Use of Resistivity to Estimate Porosity Variations in Loose Sands after Compaction by Resins Injection Treatments

In this paper we report the results of a study on the possibility of using Electrical Resistivity Tomography (ERT) techniques to verify the efficiency of soil treatments by injection of expanding polyurethane resins in loose saturated sands with high liquefaction susceptibility. In order to test the injection treatment and to monitor it through geophysical and geotechnical measurements, a small test site has been devised in the San Carlo — Sant’Agostino village, close to Ferrara (Northern Italy), a place affected by the effects of soil liquefaction during the earthquake occurred on May, 2012. This test is not meant to be exhaustive, also considering that the planned ERT survey has been greatly simplified and influenced by many constraints, but the study is intended to stimulate the interest on the possibility of using resistivity for evaluating, even quantitatively, the improvements in “mechanical” properties of liquefiable sandy horizons after compaction treatments. Electrical resistivity tomography techniques can be considered a good tool for the verification of compaction of liquefiable sandy horizons. The reduction of the voids that follows the consolidation is well indicated by the increase in resistivity that allows to highlight continuity and depth of treatment. Furthermore, the possibility of using petrophysical correlations for estimation of changes in porosity on the basis of the resistivity variations certainly makes the technique very powerful in evaluating the improvements in terms of “mechanical” properties.