Stability Assessment of Volcanic Lava Tubes Using Engineering Rock Mass Classifications and an Empirical Approach

Volcanic caves, commonly referred to as lava tubes, are typically shallow subsurface cavities formed by the cooling of a generally basaltic lava flow under a roof or crust that cools faster and acts as a thermal insulator. These cavities can serve as tourist attractions, in which case their stability must be analyzed and ensured. Empirical rock mass classification systems, in this case we have applied the Q-index have been employed to evaluate the stability of underground excavations: mines and tunnels, including natural caves. We have identified that these approaches have limitations, particularly incorporating key geometric parameters such as roof thickness and cave length. In this study we have analyzed applicability of the Scaled Span Method (SSM) to volcanics caves. This method was originally developed for the stability assessment of crown pillar stability in shallow mines. We have developed a dataset of lava tubes (caves) located in the Canary Islands (Spain), the Galápagos Islands (Ecuador), and Jordan. In this research we have conducted geomechanical characterization using the Q-system, and also the Scaled Span to evaluate stability based on cave geometry and rock mass properties. The results indicate that, in general, the SSM yields more conservative stability estimates compared to the Q-system, particularly for shallow caves with limited roof thickness. Nevertheless, discrepancies between the two approaches are observed in several cases, highlighting the limitations of directly transferring empirical methods developed for mining excavations to natural cave systems. These differences underscore the need for careful interpretation and, where appropriate, complementary stability analyses. The Scaled Span Method is useful for preliminary assessment of volcanic cave stability, especially in scenarios where potential interaction with the ground surface is expected: buildings or roads on top. However, its application requires adaptation and critical evaluation due to the fundamental differences between engineered mining excavations and natural subsurface cavities.