Site selection is, no doubt, one of the most important preliminary studies in any telescope project since the image quality of the telescope will be limited by the site quality. Traditionally, geotechnical studies have had relatively little importance, normally being carried out once the site was already selected. However, with the increase of the telescopes size and mechancial requirements, the geotechnical studies have become increasingly important. Soil characteristics can make a big difference to the cost of an ELT. A thorough geological and geotechnical investigation of the possible site for the erection of an ELT should be carried from the beginning of the site selection process. It is very important to know the geology of the subsoil to determine the geomechanical properties of the materials, to determine the most suitable layers for anchoring the foundations, and to be able to estimate construction costs.