Soil Resistivity Survey

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SOIL RESISTIVITY TESTING

This article discusses the most common soil resistivity testing method and provides some guidelines for properly collecting sufficient data for the cathodic protection system designer.

One of the most important design parameters when considering the application of cathodic protection for buried structures is the resistivity of the soil. Soil resistivity testing is an important consideration for assessing the corrosivity of the environment to buried structures. It also has a tremendous impact on the selection of anode type, quantity, and configuration. Thus, it is critical that the CP designer have accurate data on the soil conditions at both the structure and at any proposed anode system locations. The lack of sufficient soil resistivity data can render a cathodic protection system (CP system) design ineffective and can result in costly remediation efforts during commissioning.

Soil Corrosivity

Soil resistivity is the principal diagnostic factor used to evaluate soil corrosivity. When performing soil resistivity testing, there are numerous factors that can be assessed, including soil composition, moisture content, pH, chloride and sulfate ion concentrations, and redox potential.  These are all common components of a lab or in-situ soil testing program and all have an impact on soil resistivity. While a comprehensive soil testing program may be warranted, especially when performing failure analysis, for most environments the soil resistivity testing data provides an outstanding basis for assessing soil corrosivity. Below is a typical chart correlating soil resistivity with soil corrosivity.

Soil Resistivity (ohm-cm) Corrosivity Rating
>20,000 Essentially non-corrosive
10,000 to 20,000 Mildly corrosive
5,000 to 10,000 Moderately corrosive
3,000 to 5,000 Corrosive
1,000 to 3,000 Highly corrosive
<1,000 Extremely corrosive

SOURCE: Corrosion Basics: An Introduction, NACE Press Book, 2nd edition by Pierre Roberge

The purposes of soil resistivity testing is:

  • To obtain a set of measurements which may be interpreted to yield an equivalent model for the electrical performance of the earth, as seen by the particular earthing system.
  • Geophysical surveys are performed using these values as an assistance in finding depth to bedrock, core locations and other geological phenomena.
  • The degree of corrosion in underground pipelines is determined. A drop in resistivity is proportional to an indent in corrosion in subversive pipelines.

Soil resistivity influences the plan of an earthing system absolutely and is the major factor that decides the resistance to earth of a grounding system. Thus before designing and installing a new grounding system, the determined location should be tested to find out the soil’s resistivity.

Type of Soil or Water Typical Resistivity m Usual Limit m
Sea water 2 0.1 to 10
Clay 40 8 to 70
Ground well & spring water 50 10 to 150
Clay & sand mixtures 100 4 to 300
Shale, slates, sandstone etc. 120 10 to 100
Peat, loam & mud 150 5 to 250
Lake & brook water 250 100 to 400
Sand 2000 200 to 3000
Moraine gravel 3000 40 to 10000
Ridge gravel 15000 3000 to 30000
Solid granite 25000 10000 to 50000
Ice 100000 10000 to 100000

 

Soil Resistivity Testing


Wenner four-pin soil resistivity testing method

While there are several methods for measuring soil resistivity, the most common field testing method is the Wenner four-pin method (ASTM G57). This test uses four metal probes, driven into the ground and spaced equidistant from each other. The outer pins are connected to a current source (I) and the inner pins are connected to a volt meter (V) as shown in Figure 1.

When a known current is injected in the soil through the outer probes, the inner probes can be used to measure voltage drop due to resistance of the soil path as current passes between the outer probes. That resistance value R can then be converted into a soil resistivity value with the formula: ρ=2×π×a×R where “ρ” is measured in ohm-cm and “a” is the spacing of the pins in cm. This value represents the average soil resistivity at the depth equivalent to the spacing of the probes so if the probes are spaced 5 foot apart, the value derived would be equivalent to the average soil resistivity at 5 foot depth.

For cathodic protection system design, it is common to take multiple soil resistivity measurements using this methodology with various probe spacings. For shallow anode placement, it is usually sufficient to take reading readings at 2.5 ft, 5 ft, 10 ft, 20 ft, 25 ft. For deep anode applications, soil resistivity measurements may be recommended at much deeper depths corresponding with the anticipated depth of the deep anode system.

Layer Effects

It is important to note that the soil resistivity values generated from the four pin testing represent the average soil resistivity from the earth surface down to the depth, and each subsequent probe spacing includes all of the shallow resistance readings above it. For cathodic protection design purposes, it is often necessary to determine the resistance of the soil at the anode depth by “subtracting” the top layers from the deep readings. This process of “subtracting” the top layers requires some form of computational adjustment. One popular approach is called the Barnes method which assumes soil layers of uniform thickness with boundaries parallel to the surface of the earth. If the measured data indicates decreasing resistance with increasing electrode spacing, this method can be used to estimate the layer resistivities.

