Evaluating physical and mechanical changes induced in the subsoil by expanding resins injections for consolidation purposes by means 3D Electrical Resistivity Tomography and geotechnical tests

Instability phenomena in the urban environment and their consequences on infrastructures are a very topical issue in engineering practice. There are a lot of technical solutions on the market and, among these, consolidation techniques have assumed a role of primary importance. Due to the effectiveness of the intervention, the reduced invasiveness and especially the implementation and integration of monitoring techniques like the geophysical method of time-lapse 3D electrical resistivity tomography and geotechnical tests like dynamic penetration tests (Morelli et al., 2006, Santarato et al., 2011), the injections of expanding resins are one of the most established among consolidation methods. This work tries to deepen knowledge about physical and mechanical changes in the subsoil after consolidation with expanding resins. The research is divided into two main branches: experimental tests on a controlled test site and continuous data analysis on a wide archive of data, kindly provided by a specialized company operating in the market. Geological, geotechnical and geophysical test have been carried out in the test site in order to obtain a detailed picture of the physical and mechanical properties of the undisturbed state. In particular CPT tests, geotechnical laboratory testing, 2D and 3D electrical resistivity tomography, MASW were carried out. The experimental tests were carried out in three main phases. In the first one the effects induced by injections on the mechanical properties of the involved materials have been studied. In the second one the effects induced on the electrical properties and in the last one the possible relationship between the physical and mechanical properties measured after the injections have been studied. From the mechanical point of view, obtained results shows remarkable increases in load-bearing capacity of subsoils, in the ability to fill voids and the consequent migration of the interstitial water. The results also show the effectiveness of the 3D electrical resistivity tomography in the three-dimensional and temporal evolution monitoring of the phenomena taking place in the subsoil during the injections, as well as a good relationship between electrical and mechanical properties after the consolidation. Before the analysis of case histories a rigorous and careful data sorting work aims to generate datasets as homogeneous as possible. The data is classified by buildings properties (number of floors, presence of basements, topographical variations, etc..), soil properties (detected in situ by the geologist), presence of groundwater, etc.. Both 3D electrical resistivity tomography and dynamic penetration tests are performed before and after the injections. Electrical resistivity and dynamic cone resistance percentage change is then calculated. The obtained results show that there is a high percentage of cases in which the curve of the percentage change of the electrical resistivity is similar to the curve of the dynamic cone resistance percentage change. This work aims to improve our understanding on the effects induced in the subsoil by the expanding resins injections. The research has led to better understand, in quantitative terms, physical and mechanical changes that occur as a result of expanding resins injections and to increase knowledge about the response of the 3D ERT applied to the monitoring of the consolidation process. Thanks to the large amount of case histories analyzed, results clearly demonstrate a relationship between the ERT geophysical method and the dynamic cone penetration test.