Soil-Resistivity Measurement

Soil resistivity is a function of soil moisture and the concentrations of ionic soluble salts and is considered to be the most comprehensive indicator of a soil s corrosivity. Typically, the lower the resistivity, the higher will be the corrosivity as discussed in more details in Chap. 10. Typically, soil resistivity decreases with increasing water content and the concentration of ionic species. Sandy soils, for example, are high up on the resistivity scale and therefore considered the least corrosive while clay soils are excellent at retaining water and at the opposite end of the corrosivity spectrum. 

An alternating current from the soil resistance meter causes current to flow through the soil, between pins Cl and C2 and the voltage or potential is measured between pins PI and P2. Resistivity of the soil is then computed from the instrument reading, according to the following formula [1]  

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 0.5 to 1 m. 

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 therefore be avoided. 

The first set of data in Table 5.5, Set A, represents uniform soil conditions. The average of the readings shown (-960 Q cm) represents the effective resistivity that may be used for design purposes for impressed current groundbeds or galvanic anodes. 

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When making measurements in the soil box, it has to be remembered that soil samples can change from their original condition and this will have an effect on the resistivity. Soil resistivity measurements in the soil box only give accurate results with cohesive soils. However, the order of magnitude of the specific resis-... 

Since the Wenner formula [Eq. (24-41)] was deduced for hemispherical electrodes, measuring errors appear for spike electrodes. To avoid errors in excess of 5%, the depth of penetration must be less than a 5. Soil resistivity increases greatly under frost conditions. While electrodes can be driven through thin layers of frost, soil resistivity measurements deeper than 20 cm in frozen ground are virtually impossible. 

Soil resistivity measurements can be affected by uncoated metal objects in the soil. Values that are too low are occasionally obtained in built-up urban areas and in streets. Measurements parallel to a well-coated pipeline or to plastic-coated cables give no noticeable differences. With measurements in towns it is recommended, if... 

Soil resistivity surveys are often impractical in built-up areas, but in such areas impressed-current cathodic protection is usually avoided on account of the danger of interaction. Under such conditions adequate protection can be achieved by installing magnesium anodes in the pipe trench should the soil resistivity measurements made when the trench is opened indicate that this is necessary. 

Buried pipelines are subject to external corrosion from ground water and highly conductive soils. The corrosiveness of soils is often estimated based on soil resistivity measurement. The measurement is made with the Wenner four-pin method, which is used in conjunction with a Vibroground(1 and a Miller U 10-pin conductor set to determine the average electrical resistivities. A general relationship between corrosion and soil resistivity is as follows ... 

A long pipeline of 8-in. diameter is buried 6 ft underground. The potential difference between two Cu—CUSO4 reference electrodes located on the soil surface over the pipe and a point at right angles 60 ft distant, is 1.25 V. The electrode over the pipe is negative to the other, and soil resistivity measures 3000 Q-cm. 

Soil resistivity measurements should be repeated at a number of locations to establish a resistivity profile for the site. The depth of measurement can be controlled by varying the spacing between the probes. In no case should the probe length exceed 20% of the spacing between probes. 

FIGURE 10.219 The four-point method for soil resistivity measurement. (After [11].)... 

The DCG V method enables the detection of defects in the insulation by the determination of zones of inflow and outflow of polarizing current. A potential gradient is measured in the ground with a very sensitive voltmeter and two CSE electrodes placed on both sides of the investigated pipeline at distances from each other of 1-2 m. Defects can be localized with an accuracy of 10-15 cm on pipelines placed at a depth of up to 6 m. The shape and extent of the defects are determined from potential gradient graphs and soil resistivity measurements in the vicinity of the epicentre. Dur-... 

Figure 5.10 Four-pin soil resistivity measurements being made (a) field setup and (b) close-up on the instrument. (Courtesy of Tinker Rasor)...

In the two-pin (Shepard s Canes) method of soil resistivity measurement, the potential drop is measured between the same pair of electrodes used to supply the current [3]. As shown in Fig. 5.11, the probes are placed 0.3 m apart. If the soil is too hard for the probes to penetrate, the reading is taken at the bottom of two augured holes. 

Potential drop is measured across probes inserted into the soil. The resistivity is calculated using constants provided with the particular geometry of soil box being used. Due to the disturbance of the soil during sampling and possible drying out of the soil during shipment, this method of soil resistivity measurement is less likely to represent true, in-place soil resistivity than an actual field test. 

It should be noted that different devices may provide different formulas for computing resistivity, or they may display other values (e.g., resistance instead voltage and current). It is also imperative to use a resistivity meter designed for soil resistivity measurements and not simply a battery and voltmeter. Significant errors will be caused by polarization effects caused by an applied DC. 

Soil resistivity measurement is the first important step in the design of a cathodic protection system as the current requirement would differ from one soil resistivity to another for the system. It may, however, be pointed out that there is no single method available to determine precisely the degree of corrosivity caused by soils. SoU resistivity only provides a rough guide to the corrosivity of the soils. There are several methods... 

Soil resistivity can be measured by the so-called Wenner four-pin technique or, more recently, by electromagnetic measurements. The latter allows measurements in a convenient manner and at different soil depths. Another option for soil resistivity measurements is the so-called soil box method, whereby a sample is taken during excavation. Preferably sampling will be in the immediate vicinity of a buried structure (a pipe trench, for example). 

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