Vertical. This is the most common type of impressed current anode
installation. Both graphite and HSCBCI anodes are brittle and must be
carefully handled to prevent breakage. The anode cable is particularly prone
to failure if the insulation is damaged in any way and particular care must be
exercised in handling the anode leads. As impressed current cathodic
protection anodes are generally longer than sacrificial anodes, excavation of
holes for them is often more difficult. "Jetting" of bare anodes is sometimes
possible in sandy soils using equipment specially designed for this purpose.
If caving of the hole is encountered, either the use of a packaged "canned"
anode complete with backfill, or a thin metal "stovepipe" casing may be
necessary. If bare anodes are used, the backfill should be added as soon as
the anodes are placed. The backfill should be well tamped to insure good
contact with the anode. The backfill should be used to fill the hole to
within a few inches of grade unless coarse gravel is available for this
purpose. This is to allow the gasses generated during system operation to be
properly vented. A typical vertical anode installation using a bare HSCBCI
anode with backfill is shown in Figure 120. A typical vertical installation
of a "canned" HSCBCI anode is shown in Figure 121.
Horizontal. Horizontal installations of impressed current anodes
are less expensive than vertical anodes. Horizontal installations may be
necessary when obstructions or other soil conditions make augering of deep
holes difficult. Horizontal installations are also used where soil
resistivities are very low and the increased resistance of the horizontal
installation is not significant. A typical horizontal installation of a
HSCBCI anode is shown in Figure 122. A minimum of 2 feet of burial for all
cables and 3 feet of burial for the anode is recommended. The excavation
should be partially filled with backfill before the anode is placed. After
the anode is placed, the remainder of the backfill should be added and tamped
into place. If backfill is not required, soil free from stones or debris
should be used to fill the excavation. Again, it must be remembered that
impressed current anodes, and particularly the anode leads, are susceptible to
damage and must be handled carefully.
Deep Anode Beds. In some installations where interference problems
are severe, anode beds are sometimes installed deep below the surface. This
causes the current flow to become more vertical and reduces interference
between horizontally displaced structures. Deep anodes are also used where
the resistivity of the soil near the surface is high. Anodes installed deeper
than 50 feet are called "deep" anodes. Specialized equipment and skill is
required for the installation of such an anode array. Installation of deep
anode systems is described in NACE Standard RP-50-72. Type TAD HSCBCI or
center tapped 3- by 60-inch graphite are suitable for such installations.
Newly developed deep anode systems using platinized anodes show considerable
promise for such applications. A typical deep anode system using HSCBCI
anodes is shown in Figures 123 and 124.