What is soil resistivity

Soil resistivity is the average value of soil resistance per unit length and is measured in ohm-meters.

Soil resistivity is a commonly used parameter in grounding engineering calculations, which directly affects the grounding resistance of the grounding device, the ground potential distribution of the ground grid, the contact voltage and the step voltage.

Soil resistivity is an important factor in determining the resistance of the grounding body. In order to reasonably design the grounding device, the soil resistivity must be measured so that the measured resistivity can be used as the calculation parameter of the grounding resistance.

One of the methods to measure soil resistivity is to measure the grounding resistance of the grounding body. After measuring the grounding resistance of the grounding body, the soil resistivity is calculated according to the following formula.

When steel pipe or round steel is used as grounding body, ρ=2πRjL/(ln(4L/d))=RjL/(0.336lg(4L/d))Ωcm

Where L is the length of the steel pipe or round steel inserted into the ground, in meters

d is the diameter of the steel pipe or round steel, in m

Rj is the measured grounding resistance value, unit Ω. When flat steel is used as the grounding body,
ρ=2πRjL/(ln(2L^2/(bh)))=RjL/(0.336lg(2L^2/(bh)))Ωcm

Where L is the length of the flat steel, unit: m

b is the thickness of the flat steel, in m

h is the burial depth, unit is m.

The above method has a disadvantage, that is, due to the influence of grounding resistance, it may cause a large error. If the ground structure is uneven, the calculated soil resistivity will also vary with the size and burial method of the grounding body. Therefore, the four-level method shown in Figure B.1 is sometimes used for measurement.

The four electrodes are distributed in a straight line. The electrode insertion depth h should be less than 1/20 of the inter-electrode distance a. The soil resistivity can be calculated based on the indications of the ammeter A and the voltmeter V.

ρ=2paV/I

Where ρ is the calculated soil resistivity, unit Ωcm

U is the measured voltage, unit V

I is the measured current, unit is A

a is the distance between the poles, in m

Measures to reduce soil resistivity

(1) Replace the soil with a higher resistivity by replacing the soil with a lower resistivity such as black soil, clay, or sandy clay. Generally, replace the soil within 1/3 of the upper part of the grounding body and within 0.5 meters around it.

(2) Deep burial If the deep soil resistivity of the grounding point is low, the burial depth of the grounding body can be appropriately increased. Deep burial can also ignore the impact of increased resistivity caused by soil freezing and drying.

(3) External grounding uses a metal lead to bury the grounding body in a nearby location with low soil resistivity.

(4) Chemical treatment: Mixing chemical substances such as slag, charcoal powder, and salt into the soil at the grounding point, as well as using special chemical resistivity reducing agents, can effectively reduce soil resistivity.

(5) Soil conservation: Take measures to keep the soil at the grounding point moist for a long time.

(6) Treat frozen soil by adding peat to the soil at the grounding point in winter to prevent the soil from freezing, or bury the grounding body under the building.

Main factors affecting soil resistivity

(1) The influence of the concentration of conductive ions in the soil and the water content in the soil The size of the soil resistivity ρ mainly depends on the concentration of conductive ions in the soil and the water content in the soil. It is a function of the reciprocal A1 of the conductive ion concentration A contained in the soil and the reciprocal B1 of the water content per unit volume of the soil [2], that is, ffAB. In other words, the higher the concentration of conductive ions in the soil, the better the conductivity of the soil and the smaller ρ; otherwise, the resistivity is greater. For example, in a sandy river, the ρ at the bottom of the river is larger because the concentration of conductive ions at the bottom of the river is smaller due to the erosion of flowing water. The wetter the soil and the more water it contains, the better the conductivity and the smaller ρ; otherwise, the resistivity is greater. This is why the grounding resistance of the grounding body changes with the dryness and wetness of the soil. ], when the water content reaches 15-20% or more, ρ decreases very little. Soil resistivity sandy clay and sandy clay ρ influence curve

(2) Influence of soil quality The soil resistivity of different soil types is different, and the difference may even be thousands to tens of thousands times. The ρ value of different soil types at different water contents. Soil resistivity ρ of soil type Water content of soil (%) ρ (Ω·m) Gravel, crushed stone - Granite - Moisture-containing yellow sand Sandy soil Sandy clay The influence of temperature Temperature also has a great influence on soil resistivity. Generally speaking, soil resistivity decreases with increasing temperature.

(3) When the temperature drops further, ρ increases significantly; however, when the temperature rises from 0°C, ρ only decreases steadily.

(4) The influence of soil compactness The compactness of soil also has a certain influence on soil resistivity. Experiments show that when the moisture content of clay is 10%, the temperature remains unchanged, and the unit pressure increases 10 times from 1961Pa to 19610Pa, ρ can drop to 65% of the original value. Therefore, in order to reduce the stray resistance of the grounding electrode, the backfill soil around the grounding body must be compacted so that the grounding electrode is in close contact with the soil, thereby achieving the effect of reducing the soil resistivity.

(5) Influence of seasonal factors Seasonal changes will also cause changes in soil resistivity. The moisture content and temperature of the soil vary with the season. The most obvious factors affecting soil resistivity are rainfall and freezing. In the rainy season, due to the infiltration of rainwater, the ρ of the surface soil decreases and is lower than that of the deep soil; in winter, due to the freezing of the soil, the ρ of the surface soil increases and is higher than that of the deep soil. In this way, the soil changes from its original uniform structure to a layered uneven structure, causing a change in ρ. The ρ of permafrost is extremely high, which can be dozens of times higher than that of unfrozen soil. In Northeast my country, the thickness of frozen soil in winter can reach 1.6m. 2 Main measures to reduce soil resistivity Since the size of soil resistivity is directly related to the size of the grounding resistance of the grounding device, and the grounding resistance is required to be as small as possible, the soil resistivity is also required to be as small as possible.