The area of influence taking in Campo de Gibraltar and the Costa del Sol is very geologically complex, hence the importance of accurate geotechnical studies, and in particular, the need to follow expert recommendations.
Geologically, the area under study is located in the context of the Cordillera Bética or Betic System of mountain ranges, which can be divided into three major geological units, one of which is the Campo de Gibraltar Complex, appearing mainly in the provinces of Cádiz and Málaga, although there are also terrains in North Africa which are tectonic equivalents of this complex.
The Sotogrande area is located in the Campo de Gibraltar Complex, specifically, in the Manto de Aljibe Unit. This unit consists of sequences (turbidites), in other words marine sediments with a large percentage of sandstone levels.
Cohesive materials (clays) predominate, with a partly altered clay layer, all under a layer of topsoil. From the hydrogeological point of view, these soils have low permeability, with a risk of medium-high expansiveness, but low risk of subsidence. Occasionally, especially due to human urbanisation activity, we find highly altered upper strata, an anthropic filler layer which may become very powerful.
In contrast, areas inland from the Costa del Sol such as La Zagaleta are located in an area of significant tectonic contact between materials from different mantles, dominated by a large Anticlinal Fold, which brings peridotites and older materials such as schists and gneisses to the surface.
Geotechnical studies reveals the problem of contact between peridotites and schists is the generation of low-angle faults between both formations (extensional faults), with orientations along the slope, which can cause landslides.
Meanwhile, in other areas closer to the coast, such as Sierra Blanca, located in the Marbella complex, we can find different strata formed of schists, phyllites, limestones, slates, clay levels, and limestone conglomerates, which as they dissolve can form cavities, etc.
To sum up, this is an area of great geological complexity, requiring thorough and exhaustive study to identify the main risks and adapt construction foundations to the terrain, not only from the point of view of structural safety, but also considering economy.
Geotechnical studies are compulsory, and their scope and intensity are mandated in the building’s technical code, and more specifically in the basic document on structural safety in foundations. The working method used means field work must be carried out in situ, consisting of rotary core drilling for the continuous extraction of core samples, from which soil identification samples are taken; standard penetration tests (SPT) or dynamic probe super heavy tests (DPSH), and the mandatory visit of a qualified soil characterisation technician. With the results obtained and supported by laboratory work: chemical analysis, resistance, etc., a document is produced specifying the geotechnical conditions and recommended foundation types.
The Standard Penetration Test or SPT takes place within the borehole during drilling, and consists of driving a sample tube of standard dimensions 45 cm into the ground, counting the number of hammer blows needed for 15 cm to penetrate. This test provides the N-value or standard penetration resistance.
The Dynamic Probe Super Heavy test seeks to semi-quantify the permissible soil tension at different depths by obtaining the N20, corresponding to the number of blows needed to penetrate 20 cm of soil. This test does not provide any soil samples.
Laboratory tests of the altered or unaltered samples obtained enable us to determine different soil characteristics, such as particle size, chemical aggressiveness, Atterberg limits, humidity, density, expansion index (swelling potential), resistance characteristics (simple compression, direct shear, etc.).