The resistance data (R) values should be laid out in a tabular format and then converted to conductance which is simply the reciprocal of the resistance value. The change in conductance is then calculated for each subsequent spacing. That value is then converted back to a layer resistance value by taking the reciprocal of the change in conductance. Finally, the layer resistivity is calculated using ρ=2×π×a×R.

For the Barnes analysis below, the data shows that a low resistance zone exists between 60m depth and 100m depth.

TEST DATA BARNES ANALYSIS
Spacing a
(m)
Resistance
(ohms)
Conductance 1/R
(Siemens)
Change in Conductance
(Siemens)
Layer Resistance
(ohms)
Layer Resistivity
(Ohm-m)
20 1.21 0.83 1.21 152
40 0.90 1.11 0.28 3.57 449
60 0.63 1.59 0.48 2.08 261
80 0.11 9.09 7.5 0.13 17
100 0.065 15.38 6.29 0.16 20
110 0.058 17.24 1.86 0.54 68

Soil Resistivity Testing Equipment Considerations

Electrically speaking, the earth can be a rather noisy environment with overhead power lines, electric substations, railroad tracks, and many other sources that contribute to signal noise. This can distort readings, potentially resulting in significant errors. For this reason, specialized soil meter equipment that includes sophisticated electronic packages capable of filtering out the noise is critical when taking soil resistivity data.

There are two basic types of soil resistivity meters: high-frequency and low-frequency meters.

High-frequency Soil Resistivity Meters

High-frequency meters operate at frequencies well above 60 hz and should be limited to data collection of about 100 feet in depth. This is because they lack sufficient voltage to handle long traverses and they induce noise voltage in the potential leads which cannot be filtered out as the soil resistivity decreases and the probe spacing increases. These are less expensive than their Low-Frequency counter parts and are by far the most common meter used for soil resistivity testing. For CP design purposes, these are frequently used to assess soil corrosivity and for designing shallow anode applications.

Low-frequency Soil Resistivity Meters

Low-frequency meters generate pulses in the 0.5 to 2.0 hz range and are the preferred equipment for deeper soil resistivity readings as they can take readings with extremely large probe spacings. Some models can operate with spacings many thousands of feet in distance. These models typically include more sophisticated electronics filtering packages that are superior to those found in high-frequency models. For CP designs involving deep anode installations, a low-frequency meter is the preferred equipment to provide accurate data at depths below 100 ft.

Field Data Considerations

When collecting accurate soil resistivity data for cathodic protection system design, it is important that the following best practices are taken into consideration to avoid erroneous readings:

  1. Suitability of the testing location.The use of the Wenner four pin testing method requires sufficient open area to properly space the pins to collect data to the depths necessary. For deep anode cathodic protection systems this would require a minimum of three times the anticipated anode system depth.
  2. Avoidance of buried piping and other metallic objects. The presence of any buried metallic structures (piping, conduit, reinforced concrete structures, grounding systems, etc…) provides low current paths that could cause a short-cutting effect that would distort the resistance readings and yield an erroneous soil resistivity reading.
  3. Depth of the probes. It is important that the probes are properly inserted into the earth. For shallow resistivity readings, probes that are driven too deep can impact the shallow readings. Ideally, the pins should be no deeper that 1/20th of the spacing between the pins and no more than 10 cm (4 inches) deep.
  4. Avoid areas of high electrical noise. Soil testing should not be performed directly under high voltage transmission systems or near other outside sources of current in the soil such as DC light rail systems.
  5. Accurately record the test location and conditions. It is important that the location of the testing is accurately recorded along with the soil conditions and temperature at the time of testing. Testing should not be performed in frozen soil, or during periods of extreme drought or abnormally wet conditions.

 

Soil Resistivity Measurement

Soil resistivity is a function of soil moisture and the concentrations of ionic soluble salts and is considered to be most comprehensive indicator of a soil’s corrosivity. Typically, the lower the resistivity, the higher will be the corrosivity as indicated in the following Table.

 

Corrosivity ratings based on soil resistivity

Since ionic current flow is associated with soil corrosion reactions, high soil resistivity will arguable slow down corrosion reactions. Soil resistivity generally decreases with increasing water content and the concentration of ionic species. Sandy soils are high up on the resistivity scale and therefore considered the least corrosive. Clay soils, especially those contaminated with saline water are on the opposite end of the spectrum.

Field soil resistivity measurements are most often conducted using the Wenner four-pin method and a soil resistance meter. The Wenner method requires the use of four metal probes or electrodes, driven into the ground along a straight line, equidistant from each other, as shown in the following Figure. Soil resistivity is a simple function derived from the voltage drop between the center pair of pins, with current flowing between the two outside pins.

