Geotechnical zoning of the new lava field of 2021 Tajogaite volcano (La Palma, Canary Islands) and its application in the recovery of the territory for new urbanization

The eruption of the Tajogaite volcano occurred between September and December 2021 in the westside of the island of La Palma (Canary Islands) buried an area of 12,19  km², and destroyed about 1700 buildings. The need of recovering the territory, led the regional authorities to commission a work to the Volcanological Institute of the Canary Islands (INVOLCAN), consisting on the elaboration of a geotechnical map with a classification for the aptitude of the terrain in terms of bearing capacity of buildings foundations and other structures, summarized in this work. The results were applied to 3 pre-selected areas called reconstruction sectors: La Laguna, La Asomada and Las Norias-La Majada. These sectors were divided into 250 x 250 m grids on which the classification was carried out.

From the field reconnaissance, and the support of photogrammetric tools, 5 geotechnical units were identified with similar geomechanical and geotechnical behavior: Unit 1, very scoriaceous "aa" lava flows with channels and levees of moderate unevenness; Unit 2, very scoriaceous "aa" lava flows with channels and levees of steep slopes; Unit 3, "pahoehoe" surfaces with cracks and cavities; Unit 4, pyroclastic cone of the Tajogaite volcano and proximal ash deposits; Unit 5, Scattered and distal ash deposits of the main cone. Based on the geomechanical quality of the materials, they were classified into four categories according to the value of the RMR index (Rock Mass Ratio): bad (RMR 0-25); moderate (RMR 26-50); good (RMR 51-75); very good (RMR 76-100).

Within the La Laguna sector, of the 19 grids analyzed, 17 were classified as "moderate" and 2 as “good”. In the La Asomada sector, 6 grids that compose it were classified as “moderate” and in Las Norias-La Majada sector, made up of 17 grids, 15 of them were classified as “moderate” and 2 as “bad”.