Measurement specific soil resistivity

There is a direct and an indirect method of measuring specific soil resistivity. The direct method is carried out in the laboratoiy on a soil sample using a soil box as shown in Fig. 3-16. The resistivity of a soil specimen of cross-section, S, and length, I, is measured and the specific resistivity determined ... 

The most commonly applied indirect method of measuring soil resistivity using the four-electrode arrangement of Fig. 3-14 is described further in Section 24.3.1. The measured quantities are the injected current, /, between the electrodes A and B, and the voltage, t/, between the electrodes C and D. The specific soil resistivity follows from Eq. (24-41). For the usual measuring arrangement with equally spaced electrodes a = b,ii follows from Eq. (24-41) ... 

Since the Wenner rod is mechanically somewhat delicate, it is only used in loose soils or in bore holes. For all measuring rods, the specific soil resistivity is equal to the product of the measured ac resistance and the shape factor Fq, which is determined empirically. 

The measured potential contains, besides the true object/soil potential, an ohmic voltage drop that is proportional to the specific soil resistivity and the current density. 

This method gives an average in situ resistance at a depth equal to the pin spacing. Foreign buried structures may interfere with the measurements when using the Wenner method. Therefore, it is important to position the pins perpendicular to the underground pipeline [44]. If the pins are positioned parallel to the pipeline, it would result in lower resistance values than the actual resistance. If the specific soil resistance varies vertically... 

As mentioned before, soil is a physically, chemically, and biologically complex system. Factors that affect corrosion in soil, in addition to specific ions, are resistivity of soil, oxygen content, and acidity. Field measurements of soil resistivity are covered in ASTM G 57, Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method, which is the most widely used test, and using the proper meter produces accurate and reproducible results. Conducting field measurements of soil pH is covered in ASTM G 51, Test Method for pH of SoU for Use in Corrosion Testing. The corrosion resistance of lead and its alloys depends mainly upon the presence of silicate, carbonate, and to a lesser extent sulfates, in contributing to the passive film formation. 

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-... 

Extensive and cost - intensive field measurements confirm that there is a substantial reduction of CP current magnitude following initial CP system deployment. Thus, there exists a need for repetitive field measurements in order to establish and confirm the scope of CP protection for the object of interest [10,11], Field data shown in this paper confirm the existence of the correlation between CP current and duration of system deployment, until the point where full system polarization occurs. Specifically, the case of extreme desert conditions necessitates a high soil resistivity, where the CP current decays... 

The salinity may be determined directly as a total content of water-soluble salts, or as a sum of contents of particular ions. The measurement of the specific electric resistance of the soil is a very rough, but simple and rapid method. It is used for soil surveys on a wide scale. For achieving more precise results, it is possible to use a method standardized by measuring the resistance in saturated soil paste (the soil paste is prepared by mixing a soil sample with distilled water to obtain a moderately liquefied paste) and the results are corrected with respect to a temperature of 16°C. Tables are used to convert the measured electrical resistance values to the soil concentration. For more precise data, it is possible to separate the solution from the saturated soil paste and to measure the specific electric conductivity. The solution separated from the soil paste is called the saturated soil extract. 

Of course, the erosion resistance of soils is determined by the properties of the soil itself, particularly by the interaction of the soil particles as influenced by particle size, soil moisture content, soil stickiness, etc. Also, soil erosion is determined to a great degree by the climatic conditions. Hence, measures to combat erosion must involve techniques that are applicable to the specific conditions in any given case. 

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