Stability Analysis of Lava Tunnels on Santa Cruz Island (Galapagos Islands, Ecuador) Using Rock Mass Classifications: Empirical Approach and Numerical Modeling

The cavities or lava tubes in the Galapagos Islands were formed by the differential cooling of the basaltic flow of the volcanoes surrounding these islands. In this article, a stability analysis was carried out to determine the degree of safety of different lava tubes using three methods: two empirical ones based on geomechanical classifications and one strain–strain (Hoek–Brown failure criterion). The methodology used consisted of the following phases: (i) compilation of information based on existing geomechanical mapping; (ii) geomechanical classification of the rock mass using Barton’s Q index and rock mass rating; (iii) steady state qualification using the geotechnical index of cavities (GCI); (iv) numerical modeling applying the Hoek–Brown criterion; (v) comparison of methodology and discussion of the results. The data obtained indicate that the methodologies used to evaluate the stability of the lava tubes have high reliability since they allowed the characterization of the different lava tubes. As the final “product” of the investigation, a graph was drawn up in which the empirical observations and the safety factors obtained with the numerical analysis (stress–strain) were superimposed, classifying the lava tubes as stable and non-stable. It can be concluded that the characterization methodologies used in this article can be applied to similar cases and fill a gap in rapid preliminary analyses of the degree of stability and risk of cave collapse