This article exposes one of the main engineering problems in the design and construction of the upper reservoir of El Hierro hydro-wind plant, relative to the high settlement of the bottom in the southeast area. The high settlements measured during the construction phase are consequence of the geological-geotechnical settings of the site, a natural depression of a volcanic crater occupied by highly deformable soils derived from the weathering of volcanic materials. Ground improvement was carried out by partial preloading (mobile dune), preceded by the execution of a trial embankment in the southeast area of the reservoir, where the greatest thickness of deformable soils was identified.

On El Hierro (Canary Islands, Spain), the company Gorona del Viento El Hierro S.A. carried out the construction of a Wind-Pumped Hydro Power Station in order to achieve the energy autonomy on the island through 100% renewable energy. The station consists of two man-made water reservoirs, whose design was adapted to the island topography and emplaced in a valley and an inactive crater. In order to study the subsurface of both reservoirs, the characteristic heterogeneity of volcanic materials was taken into account. Therefore, a research campaign combining different methods was proposed; mechanical investigations traditionally applied in engineering (rotary boreholes, dynamic probing super heavy tests and pits) and geophysical methods (seismic refraction tomography and electrical resistivity tomography) were both utilized. The aims were to determine the geological structure of each site, detect possible anomalies or structures (i.e., faults, volcanic cavities, etc.), identify the different lithologies, determine the soil thickness and depth to bedrock, predict the excavatability of the materials, and obtain data about the physical properties and mechanical behaviour of the ground in an effort to carry out a geotechnical site characterization of both sites. At the site of the upper reservoir, electrical resistivity tomography recorded a high thickness of low resistivity soils, which were subsequently investigated by deep boreholes in order to sample and test them properly, since they could generate high settlements during the construction of the reservoir. At the site of the lower reservoir, the existing palaeorelief in the basaltic bedrock was detected by seismic refraction tomography, which allowed for the selection of the appropriate excavation machinery for the planned earthworks. The geological interpretation of the P-wave velocity models and electrical resistivity models are shown in this paper, as well as a correlation between geotechnical parameters and geophysical properties of volcanic materials and recent deposits, which will be useful for other civil projects which may be carried out in the future at sites with similar geological conditions.

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