An Introduction to Soil Resistivity

The design of a grounding system requires reliable figures for soil resistivity. Geoelectric exploration provides the information needed to determine the soil resistivity. This exploration uses arrays of electrodes arranged on the surface of the ground. One of the most popular techniques in America is the four-terminal or Wenner’s method.

Every kind of conducting material has a sort of inherent built-in influence on resistance, called its resistivity. Resistivity is a characteristic parameter of conductive media. Conductors have low resistivity; insulators have high. The units for measuring resistivity are ohm-m and ohm-cm.

The electrical resistivity of the soil characterizes its ability to oppose the passage of a current. It affects various fields such as electric power systems, electronics, environmental, geotechnical, groundwater, agriculture, and underground resource exploration.

Electrical grounding systems can be straightforward, such as a vertical rod, or a horizontal conductor buried at a certain depth, or complex such as the grounding mats in transmission and distribution substations. The crucial role of a grounding system is protecting people against electrical shocks. The design of a grounding system requires a knowledge of the electrical response of the soil.
 

The Resistivity of the Soil

The resistivity characterizes the electrical behavior of the soil. Soils are homogeneous and isotropic when the resistivity is equal at any point and direction. 

Homogeneous soils are very rare. Usually, there are variations in resistivity both laterally and in-depth. 

Resistivity variations depend mainly on the following features:

• soil type
• material mixture
• moisture content
• mineralogical composition
• salt concentration
• degree of saturation
• pH
• temperature
• porosity
• compaction
• stratification
• rainfall

 

Some of these factors depend on the geology of the place, and their variation is very long term. That is why they are considered constant for grounding system design.

Other factors, like moisture and salt content, temperature, and compaction, are variable and will affect the performance of the grounding systems throughout their lives.

 

The Resistivity of Different Types of Soil

There are several methods of classification based on specific properties of soils, such as strength, plasticity, texture, and other characteristics. These classifications help the engineers to recognize the features of a  given land. The Unified Soil Classification System (USCS), proposed by the Austrian Engineer Arthur Casagrande in 1947, is widely used in the United States.

 

Fundamentally, the soil is:

• Gravel, if the #4 sieve retains more than 50% (by weight),
• Sand, if more than 50% passes the #4 sieve, but the #200 sieve retains more than 50%, and,
• Fine-grained (silt or clay), if more than 50% passes the #200 sieve.

 

If less than 5% passes the #200 sieve, the material is a well-graded or poorly-graded sand or gravel with the symbols,

 

GW, GP, SW, SP, where,

G = gravel; S = sand; W = well-graded; P = poorly-graded; 

If more than 12% passes the #200 sieve, the material symbols are,

GM, GC, SM, SC, where,

M = contains silt; C = contains clay.

Soils are fine-graded when more than 50% passes the #200 sieve.

With liquid limits <50%,

ML, OL, CL, where,

M = silts; O = organic soils; C = clay; L = less than 50%.

With liquid limits >50%,

MH, OH, CH, where,

H = higher than 50%.

 

Table 1 shows the factors to classify a soil, following the USCS, with some selected resistivity values.

Table 1 The unified soil classification system and selected resistivity values

Major Divisions		Group Symbol	Typical Names	Resistivity Range
ReCoarse-grained soils (more than half of the material is larger than #200)	Gravel and Gravelly Soils	GW	Well-graded gravels or gravel-sand mixtures, little or no fines	60,000-100,000
		GP	Poorly graded gravels or gravel-sand mixtures, little or no fines	100,000–250,000
		GM	Silty gravels, gravel-sand-silt mixtures
		GC	Clayey gravels, gravel-sand- clay mixtures	20,000–40,000
	Sand and Sandy Soils	SW	Well-graded sands or gravelly sands, little or no fines
		SP	Poorly graded sands or gravelly sands, little or no fines
		SM	Silty sands, sand-silt mixtures	10,000–50,000
		SC	Clayey sands, sand-silt mixtures	5,000–20,000
Fine-grained soils (more than half of the material is smaller than #200)	Silts and Clays (liquid limit < 50)	ML	Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity	3,000–8,000
		CL	Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays	2,500–6,000
		OL	Organic silts and organic silt- clays of low plasticity
	Silts and Clays (liquid limit > 50)	MH	Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts	8,000 – 30,000
		CH	Inorganic clays of high plasticity, fat clays	1,000 – 5,500
		OH	Organic clays of medium to high plasticity, organic silts
Highly organic soils		Pt	Peat and other highly organic soils

 

Table 2 exhibits resistivity values for some types of soils and rocks, expressed in common terms and measured by different methods. Observe the full range of values for each type of soil. The actual resistivity can vary within the maximum and minimum values quoted there.

