# Stabilization Methodology in Foundation Soils by ERT-3D Application in Estepona, South Spain
## 5 Conclusions and Follow-Up  
The ERT-3D technique proposed in this research proved to be versatile, fast, and cost-effective for detecting subsurface anomalies. In the present case study, the researchers were able to analyze the subsurface to a depth of 5.80 m, deep below the severely damaged building. Nevertheless, anomalies only existed at a depth of 3.90 m, below the foundation of load-bearing wall B. When the subsoil has holes, those spaces are filled with air or with water which, when evaporated, are dielectric. This means that the terrain presents a strong gradient anomaly and very high resistivity values, which causes serious damage to the foundation and structure of the building. Our methodology solves this problem, by reducing the resistivity of these initial anomalies (cavities) by means of the combined growth injection in two passes, with the control of resistivities through 3D tomography, and returning the stabilization of the foundation of the building in a non-invasive way. It has been applied to a case study that has been totally effective, where initially we had resistivities up to 3000 \(\Omega\) m and it was effectively reduced with our methodology to 55 \(\Omega\) m.  
In the present case study, to implement the methodology, the subsoil cavities and weaknesses were first identified and located beneath the building's foundations. Secondly, a consolidation grout injection campaign was carried out, enhancing the subsoil conditions to an optimum result. The consolidation fluid injections eliminated the high value of initial subsoil resistivities (up to 3000 \(\Omega\) m) and moved those resistivities to values between 55 and 1000 \(\Omega\) m, filling in those cavities. Through this process, ERT-3D showed itself to be a fundamental, minimally invasive tool for research. It was useful both during the project  
Figure 11: Current state of the main building. Image taken after the intervention and application of the methodology. Had this methodology not been used, the only solution would have been demolition of the building.  
phase and in the follow-up, underpinning work when the consolidation injections were placed under the foundations.  
The limitations of our methodology increase with the depth of the altered subsoil, from 5.80 m. As has been shown in our research, it is difficult to have a reliable electrical section of resistivities to intervene later with consolidation injections. Although it would be necessary to use another geophysical methodology and another type of deep consolidation intervention using a pile or micropyle, our methodology is appropriate for emergency surface consolidations in disturbed porous subsoils. As they work safely outside the affected building, this methodology offers researchers an accurate lithological model and correctly highlights subsurface anomalies. Hence, ERT-3D allows them to determine the causes of the instability of the foundation and building and how it can be later consolidated and stabilized, in addition to monitoring the chosen solution.  
Conceptualization, A.G.-M. and R.C.; methodology, A.G.-M.; software, A.G.-M. and M.F.-H.; validation, A.G.-M., J.I.Y. and M.F.-H.; formal analysis, A.G.-M. and M.F.-H.; investigation, A.G.-M., J.I.Y. and R.C.; writing--original draft preparation, A.G.-M. and R.C.; writing--review and editing, R.C., A.G.-M.; visualization, R.C. and M.F.-H.; supervision, R.C. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.