Derivation of empirical relationships between geotechnical parameters and resistivity using electrical resistivity tomography (ERT) and borehole data at Sohag University site, upper Egypt.

The new site is located in desert region of rugged topography. Quaternary boulders, gravels, sands and clays intercalation are the main rock types in the area. Geophysical evaluation of the subsurface lithology and structures using 2D and 3D electrical resistivity tomography (ERT) of the proposed site for construction was the main target of the present study, in addition finding empirical relationships between rock's electrical resistivity and some geotechnical parameters are also another target. Four geoelectrical resistivity layers were discriminated after the 2D inversion. Inspection of all 2D inverted profiles and 3D electrical resistivity depth slices showed rapid lateral and vertical variations in electrical resistivity values indicating heterogeneity in soil section. Clayey gravels or clayey sands and clays facies are predominant and hence swelling actions of the foundation beds is highly expected. The results of electrical resistivity tomography (ERT) were correlated with that inferred from the boreholes within the investigated area. The geotechnical parameters: liquid limit L.L%, plastic limit P.L%, free swelling F·S% and density Qu were calculated for representative samples collected from drilled boreholes. All samples showed considerably high L.L%, low to moderate P.L% with high F·S% which characterizes clayey soil. A trial for finding empirical relationships between the geotechnical properties and the rock electrical resistivity (L.L%, P.L%, F·S% and Qu) were carried out. These relations revealed that there is a considerable correlation between them when the rock has a resistivity more than 400 Ω m and poor correlation when the electrical resistivity is less than 400 Ω m. Accordingly, inferring applicable empirical equations between rock electrical resistivity and geotechnical parameters can be strong in the dry successions with a limited clays content, but in the wet or clayey soil materials the correlation will be weak and risky in application. From all obtained results and relationships, highly engineering awareness should be taken into account during establishing any construction in this site. • Establishing a new building needs high awareness by subsurface geologic features. • The results show rapid resistivity variation indicating heterogeneity in soil/rock. • Empirical equations between resistivity and geotechnical parameters were calculated. • The equations show a good correlation when the soil/rock has resistivities >400 Ωm. • The correlation is strong in dry sequences with limited clay and moisture contents. [ABSTRACT FROM AUTHOR]