The consolidation and stabilization of foundation soils through the injection of expanding polyurethane resin under a non-invasive diagnostic check by 3D-4D-ERT

Vertical differential settlement of buildings is produced by both natural and/or anthropogenic disturbances of the foundation soil, such as, for example, differential consolidation, circulation of water due to rupture of water pipes, seepage, suction, vibrations induced by vehicle traffic, excavations, construction defects and an incorrect design/execution of the work. Techniques of soil consolidation and stabilization by means of expanding resin injections are being advantageously employed to mitigate or even solve this issue. The resin used is a two-component polyurethane compound that, once its chemical reaction is completed, reaches an inert and lightweight solid state so that it is environment-safe and perfectly compatible with the injected soil. The resin is injected directly into the critical volumes by means of small diameter pipes. After the injection, the resin expands rapidly in the soil through an irreversible exothermic chemical reaction, so that it actively counteracts the settlement and its causes. Indeed, the observed effects are threefold: i) filling voids; ii) compacting soil; iii) reducing/removing interstitial water. Among the possible procedures to monitor soil consolidation and stabilization by injections of expanding resins, a procedure is hereafter described which involves the use of Electrical Resistivity Tomography (ERT). The rationale for the use of this noninvasive imaging technique is that electrical resistivity is a physical property sensitive to grain size, porosity and water content of sediments and to voids. Therefore, it can both acquire information about the subsurface structure, also from volumes of difficult access (e.g., below the foundations and the floor of the building), which is necessary for the consolidation project, and it can monitor the injection work in progress, in addition to traditional geotechnical testing. Specifically, the process is divided into several steps. A 3D-ERT is acquired first, covering both the volumes affected by the settlement and nearby stable volumes of soil underlying the footprint of the building, to accurately map the different resistivity that characterizes the anomalous volumes. The 3D resistivity model is integrated, wherever necessary, by geotechnical investigations, mainly penetration tests and sample identification, so as to optimally design the injection points, in terms of position, depth and number. During injections, the 3D-ERT and some geotechnical tests are repeated to check progressively the effects obtained by the treatment, so as to allow, if necessary, to modify the injection scheme until reaching the predetermined target, i.e., obtaining the greatest uniformity of chemical and physical characteristics between the stabilized volumes and the nearby stable ones, which are used as a reference. In some sites, where the settlement was stabilized by the above procedure, it was also possible to repeat a 3D-ERT after several years: results confirmed that the situation of the subsurface just after the injections had remained almost unaltered and the effectiveness of the procedure over time was demonstrated. The above approach is described hereafter and is accompanied by the description of the results from a specifically planned test site and from two work sites, selected as examples.