LABORATORY TESTS ABOUT RESISTIVITY VARIATION IN SOIL, IN CONNECTION WITH ROOT PRESENCE

Knowledge about root system distribution plays an important role in slope stability studies, as this factor grants an increase in soil geotechnical properties (cohesion and angle of friction) and determines a different underground water circulation. Published studies about in situ application of Electrical Resistivity Tomography analysis show how the root presence affects the measurable soil resistivity values, confirming the suitability to investigate the application of such technique, aiming to estimate root density in soil with an indirect and non-invasive method. This study, laboratory-based and led on reconstructed samples in controlled condition, aims to find a correlation between the resistivity variations and the various factors that can affect them (humidity, bulk density, presence of foreign bodies). The tests involved a clay-loam soil taken from Quaracchi (Florence, Italy), in an experimental fir-wood (Picea abies) owned by the Scuola di Agraria of Università degli Studi di Firenze, the first-chosen site for field applications of Electrical Resistivity Tomography. The soil has been dried out in a lab stove, sieved at 2 mm, and then placed in a Lexan box (30´20´20 cm), without compaction. Inside the sample have been inserted 3 series of 4 iron electrodes, at three different depths. Resistivity measures are conducted on the three levels using a Syscal R2 with electrodes connected in a Wenner configuration. Root presence is simulated inserting 48 bamboo sticks (simple geometry, replicable root density). The tests were repeated in time, monitoring the natural variations in humidity (evapotranspiration) and bulk density (compaction). The first results show an increase in resistivity with the decrease in mean soil humidity and increase in bulk density. Root presence, on equal terms, entails higher soil resistivity values whose trend appears, on first impression, to be fixed in an exponential law in relation to humidity.