What the test measures
ASTM D6938 covers measuring the in-place density and water content of soil and soil-aggregate with a nuclear gauge. The gauge sends gamma radiation into the ground to estimate the wet density, and uses neutrons to estimate the water content. A technician takes readings at a spot you choose and the report lists the results.
The standard allows three measurement methods. In direct transmission (method A) a probe is lowered into a hole to a set depth. In backscatter (method B) the gauge sits on the surface. In backscatter with air gap (method C) a spacer is used. Reports usually state which method and which probe depth were used.
The fields on a typical report
Layouts differ between labs and gauge software, but most reports carry the same information:
| Field | What it tells you |
|---|---|
| Project, date, technician, gauge ID | Traceability. You need these if a result is ever questioned. |
| Location (station, offset) and elevation or lift | Where the test was taken, so a failed spot can be matched to its retest later. |
| Material and Proctor reference | Which soil type and which moisture-density curve the result is compared against. |
| Test method and probe depth | Direct transmission or backscatter, and the depth of the reading. |
| Wet density | Total unit weight of the soil as measured, including water. |
| Moisture content | