Wenner 4-Pin Soil Resistivity
Apparent soil resistivity from a Wenner 4-pin (four-electrode, equal-spacing) earth test, the field measurement behind every ground-grid and driven-rod design: rho = 2 x pi x a x R, where a is the equal probe spacing and R is the earth-tester reading. With a converted to meters the result is ohm-meters (x100 for ohm-cm, the unit the grounding-electrode / Dwight tile wants). A 10 ft (3.048 m) spacing reading 5 ohms is 95.8 ohm-m (9,575 ohm-cm); a wider spacing probes deeper, so a set of readings at increasing spacings maps resistivity vs depth and reveals layering. Assumes the electrode depth is small vs the spacing and the 4 pins are equally spaced in a line. Resistivity swings widely with moisture, temperature, and season; the IEEE 81 / ASTM G57 method and the engineer of record govern the design value.
Formula and source
rho = 2 x pi x a x R, with the probe spacing a converted to meters (a_m = a_ft x 0.3048) and R the earth-tester reading (ohms); resistivity in ohm-m, and x100 for ohm-cm.
Wenner four-electrode (equal-spacing) soil-resistivity test method by name (IEEE 81 / ASTM G57); the field measurement and the engineer of record govern the design value.
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