Resistivity method applied to the investigation of earth embankments

Electrical resistivity is one of the most important and interesting physical parameters of an earth embankment in that it can be related to the lithology and geomechanical properties. By applying the electrical profiling method, resistivity measurements were made along two typical sections of river embankment: one along the Danube embankment and the other along the Tisa embankment. The resistivity was measured during the two hydrologically extreme periods, at low and high water level. Statistical analysis of the data made it possible to give an interpretation of the geomechanical conditions along the two sections studied. The attempt to use geophysical methods in investigating the condition of earth embankments is based on the assumption that the geophysical parameters of the embankment vary with the hydrological state which controls its geochemical properties. Two factors are favourable for the geophysical investigation of an embankment: the body has a regular, constant shape and much is known about its lithology and the hydrogeological situation. Naturally, the embankment includes heterogeneous lithological and geomechanical zones of various dimensions. These zones may reasonably be expected to give rise to geophysical anomalies, whose amplitudes will vary with the hydrological state of the embankment. The moisture content of the embankment is controlled by variations in the river water level, the water table on the landward side of the embankment and the rainfall. The effects of moisture will be amplified in less compact or geomechanically weaker zones. Compared to other geophysical parameters, resistivity is the most sensitive to moisture content. Figure 1 shows the relationship between the resistivity of the embankment material, which consists of clay and sand, and the moisture content measured in the laboratory. Extremely high resistivity values were obtained for dried cores, and the lowest resistivity values were obtained for wet and water-saturated cores. It may be concluded that systematic measurement of resistivity throughout an embankment would have a high probability of locating zones with the highest moisture variation. Naturally, these changes will be associated with changes in porosity or geomechanical properties (relative compactness) of the embankment.