Three-dimensional Electrical Resistivity Tomography to control the injection of expanding resins for the treatment and stabilization of foundation soils.

In this paper we describe how time-lapse three dimensional Electrical Resistivity Tomography (3D ERT) can be advantageously integrated in the standard practice of shallow polyurethane resin injections, to improve the reliability of ground treatment and to reduce its costs. Settlement of buildings has several causes, which are generally triggered by movement of fluids in the subsurface. The goal of the injection of expanding resins is to occupy voids and displace water so that the continuity and stiffness of terrains around and below the settled foundation are re-established. In order to control the effectiveness of the consolidation beneath the building, non-invasive geophysical methods such as Seismics, ERT or GPR can be used. Among these, ultimate 3D ERT developments provide the best solution for this application because of the fast imaging capability, the extreme reliability and repeatability of the measurements and the high correlation of electrical resistivity to soil moisture, which is known to be the most common cause of settlement. Given the often limited accessibility of work sites, a successful practical approach was focused on innovative arrangements of surface electrodes (inside and outside of buildings) and acquisition geometries, combined with appropriate measurements and processing procedures, that enable the effective 3D imaging of the foundation soils, before, during and after the injection treatment. Details on the field procedures, validated by a sensitivity and resolution analysis, are introduced through the description of a case-history in which the pre-injection ERT measurements, combined with structural and geotechnical evidences, allow to investigate the status of foundations and lithological eterogeneities. Time-lapse ERT surveys control soil homogenization and fluid movements and provide a quasi real-time feedback for modifying and modulating the parameters of treatment, which would be very difficult with direct methods. The illustrated case-history has been purposely selected since it represents the most common practice occurring throughout Italy and allows to assess and to discuss the current limitations and the future developments