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<p>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  km<sup>2</sup>, 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.</p><p>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).</p><p>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”.</p>

<p>Between September and December 2021, a Strombolian fissure eruption with phreatomagmatic pulses occurred in the Westside of the Island of La Palma (Canary Islands, Spain), giving rise to a volcanic edifice called Tajogaite and whose cone reached a height of 1121 m. above sea level (200 m above the pre-eruption topography) with a volume of 34 Mm<sup>3</sup> consisting of the alignment of 6 different craters of 557 meters in length, direction N130ºE.</p><p>In order to analyze the stability of the volcanic edifice and estimate the possible risk of lahars from pyroclastic materials lying on the slopes of the volcano and the surrounding areas, a series of investigations have been carried out consisting on historical series of rainfall data, thickness estimation  of the ashes deposits, laboratory tests on in situ samples, density, friction angle and cohesion of the soils determinations, as well as a geophysical survey using MASW seismic technique geotechnical properties of the slope layers in the first 30 meters of the volcano slopes. Based on these investigations, a stability analysis of the slopes of the volcanic edifice has been carried out considering different rainfall scenarios. Slope stability calculations were carried out using the Slide V6 program, from geotechnical data obtained in laboratory and from limit equilibrium back analysis conditions. Two main scenarios were analyzed including the most probable and the least probable rainfall, corresponding to the return period of  10 years and 50 years respectably.</p><p>Risk of lahars had been indentify as possible for the 50 years return period, as a consequence of both the instability of the cone slopes and the surrounding ash deposits, associated with very intense rains. These data has been incorporated to the land planning of reconstruction development, particularly the adoption of preventive measures in case of intense rainfall, as well as for the design of drainage infrastructures.</p>

<p>Lava flows emitted by the Tajogaite eruption  (La Palma, Canary Islands) have covered an approximate area of 12 million m<sup>2</sup>, with thicknesses of up to 80 m. Type Aa flows represent more than 90% of the surface, the rest being Pahoe-hoe type. The urgent need to rebuild the affected roads has been the highest priority, along with territorial planning with urban proposes, infrastructures and agricultural development, being the lava temperature one of the most critical restrictive condition.</p><p>Geological, geotechnical and thermal investigations have been carried on including 4 boreholes up to 10 m deep in which a detailed geotechnical characterization and systematic measurement of temperatures at different depths. Temperatures measurements on the lava surface were also recorded using drones and in situ thermocouples. Detailed maps of lava flow thicknesses and geological cartographies, including fractures and volcanic cavities have been made. An empirical model of lava cooling rates has been developed based on geological, geotechnical data and temperature measurements.</p><p>Additionally a new cooling rate model has been developed as a function of depth, based on exponential equations in accordance with Newton's cooling law that relate temperature to time and whose main parameter is the cooling coefficient. This model has been applied to different lava depths. The results obtained indicate an acceptable agreement between the numerical models and the recorded data of temperatures. These results have been applied to the reconstruction of infrastructures and urban planning, allowing to identify favorable areas for infrastructures reconstruction and land use planning.</p>