# Stabilization Methodology in Foundation Soils by ERT-3D Application in Estepona, South Spain
## 1 Introduction  
The success of architectural structures, which are built directly on the earth's surface, depends, among other factors, on the support offered by the foundation materials bearing the structures' loads [1, 2, 3, 4, 5, 6]. In turn, the ability of a building's foundation to offer the necessary support for architectural structures depends on the bearing capacity of the soils underneath, and, if the upper layer has heterogeneous physical properties, it could cause spatial variability in the foundation material's strength. Spatial variability of the soil's bearing capacity puts stress on poorly supported architectural structures. Associated structural failure could occur as total, partial, or differential settlement, or even total collapse of the structure, with differential settlement in the foundations being one of the worst problems [7, 8, 9, 10]. The global prevalence of failure and collapse, with the associated loss of life and property, has made it necessary to ensure that buildings are properly constructed [11]. It is common to find many stress-induced cracks and other defect-related issues due tothe various structures and foundations of historic, public, and private buildings, and some of these problems are disasters in the making [12; 13; 14]. This necessitates proposing a non-invasive intervention methodology for the subsoil of these buildings to avoid collapse. One of the most used non-invasive techniques for subsoil exploration is geophysics and, more specifically, electrical resistivity tomography, in both two and three dimensions [15; 16; 17; 18; 19; 20; 21], and metrological perspectives of tomography in civil engineering [22]. In recent years, this method has proven to be an efficient tool, not only to monitor wall degeneration [23], but also to detect other types of problems in building foundations, such as subsoil degradation [14; 24; 25]. Spatial variability in load-bearing rock/soil beneath a building puts stress on poorly supported structures, causing failure [10; 26]. Therefore, a geophysical investigation offers a faster, non-invasive means of obtaining detailed, credible information about the subsurface under a building. Electric tomography can also image the ground's distribution and structural deformation, both of which offer credible information regarding the strength the rock/soil is likely to offer a building [24; 27; 28]. Electrical resistivity imaging has, for decades, been very effective in illuminating the subsurface, and apt at providing information about the soil's physical properties for economic, environmental, and geological engineering. As mentioned, this technique is a useful and non-invasive method to diagnose subsoil problems in shallow foundations of buildings [3; 29; 30; 31; 32], which represent 25% of the claims reported in Spain, according to recent data provided by Aseguradora Mutua de Arquitectos Superiores, a public limited company (ASEMAS S.A.). However, the three-dimensional geophysical research approach ERT 3D [33; 34; 35] is better able than the two-dimensional approach ERT 2D to characterize the subsurface and determine heterogeneity in measured rock properties along the vertical (\(Z\)) and orthogonal horizontal (\(X\), \(Y\)) axes [36; 37].Thus, the determination of the variation in the soil properties along the three orthogonal directions would allow the evaluation of the spatial variation in the strength of the foundations, imposed by the heterogeneous properties of the soil [38]. Electrical resistivity tomography is an appropriate procedure for detecting and controlling underground consolidation and stabilization, specifically the ERT-3D tomography technique [39; 40; 41; 42]. As mentioned above, the method consists of setting out parallel lines of observation, which cover the study area, to obtain underground data; however, the key point lies in processing the data.