Wenner four pin soil resistivity test set-up

An alternating current from the soil resistance meter causes current to flow through the soil, between pins C1 and C2. The voltage or potential is then measured between pins P1 and P2. The meter then registers a resistance reading. Resistivity of the soil is then computed from the instrument reading, according to the following formula:

where:

ρ is the soil resistivity (ohm-centimeters)

A is the distance between probes (centimeters)

R is the soil resistance (ohms), instrument reading

π equals 3.1416

The resistivity values obtained represent the average resistivity of the soil to a depth equal to the pin spacing. Resistance measurements are typically performed to a depth equal to that of the buried system (pipeline) being evaluated. Typical probe spacing is in increments of 0.8 m (2.5 ft).

If the line of soil pins used when making four-pin resistivity measurements is closely parallel to a bare underground pipeline or other metallic structure, the presence of the bare metal may cause the indicated soil resistivity values to be lower than it actually is. Because a portion of the test current will flow along the metallic structure rather than through the soil, measurements along a line closely parallel to pipelines should be avoided.

When making soil resistivity measurements along a pipeline, for example, it is good practice to place the line of the pins perpendicular to the pipeline with the nearest pin at least 4.5 meters from the pipe or even further, if space permits. Soil resistivity data taken by the four-pin method should be recorded in tabular form for convenience in calculating resistivity and evaluating results obtained. The tabular arrangement may be as shown here.

Format for recording soil resistivity measurements

With experience, much can be learned about the soil structure by inspecting series of readings to increasing depths. The recorded values from four-pin resistivity measurements can be misleading unless it is remembered that the soil resistivity encountered with each additional depth increment is averaged, in the test, with that of all the soil in the layers above. The indicated resistivity to a depth equal to any given pipe spacing is a weighted average of the soils from the surface to that depth. Trends can be illustrated best by inspecting the sets of soil resistivity readings such as in the following example.

Examples of soil resistivity readings using 4 pin method

The first set of data, Set A, represents a uniform soil conditions. The average of the readings shown (~960 ohm-cm) represents the effective resistivity that may be used for design purposes for impressed current groundbeds or galvanic anodes.

Data Set B represents low-resistivity soils in the first few feet. There may be a layer of somewhat less than 1000 ohm-cm around the 1.5 meters depth level. Below 1.5 meters, however, higher-resistivity soils are encountered. Because of the averaging effect mentioned earlier, the actual resistivity at 2.3 meters deep would be higher than the indicated 1250 ohm-cm and might be in the order of 2500 ohm-cm or more.

Even if anodes were placed in the lower-resistivity soils, there would be resistance to the flow of current downward into the mass of the earth. If designs are based on the resistivity of the soil in which the anodes are placed, the resistance of the completed installation will be higher than expected. The anodes will perform best if placed in the lower resistance soil. The effective resistivity used for design purposes should reflect the higher resistivity of the underlying areas. In this instance, where increase is gradual, using horizontal anodes in the low-resistivity area and a figure of effective resistivity of ~2500 ohm-cm should result in a conservative design.

Data Set C represents an excellent location for anode location even though the surface soils have relatively high resistivity. It would appear from this set of data that anodes located >1.5 meters deep, would be in low-resistivity soil of ~800 ohm-cm, such a figure being conservative for design purposes. A lowering resistivity trend with depth, as illustrated by this set of data, can be relied upon to give excellent groundbed performance.

Data Set D is the least favorable of these sample sets of data. Low-resistivity soil is present at the surface but the upward trend of resistivity with depth is immediate and rapid. At the 2.3 meters depth, for example, the resistivity could be tens of thousands of ohm-centimeters. One such situation could occur where a shallow swampy area overlies solid rock. Current discharged from anodes installed at such a location will be forced to flow for relatively long distances close to the surface before electrically remote earth is reached.

As a result, potential gradients forming the area of influence around an impressed current groundbed can extend much farther than those surrounding a similarly sized groundbed operating at the same voltage in more favorable locations such as those represented by data Sets A and C.

In some areas, experience will show that soil resistivity may change markedly within short distances. A sufficient number of four-pin tests should be made in a groundbed construction area, for example, to be sure that the best soil conditions have been located. For groundbeds of considerable length (as may be the case with impressed current beds), four-pin tests should be taken at intervals along the route of the proposed line of groundbed anodes. If driven rod tests or borings are made to assist in arriving at an effective soil resistivity for design purposes, such tests should be made in enough locations to ascertain the variation in effective soil resistivity along the proposed line of anodes.

Summary

Soil resistivity testing with accurate collection of data is the best indicator of the corrosivity of the soil for buried metallic structures and has a significant impact on the design of cathodic protection systems. The most common test methodology for field collection of soil data is the Wenner four pin method. When properly collected, and using appropriate analytical techniques, the soil resistance field data can provide an accurate assessment of soil resistivity values for use in designing an appropriate cathodic protection system.

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