Page 125 - Corrosion Engineering Principles and Practice

100 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 101

The first set of data in Table 5.5, Set A, represents uniform soil
conditions. The average of the readings shown (∼960 Ω cm) represents
the effective resistivity that may be used for design purposes for
impressed current groundbeds or galvanic anodes.
Data Set B represents low-resistivity soils in the first few feet.
There may be a layer of somewhat less than 1000 Ω cm around the
1.5 m depth level. Below 1.5 m, however, higher-resistivity soils are
encountered. Because of the averaging effect the actual resistivity at
2.3 m deep would be higher than the indicated 1250 Ω cm and might
be in the order of 2500 Ω cm or more. Even if anodes were placed in
the lower-resistivity soils, there would be resistance to the flow of
current downward into the mass of the earth.
If designs are based on the resistivity of the soil in which the
anodes are placed, the resistance of the completed installation will
be higher than expected. The anodes will perform best if placed in
the lower resistance soil. However, the effective resistivity used for
design purposes should reflect the higher resistivity of the underlying
areas. In this instance, where increase is gradual, using horizontal
anodes in the low-resistivity area and a figure of effective resistivity
of ∼2500 Ω cm should result in a conservative design.
Data Set C represents an excellent location for anode location
even though the surface soils have relatively high resistivity. It would
appear from this set of data that anodes located >1.5 m deep, would
be in low-resistivity soil of ∼800 Ω cm, such a figure being conservative
for design purposes. A lowering resistivity trend with depth, as
illustrated by this set of data, can be relied upon to give excellent
groundbed performance.
Data Set D is the least favorable of these sample sets of data.
Low-resistivity soil is present at the surface but the upward trend of
resistivity with depth is immediate and rapid. At the 2.3 m depth, for
example, the resistivity could be tens of thousands of ohm-
centimeters. One such situation could occur where a shallow swampy
area overlies solid rock. Current discharged from anodes installed at
such a location will be forced to flow for relatively long distances
close to the surface before electrically remote earth is reached. As a
result, potential gradients forming the area of influence around an
impressed current groundbed can extend much farther than those
surrounding a similarly sized groundbed operating at the same
voltage in more favorable locations such as those represented by
data Sets A and C.
Alternate Soil Resistivity Methods
In the two-pin (Shepard’s Canes) method of soil resistivity
measurement, the potential drop is measured between the same pair
of electrodes used to supply the current [3]. As shown in Fig. 5.11, the
probes are placed 0.3 m apart. If the soil is too hard for the probes to
penetrate, the reading is taken at the bottom of two augured holes.