Geotechnical Engineering

DHARM

750 GEOTECHNICAL ENGINEERING

Table 18.7 Typical Values of Electrical Resistivity of Soils and Rocks

(1 to 8 from IS: 1892-1979 Appendix B)

S.No. Material ρ (Ohm/m)

1. Limestone (Marble) 1012
   2. Quartz 1010
   3. Rock-salt 106 – 10^7
   4. Granite 5000 – 10^6
   5. Sandstone 35 – 4000


2. Moraines 8 – 4000


3. Limestones 120 – 400


4. Clays 1 – 120


5. Saturated inorganic clay or silt 10 – 50


6. Saturated organic clay or salt 5 – 20


7. Dry clays, silts 100 – 500


8. Dry sands, gravels 200 – 1000

In electrical profiling, an electrode spacing is selected, and this same spacing is used in
running different profile lines across an area, as in Fig. 18.18 (a).

In electrical sounding, a centre location for the electrodes is selected and a series of
resistivity readings is obtained by systematically increasing the electrode spacing, as shown in
Fig. 18.18 (b). Thus, information on layering of materials is obtained as the depth of informa-
tion recovered is directly related to electrode sparing. This method is capable of indicating
subsurface conditions where a hard-layer underlies a soft layer and also the situation of a soft
layer underlying a hard layer.

D 1

D 2

D 3

D

D

D

D

D

D

D

D

D

D

D

D
12 3 4
(a) Profiling arrangement (b) Sounding arrangement
Fig. 18.18 Electrode arrangement for electrical profiling and electrical sounding
The data may be plotted as electrode spacing versus apparent resistivity either in arith-
metic or in logarithmic co-ordinates. A change in the curve indicates change in strata.

18.7 THE ART OF PREPARING A SOIL INVESTIGATION REPORT

A report is the final document of the whole exercise of soil exploration. A report should be
comprehensive, clear and to the point. Many can write reports, but only very few can produce