Stability assessment of volcanic lava tubes in the Galápagos using engineering rock mass classifications and an empirical approach

The stability of the lava tubes of the Galapagos Islands has been analysed from an engineering perspective. Lava tubes are natural caves originated by differential cooling between the centre and the outer part of a moving lava flow. Although these cavities are highly frequented in the Galapagos and the rest of the world, there are only a few studies that treat the stability and security of such caves. In the present study, the stability and security of some of the most representative show-caves in volcanic tubes, in the Galapagos islands of Santa Cruz and Isabela, are evaluated. We have used an internationally accepted empirical methodology for the evaluation of tunnels and underground man-made constructions. The methodology used in this general stability analysis is a geomechanical classification, also known as the Q index and the scaled span methods. Of the studied caves, some result to be relatively stable, while others demonstrate (back analysis) signs of instabilities with unravelling of blocks from the top and detached slabs in walls. The methodology has been also applied in relatively stable parts of cavities in order to verify that this methodology works as a preliminary assessments. The obtained data indicate that these methodologies, which originally were created for man-made underground works, are a useful tool in a preliminary assessment the stability of natural caves such as lava tubes.