For underground structures requiring cathodic protection, the location and nature of the site where the anode is placed needs careful consideration. A low soil resistivity, which would otherwise be classified as a highly corrosive soil, is not the only factor which determines the location of the anode. Other factors to be considered include the presence of foreign metallic structures, accessibility and availability of a power source. The location which is specifically prepared to house a single or a combination of anodes is called a groundbed.
Impressed anode groundbeds: Once a location is selected and the soil resistivity is determined, the engineer needs to design the type of groundbed and choose anode material and combination. Types of groundbeds are classified as: shallow vertical, shallow horizontal or deep well. Anode materials used for underground impressed current systems are generally graphite or high silicon cast iron. In the groundbed, it is preferred for the anode to be surrounded by a carbonaceous backfill. The backfill particles help to reduce anode resistance to earth, extend anode life by allowing anodic reactions to occur on their surface and provide a porous structure so the gases produced can escape.
Shallow Vertical Groundbed: The individual or multiple anode groundbed resistances can be determined using Dwight formula or can be determined graphically.
Shallow Horizontal Groundbeds: Sometimes due to unfavorable soil conditions shallow horizontal groundbeds are preferred. Again earth resistances can be determined by calculation.
Deep Well Groundbeds: Where soil resistivity at the surface are very high, a series of vertical anodes are sometimes installed in deep well groundbeds (up to 100 m and more deep). To determine the depth of the well it may be necessary to carry out a soil resistivity survey. Using the four-pin method, soil resistivity variation with depth can be determined by changing the pin spacing. Deep well groundbeds can take different forms.
A basic design incorporates the use of a steel casing to prevent the collapse of the drilled hole. Several anodes attached together with a rope are placed inside the casing. The remaining space is then filled with carbonaceous material. Once the groundbed becomes operative the steel casing will be consumed. After the pipe corrodes away the anode and backfill become active. Deep wells are generally fitted with a vent to allow gases to escape. Gas entrapment tends to increase the groundbed resistance. It should be added that in certain rock formations anodes have been installed satisfactorily without a steel casing. Although deep well groundbeds provide good current distribution they are expensive to construct because of the cost of the drilling. Careful design is also necessary because anode failures cannot be easily rectified.
Sacrificial Anode Groundbeds: In certain situations, for example in reducing stray current effects, a sacrificial system may be specified to protect underground structures. The backfill used with these anodes is different from that described for impressed anodes. A typical backfill contains a mixture of clay and gypsum. The function of this chemical backfill to provide conditions favorable to anode dissolution. It also helps to reduce the groundbed resistance. Groundbed resistances can be calculated using the same procedure adopted for impressed current anodes. Individual galvanic anodes in a horizontal groundbed are generally not used. For this type of groundbed a continuous galvanic anode strip is found to be practical.
Remote and Close Groundbeds: Since a groundbed is a source of current, there is a field (potential) gradient associated with it. When the voltage gradient in the soil near the structure is far from the voltage gradient of the groundbed, the groundbed is referred to as remote. However when the voltage gradient in the soil near the structure overlaps with that of the groundbed, the groundbed is referred to as close. This is because the groundbed is near enough to interfere significantly with the field gradient around the structure.