Characterisation of soils with stony inclusions using geoelectrical measurements

Characterisation and sampling of coarse heterogeneous soils is often impossible using common geotechnical in-situ tests once the soil contains particles with a diameter larger than a few decimetres. In this situation geophysical techniques - and particularly electrical measurements - can act as an alternative method for obtaining information about the ground characteristics. This paper deals with the use of electrical tomography on heterogeneous diphasic media consisting of resistive inclusions embedded in a conductive matrix. The adopted approach articulates in three steps: numerical modelling, measurements on a small-scale physical model, and field measurements. Electrical measurements were simulated using finite element analyses, on a numerical model containing a random concentration of inclusions varying from 0 to 40 %. It is shown that for electrode spacing 8 times greater than the radius of inclusions, the equivalent homogeneous resistivity is obtained. In this condition, average measured resistivity is a function of the concentration of inclusions, in agreement with the theoretical laws. To apply these results on real data, a small-scale physical model has been built, where electrical measurements were conducted both on the model and on each phase. From these laboratory measurements, a very satisfying estimation of the percentage of inclusions has been obtained. Finally, the methodology applied to a real experimental site composed of alluvial fan deposits made of limestone rocks embedded in a clayey matrix. The estimated percentage of rock particles obtained via electrical measurements was in accordance with the real grain size distribution