Distribution of Soil Conductivity and Its Relation to Underground Corrosion

This article discusses results and consequences arising singly or jointly from the linear soil conductivity plot, the logarithmic normal distribution of soil conductivity, and the relation between underground corrosion and soil conductivity, including the following: the cumulative probability chart provides an almost ideal way to display soil resistivity data; the logical definition of a statistically uniform soil is shown to be one in which soil conductivities conform to a distribution that is logarithmically normal; a simple, compact, and accurate way to describe a statistically uniform soil is provided; the analyses prove beyond a reasonable doubt that underground corrosion, however measured (even qualitatively), is related in a uniform soil to soil conductivity; the linear form of the relationship between the corrosion as measured by loss in weight and soil conductivity may be deduced from Ohm's law; a basis for classification of soils with respect to corrosive quality as gaged by soil conductivity (or resistivity) is virtually dictated; the pivot of such classification would be a resistivity of 1,000 ohm‐ern (equivalent to a conductivity of 1,000 μmho/cm); the partitions between corrosive classes would occur as a consequence of the logarithmic normal distribution at multiples of two (or one‐half) of the pivot; for example, resistivity (or conductivity) boundaries would be 250,500, 1,000, 2,000, and 4,000 ohm‐ern (or micrornhos per centimeter), respectively; the soil parameters of incomplete sets of resistivities measured for example, by probe‐type instruments which do not indicate resistivities above 10,000 ohm‐ern can be determined in statistically uniform soils by the analytical procedure, and if necessary, missing data can be estimated with a high degree of probability; the analytical procedure provides a rational basis for the exercise of engineering judgment on problems of underground corrosion; for example, the prediction of probable current strengths obtainable from galvanic anodes, their life, and related items; the notion of a statistically uniform soil and the linear conductivity plot serve together to resolve noncon‐ forming sets of resistivity measurements into two or more statistically uniform soils; the concept of a statistically uniform soil and the use of a linear conductivity plot serve to correlate conductivity with such variables as elevation, drainage, vegetation, alkali, rocks, soil texture, and the like; a method is provided whereby one may choose a suitable soil area within which to conduct experiments to relate numerous soil characteristics or measurements, such as redox potentials and pit depths; the conductivity right‐of‐way plot provides an engineering basis for design of cathodic protection, which should be centered in areas of higher conductivity; there is a growing necessity to maintain minimum current waste, to minimize damage to coatings by the current, to avoid cathodic interference, and to provide maximum protection to a pipeline at minimum cost; it is profitable to explore the nature of the exposure of any proposed underground structure before construction; the preliminary step is a soil conductivity survey, which may or may not be followed by chemical analyses, depending upon results of the preliminary survey and upon the composition of the proposed structure.