruction of the structure of the material starts to take place, in such a way that when the lower collapse line is crossed, the transition zone is entered and this, in turn, is externally limited by the upper collapse line. The failure occurs within this transition zone. By the time the stress reaches the upper collapse line the structure of the material has already been completely destroyed. The material can now actually be regarded as a soil. From this time the material will behave as a soil and could thus have one peak strength and another residual strength. The behaviour will either be stable or unstable and this will depend upon what point it has arrived when it reaches the upper collapse line. Where homogeneous materials are concerned, the lower and upper collapse lines are in exactly the same place, and the transition zone disappears. The presence of both upper and lower collapse lines has been postulated in this description; that is to say, it has been assumed that the material is heterogeneous. These lines, which are in fact no more than the boundaries for the behaviour domains, depend upon the stress path.

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