Caveat:

This ELTECH Groundbed Design program creates a design based strictly on parameters entered. There are no safety factors or other allowances. For example, the volume of the coke column is based on the entered dimensions of the hole with no allowances for normal variations which occur during drilling.

The depth of backfill cover at the bottom of the hole is 5 feet in all deep groundbed designs produced by this calculator.

Center-to-Center spacing for anodes is not permitted to be less than 10 feet.

If the design involves multiple strings, it is assumed that cable length above ground is the same for each string.

Explanations:

Inactive Length:
Inactive length is the length from grade to the top of the backfill column. This portion of the hole is normally cased and filled with material to provide an environmental seal.

Number of Strings:
The required number of strings is calculated from the ampacity of the chosen cable, anode current output, and the number of anodes. If the designer has requested multiple anodes per string for tubular anodes, the required number of strings is calculated. Otherwise the number of strings is equal to the number of anodes.

Minimum GroundBed Length:
Intially minimum groundbed length is calulated from the hole diameter, output current, and maximum backfill to soil current density. This initial length may be increased to accomodate the required number of anodes at a minimum center-to-center spacing of 10 feet. The minimum groundbed length may also be increased to lower total groundbed resistance to a resistance smaller than the total groundbed resistance specified on the entry form.

Spare Length:
Spare length is hole length required to contain the amount of backfill consumed during the design life of the groundbed.

Total Groundbed Length :
Total groundbed length is the total length of the backfill column, including the bottom cover, minimum groundbed length, and the spare length.

Anode Positon:
Anode position is expressed by providing:

1) depth to the top of the top anode;
2) depth to the bottom of the bottom anode;
3) center-to-center spacing for anodes.

These dimensions are based on the number of anodes in the design and the final determination of minimal groundbed length.
These dimensions uniformly distribute the anodes in the area label minimal backfill length on the above diagram. An equal length of the minimal groundbed lenth is exposed to each anode.

Resistance:
Total groundbed resistance, not circuit resistance, is calculated in this design program.
Total groundbed resistance includes resistance of the cable, resistance of LIDA^® anode to backfill, and backfill to soil resistance.
Total groundbed resistance at startup takes into account the entire length of the backfill column at the time of installation.
Total groundbed resistance at end of life is based on the assumption that the spare backfill will have been consumed. The length of the backfill column at end of life will be the minimal groundbed length plus the 5 foot backfill cover at the bottom of the hole and it is this length which is used to compute total groundbed resistance at end of life.

Backfill Current Density:
Maximum backfill current density is used to calculate the minimal groundbed length. Furthermore the design maintains a backfill current density less than the maximum for the design life of the groundbed.
Backfill current density at startup will be smaller because the spare backfill adds length and hence surface area to the backfill column.
Backfill current density at the end of design life will normally be close to the maximum because it is assumed that all of the spare backfill has been consumed.
Backfill current density at the end of design life should always be slightly less than the maximum because the 5 foot bottom cover is still part of the backfill column.
Other factors in the design may force the backfill column to be so long that backfill current density is always much lower than the maximum allowed.

Rectifier Operating Voltage:
Ohms Law is used to compute rectifier operating voltage using total groundbed resistance and the current output. Two volts is then added to this result to accomodate back-voltage.

Volume:
Calculation of the required volume of backfill is based on volume of the backfill column minus the volume of wire, anodes, and vent pipe. Good engineering practice mandates that this volume be adjusted to take into account irregularities in the hole and waste which might occur during installation.