4D cross-borehole Electrical Resistivity Tomography to control resin injection for ground stabilization: a case history in Venice (Italy)

Settlements of building foundations are generally due to water content changes in the shallow subsurface, both by natural and man-made causes. Although resin injection is revealed to be a satisfactory solution for ground consolidation, a continuous monitoring of the process is needed to achieve optimal results. In order to control the injection of expanding resins, a field procedure is developed, based on the use of time-lapse three-dimensional (4D) Electrical Resistivity Tomography (ERT). The choice of electrical resistivity, as a parameter for designing and monitoring the consolidation work, is based on the key assumption that this physical property is the most sensitive to water content changes in soils. During the injection stage, repeated ERT acquisitions allow the injection process to be controlled and the injection schedule and its parameters to be modified, whenever necessary. In this paper the procedure and its results are illustrated, through a case history in Venice (Italy), where salt-water bearing soils also had to be taken into account. Careful analysis of electrode array configurations and parameters had therefore to be performed in advance. Horizontal and vertical sections from the resulting 3D resistivity models show, through a noticeable local increase of the resistivity at and nearby the injection points, that re-homogenization of soil is successfully achieved. Repeated 3D ERT measurements, carried out three and a half years after the consolidation work, show that stabilization of the subsoil below and around settled foundations is achieved, as also confirmed by comparing suitable extensimeter measurements on overlying structures, carried out before and after the treatment