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 MIL-HDBK-419A
where r and h are measured in centimeters. If h < 6.2 r, the resistance to earth will be greatly influenced by
the resistivity of the granite underlayment; if h > 6.2 r, the resistance approaches that for homogeneous earth
with resistivity,
2.6.4.2 Vertical Rod.
When a vertical rod is driven through a high resistivity superficial (upper) layer into a lower resistivity subsoil,
an adjustment can be made to the resistance to earth expression for homogeneous soil by substituting a reduced
"effective length" of the ground rod. Letting
be the effective length (2-3)
(2-34)
where
= physical length of rod,
= resistivity of upper layer,
= resistivity of subsoil, and
h = depth of upper layer.
the effective length of the rod is reduced to
When the subsoil has a higher
Note that if
resistivity than the top layer of soil
the current discharged through a slender vertical rod with length
equal to the thickness of the superficial layer of soil will tend to remain in the superficial layer of soil. The
"mean path" of the superficial layer current, that is the radial distance at which half the discharge current has
entered the deeper soil, is approximately (2-3)
(2-35)
If the dimensions of the earth electrode subsystem are large compared to the thickness of the upper stratum,
the upper layer becomes insignificant and the resistance to earth can be computed as through the soil were
the resistivity of the subsoil.
homogeneous with resistivity equal to
2.6.4.3 Grids.
A useful approximation for the resistance-to-earth of a horizontally extensive electrode system is given by
Equation 2-27.
If the soil has a superficial layer with resistivity
and a subsoil with resistivity p2, the resistance to earth of
a grid in the superficial layer is given by (2-3)
(2-36)
2-33
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