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 MIL-HDBK-419A
The discontinuity in the temperature curve (Figure 2-3(b)), indicates that at below freezing temperatures the
soil resistivity increased markedly. This undesirable temperature effect can be minimized by burying earth
electrode subsystems below the frost line.
2.4 MEASUREMENT OF SOIL RESISTIVITY.
2.4.1 General. It is not always possible to ascertain with a high degree of certainty the exact type of soil
present at a given site. Soil is typically rather nonhomogeneous; many types will be encountered at most
locations. Even with the aid of borings and test samples and the use of Table 2-3, the resistivity estimate can
easily be off by two or three orders of magnitude. When temperature and moisture variations are added to the
soil type variations, it is evident that estimates based on Table 2-3 are not sufficiently accurate for design
purposes. The only way to accurately determine the resistivity of the soil at a specific location is to measure
it.
2.4.2 Measurement Techniques. The most commonly used field methods for determining soil resistivity employ
the technique of injecting a known current into a given volume of soil, measuring the voltage drop produced by
the current passing through the soil, and then determining the resistivity from a modified form of Equation 2-1.
2.4.2.1 One-Electrode Method. To illustrate the principles of this technique, first visualize a metal
hemisphere buried in the earth as shown in Figure 2-4. In uniform earth, injected current flows radially from
this hemispherical electrode. Equipotential surfaces are established concentric with the electrode and
perpendicular to the radial directions of current flow. (Regardless of the shape of an electrode, it can be
approximated as a hemispherical electrode if viewed from far enough away.) As the current flows from the
hemisphere, the current density decreases with distance from the electrode because the areas of successive
shells become larger and larger. The current density within the earth, at a given distance x from the center of
the electrode is
(2-3)
where
I = current entering the electrode and
= area of the hemispherical shell with radius x.
At the point x the electric field strength can be obtained from Ohm's law:
(2-4)
where
is resistivity of material.
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