ASTM G187: Measurement of Soil Resistivity Using the Two-Electrode Soil Box Method

What the ASTM G187 test measures

ASTM G187 measures the electrical resistivity of a soil sample in the laboratory using a two-electrode soil box. A sample of the soil is packed into a box of known dimensions with electrodes at each end, current is passed through it, and the resistance is converted to resistivity in ohm-centimeters. The test is run as received and again with the soil saturated, so the minimum resistivity, the corrosive worst case, is reported.

It is the method mechanically stabilized earth wall specifications, pipeline corrosivity assessments, and many DOT backfill requirements name for the resistivity value that steel reinforcement and buried structures are qualified against.

How the ASTM G187 test works, step by step

The soil is passed through a sieve to remove large particles and packed into the calibrated soil box in the as-received condition. A resistance meter applies alternating current between the end plates, or uses the four-terminal configuration of the box, and the reading is converted using the box constant. Distilled water is then added in increments, the soil is remixed and repacked, and the measurement repeated until the resistivity reaches a minimum. The minimum value is reported along with the as-received value and the moisture content at each.

What the numbers mean

The same corrosivity ratings used for ASTM G57 apply: under 1,000 ohm-cm is severely corrosive and values above 10,000 are generally benign. MSE wall specifications commonly require a minimum of 3,000 ohm-cm at saturation for metallic reinforcement, together with pH between 5 and 10, chlorides under 100 ppm, and sulfates under 200 ppm. A backfill failing resistivity is disqualified regardless of how well it compacts.

What can throw the result off

Contaminated sample containers, tap water instead of distilled water, an uncalibrated box, and a sample dried and oxidized before testing all shift the result. Because salts control conductivity, any contamination with de-icing salt, cement, or fertilizer during sampling will produce an unrepresentatively low resistivity.

How we help with ASTM G187 testing

We run soil box resistivity with pH, chloride, and sulfate determinations as a backfill electrochemical package for MSE walls, reinforced soil slopes, and buried pipe, and report against the specification limits so approval decisions are direct.