 

Table 2 Resistivity of different soils

SOIL TYPE	Minimum (Ω∙cm)	Average (Ω∙cm)	Maximum (Ω∙cm)
Ashes, brine, waste	590	2,370	7,000
Clay	340	4,060	16,300
Same as above with varying proportions of sand and gravel	1,020	15,800	135,000
Gravel rock, sand, stones with little clay	59,000	94,000	458,000
Granite, basalt		1,000,000
Slate	1,000		10,000
Limestone	500		400,000
Marl	100		5,000
Sandstone	2,000		200,000
Shist, shale	500		10,000
Soil, chalky	10,000		1,000,000
Sea water	20	100	200
Lake water		20,000	20,000,000
Tap water	1,000		5,000
Fertile land hills		3,000
Coastal land, dry, flat, sandy	30,000	50,000	500,000

 

Geological investigations give valuable information to have an idea of the range of resistivity values. Still, the reliable design of a grounding system requires measurements at the site.

In addition to the variation due to the soil type, the resistivity will change by several orders of magnitude with small fluctuations in moisture content, salt content, and temperature.

 

The Effect of Moisture Content on Soil Resistivity

Moisture content is one of the factors that control soil resistivity. It is stated as a percentage of the weight that the soil has when it is dry.

Table 3 shows that resistivity decreases rapidly as dry soil becomes wetter. But when the soil becomes saturated, additional increases in moisture do not produce significant decreases in resistivity. The limit value in Table 3 is approximately 20%, above which there would no longer be substantial reductions in resistivity.

 

Table 3 Effect of moisture content on soil resistivity

Moisture content  (% by weight of the dry soil)	Top soil (Ω∙cm)	Sandy loam (Ω∙cm)
0	>10⁹	>10⁹
0.25	250, 000	250, 000
5	165, 000	43, 000
10	53, 000	18, 500
15	19, 000	10, 500
20	12, 000	6, 300
30	6, 400	4, 200

 

Effect of Salt Content on Soil Resistivity

Moisture alone is not sufficient to achieve low resistivity. The soil must contain mineral salts in such a way to form an electrolyte to conduct electricity.

The concentration of dissolved salts can vary naturally due to the effect of rainfall and the chemical elements found in the upper layers of the soil. When rainwater penetrates the soil, it drags in new chemical elements or dilutes the concentration of existing ones.

The mineral salts may be natural or may have been introduced artificially in the soil to improve its conductivity.

Mineral salts have the most significant impact on reducing resistivity, and that is why they are the first choice when treating the soil to improve its electrical characteristics.

 

Effect of Temperature on Soil Resistivity

The temperature coefficient for the soil is negative, i.e., the lower the temperature, the higher the resistivity.

Table 4 shows that at temperatures above freezing, changes in resistivity are small per ⁰C. At the freezing point, 0 ⁰C, there is a discontinuity. The resistivity has a significant increase from the liquid phase to the solid phase of water.

 

Table 4 Effect of temperature on soil resistivity Sandy loam, 15.2% moisture content

Temperature (⁰C)	Resistivity (Ω∙cm)
20	7, 200
10	9, 900
0 (Water)	13, 800
0 (Ice)	30, 000
-5	79, 000
-15	130, 000

Severe increases in resistivity occur as the soil temperature decreases past the freezing point. This is the reason for ensuring that grounding electrodes extend below frozen soil in cold regions.

 

A Review of Soil Resistivity 

Resistivity is a characteristic of conductive materials, closely associated with resistance. The resistance of the grounding electrodes depends significantly on the soil resistivity. Therefore, a reasonable approximation of the resistivity will allow for a proper design.

Soil resistivity values are one of the elements in determining the depth of the grounding electrode to obtain an acceptable resistance to ground.

The soil descriptions used in geotechnical and geophysical studies are also useful for geoelectric studies. A widely used soil classification system in the United States is the USCS.

Soil resistivity varies with the seasons, and many elements influence its value. Particularly noteworthy are the temperature, moisture content, and the presence of minerals and dissolved salts. Soil, without an electrolyte present, can become an excellent insulator.

Low temperatures produce significant increases in resistivity, so the recommendation is to place the grounding electrodes below the area where the ground freezes.

Because temperature and moisture content are more stable in the deeper portions of the soil, it is important to place the grounding electrodes there.

Grounding electrode designs are very involved in places where the resistivity is high.