Wenner 4-Pin Soil Resistivity

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 a grounding-electrode (Dwight) calculation 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.

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.

The Wenner relation rho = 2 pi a R is public (the equal-spacing four-electrode formula); the probe spacing and the earth-tester reading are the field measurements, and the method is IEEE 81 / ASTM G57.

Estimate. AHJ and licensed professional govern.

Field names used by the API: probe_spacing_ft, meter_resistance_ohm, resistivity_ohm_m, resistivity_ohm_cm

• Wenner equal-spacing rho = 2 pi a R, electrode depth small vs spacing a; a in meters -> ohm-m (x100 -> ohm-cm)IEEE 81 / ASTM G57
• Depth sounding + seasonal spacing a ~ effective depth; repeat at increasing a for layering; use the worst seasonal (dry/cold) value for designgrounding-design practice