Resistivity of soils

The requirements with respect to current distribution and anode placement vary with the resistivity of soils or the electrolyte involved. 

Nominal area of conductor (mm-) Rating factor fo C cinAV r value of thermal resistivity of soil in ... 

Table A16.26 Rating factors for variation in thermal resistivity of soil, three single-core cables laid directly in the ground (three cables in trefoil, touching)...

Rating for value of ibernial resistivity of soil in C cmAV... 

From the above the resistance to ground of a plate grounding is inversely proportional to the square root of the linear dimension fA) of the plate. The variation in resistance with the size of the plate is shown in Figure 22.2. Considering the resistivity of soil as 10 k2m, since the ground resistance is proportional to the resistivity of soil, there would be different parallel curves for the ground resistance for different values of resistivity of soil. 

Figure 22.2 Variation in resistance to ground with the linear dimensions lor a plate gounding, for the same resistivity of soil...

The ground resistance is a function of the area occupied by the grounding station and the stratification of the soil. The stratification of the soil is usually of a non-uniform nature and may vary the resistivity of soil vertically as well as horizontally, thus varying the resistance of soil. The minimum value of ground resistance (resistance of the grounding station) at a certain depth h from the ground surface may be expressed by... 

A simple way to measure the resistivity of soil is a four-pin method in which four probes are drilled into the ground along a straight line at equal distances a and depth b. Then a voltage V is applied to the two inner probes and a current, T, is measured in the two outer probes (Figure 22.16). This test can also be conducted with the help of a ground icster as discussed in Section 22.3, which normally also has a provision for ihis test. 

R = station ground resistance in p = average resistivity of soil in 2m... 

The data in Table 4-1 show the considerable influence of the electrical resistivity of soil. This is particularly so in categories 2, 7, and 10. From a profile of the soil resistance along the course of a pipeline with welded connections or with electrically conducting thrust couplings, one can readily recognize anodic areas, and... 

Where there is available ground and the specific resistivity of soil in the upper layers is low, the anodes are laid horizontally [3]. A trench 0.3 to 0.5 m wide and 1.5 to 1.8 m deep is dug with, for example, an excavator or trench digger (see Fig. 9-2). A layer of coke 0.2-m thick is laid on the bottom of the trench. The impressed current anodes are placed on this and covered with a 0.2-m layer of coke. Finally the trench is filled with the excavated soil. No. IV coke with a particle size of 5 to 15 mm and specific gravity of 0.6 t m" is backfilled at a rate of 50 kg per meter of trench. The anodes are connected in parallel and every three to four anode cables are connected to the anode header cable by a mechanical cable crimp encapsulated in an epoxy splice kit to give an economical service life at high current output. 

A number of standards exist for the determination of some of these parameters. BS 1377 Part 3 1990 refers to methods of tests for soils for civil engineering purposes, and Part 9 refers to these and corrosivity tests in situ. It is significant that the standard draws attention to the fact that the results of the tests that are described should be interpreted by a specialist. ASTM tests for pH and resistivity of soil used for corrosion testing are covered by G51 1977(R1984) and G57 1978 (R1984), respectively. 

In general, most soil and rock minerals are electrical insulators (high resistivity) and, as a result, the flow of current is conducted primarily through the moisture-filled pore spaces within this matrix. Therefore, the resistivity of soils and rocks is predominantly controlled by the amount of pore water, the porosity and permeability of the system, and the concentration of dissolved solids in the pore water. 

Doelman, P. (1986). Resistance of soil microbial communities to heavy metals. In Microbial Communities in Soil, ed. Jensen, V., Kjoller, A., and Sorensen, C. H., Elsevier Applied Science, London, 369-384. 

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

The resistance of soil organic matter to biological decomposition under natural conditions has long been known but probably somewhat underestimated until the advent of C dating. Results obtained recently by this technique show quantitatively the age of soil carbon, and by selection of samples and their fractionation the technique can be used to throw light on the reasons for the stability. 

The stability, or resistance of soil organic matter to biological and chemical attack, constitutes one of its most important and puzzling characteristics. In the numerous chemical studies that have been conducted it is common for only 20—50% of the organic C and N substances to be released as comparatively simple compounds that can be identified. The percentage of the C and N that is apparently bound up in complexes is dependent to some extent on the organic matter itself (proportion of humic and non-humic materials), and even more on the methods used to fractionate the organic matter, and to separate it from colloidal clay. 

Mitchell, J.K. 1964. Shearing resistance of soils as a rate process. Journal of the Soil Mechanics and... 

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