Earthpit Design

In general, the depth of an earth pit is 10 feet. But it is not the same for all the applications; it depends upon the factors like soil resistivity, soil condition, moisture, dissolved salts, climatic conditions, physical composition, location of earth pit, the effect of grain size and its distribution, the effect of current magnitude and its duration, are available.

Soil Resistivity: It is the resistance of the soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pit depends on soil resistivity. It is the resistance of the soil to the passage of electric current. It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size and distribution, seasonal variation, current magnitude etc.

Soil Condition: Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry. Soil resistivity is measured in ohm-meters or ohm-cm. 

            Soil plays a significant role in determining the performance of electrodes. Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity is highly corrosive. If soil is dry the soil resistivity value will be very high. If soil resistivity is high, the earth resistance of the electrode will also be high.

Moisture: Moisture has a great influence on the resistivity value of soil. The resistivity of soil can be determined by the quantity of water held by the soil ad resistivity of the water itself. Conduction of electricity in the soil is through the water.

The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And the further increase of moisture level in the soil will have little effect on soil resistivity. In many locations, the water table goes down in dry weather conditions.

Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity.

Dissolved salts: Pure water is a poor conductor of electricity. The resistivity of soil depends on the resistivity of water which in turn depends on the amount and nature of salts dissolved in it. A small quantity of salts in water reduces soil resistivity by 80%. Common salt is most effective in improving the conductivity of the soil. But it corrodes metal and hence discouraged.

Climate conditions: Increase or decrease of moisture content determines the increase or decrease of soil resistivity. Thus in dry whether resistivity will be very high and in monsoon months the resistivity will below.

Physical composition: Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of 4-150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter.

                                    Location of earth pit:

The location also contributes to resistivity to a great extent. In a sloping landscape, or inland with made up of soil, or areas which are hilly, rocky, or sandy, water runs off and in dry weather conditions, the water table goes down very fast. In such a situation backfill compound will not be able to attract moisture, as the soil around the pit would be dry.

            The earth pits located in such areas must be watered at frequent intervals, particularly during weather conditions.

            Though backfill compound retains moisture under normal conditions, it gives off moisture during dry weather to the dry soil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is naturally not well-drained.

Effect of grain size and its distribution: The grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil.

Soil Resistivity Measurement

Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines the decrease or increase of soil resistivity. Thus in fry weather resistivity will be very high and during the rainy season the resistivity will be low.

Effect of current magnitude: Soil resistivity in the vicinity of the ground the electrode may be affected by current flowing from the electrode into the surrounding soil. The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and duration will cause significant drying and thus increase the effect of soil resistivity.

Area available: Single electrode rod or strip or plate will not achieve the desired resistance alone.

       If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distance between the electrodes must be equal to the driven depth to avoid overlapping of the area of influence. Each electrode, therefore, must be outside the resistance area of the other.

Current magnitude: A current of significant magnitude and duration will cause significant drying conditions in soil and thus increase the soil resistivity.

it is important to properly analyze the earth and soil conditions before proceeding for earth pits design and installation.