User manual

GMC-I Messtechnik GmbH 43
Geologic Evaluation
Except in extreme cases, the ground is measured down to a
depth which is roughly equal to probe distance d.
This makes it possible to arrive at conclusions regarding the
ground’s stratification by varying probe distance. Layers which
are highly conductive (water table), into which earth electrodes
should be installed, can thus be discovered within a region which
is otherwise poorly conducting.
Soil resistivity is subject to considerable fluctuation which may be
due to various causes such as porosity, moisture penetration,
concentration of dissolved salts in the ground water and climatic
fluctuation.
Characteristic values for ρ
E
relative to season (soil temperature
and the soil’s negative temperature coefficient) can be approxi-
mated quite closely by means of a sinusoidal curve.
Soil Resistivity ρE Relative to Season Without the Effects of Precipitation
(earth electrode depth < 1.5 m)
A number of typical soil resistivity values for various types of
ground are summarized in the following table.
Soil Resistivity ρ
E
with Different Types of Soil
Calculating Dissipation Resistance
Formulas for calculating dissipation resistance for common types
of earth electrodes are included in this table.
These rules of thumb are entirely adequate for actual practice.
Formulas for Calculating Dissipation Resistance R
A
for Various Earth
Electrodes
R
A
= dissipation resistance (Ω)
ρ
E
= soil resistivity (Ωm)
I = length of the earth electrode (m)
D = diameter of a ring earth electrode, diameter of the equivalent surface
area of a mesh earth electrode or diameter of a hemispherical earth
electrode (m)
F = surface area (sq. meters) of the enclosed surface or a ring or mesh
earth electrode
a = Edge length (m) of a square ground plate; a is replaced with the fol-
lowing for rectangular plates: √
bxc, where b and c are the two
sides of the rectangle.
J = volume (cubic meters) of an individual foundation footing
Type of Soil Soil Resistivity ρ
E
[Ωm]
Marshy ground 8 … 60
Arable soil, loamy and clayey
soil, moist gravel
20 … 300
Moist sandy soil 200 … 600
Dry sandy soil, dry gravel 200 … 2000
Rocky ground 300 … 8000
Rock 10
4
… 10
10
+ρ
E
(%)
10
20
30
-10
-20
-30
Jan. MarchMay July Sept. Nov.
Number Earth Electrode Rule of Thumb Subsidiary Variable
1
Earth strip (star type earth
electrode)
—
2
Earth rod (buried earth
electrode)
—
3 Ring earth electrode
4 Mesh earth electrode
5 Ground plate —
6
Hemispherical earth elec-
trode
R
A
2 ρ
E
⋅
I
----------=
R
A
ρ
E
I
----=
R
A
2 ρ
E
⋅
3D
----------=
D 1,13 F
2
⋅=
R
A
2 ρ
E
⋅
2D
----------=
D 1,13 F
2
⋅=
R
A
2 ρ
E
⋅
4,5 a⋅
----------=
R
A
ρ
E
π D
⋅
--------=
D 1,57 J
3
⋅=