Trenches may be necessary when crossing roads or installing in areas where water tables are close to ground level....
There are two kinds of below grade CGIT installations; direct-buried and trenched. Direct buried has the advantage of lower cost installations in open areas. Either way, below grade installations are more aesthetically pleasing, provide less above ground congestion, and result in lesser right-of-way distances.

Trenched
Road crossings are the main consideration when choosing trenched installations. The Ontario Hydro Project is a perfect example of the advantages of trenched installation. The layout consisted of two 550kV circuits. Each circuit crossed the path of the plant’s main access road. Placing the CGIT in trenches allowed for an undisturbed road routing as well as access to the trench for easy installation and maintenance of the bus between the substation and connecting overhead line. The photo above shows a typical combined trenched and above ground installation.

Direct-Buried
The aluminum sheath of a CGIT  transmission line, while made of a corrosion resistant aluminum alloy, is still subject to corrosion when buried in the earth. To prevent corrosion of  the aluminum relative to the earth such that it becomes cathodic, its potential relative to the earth is made negative.

A zinc anode is utilized to this cause and is in effect sacrificed and corrodes in place of the aluminum. A Cu-CuSO4 half-cell is used to meter the potential of the buried metal compared to the earth. Polarization cells are also utilized to provide a special means of grounding the system for AC fault currents. In order to prevent underground currents from transmitting into the sheath of the bus, an extruded polyethylene coating is applied to the underground portion of the CGIT enclosure. A complete coating, cell and metering set up is provided for each end of the underground line for every CGIT direct-buried installation.

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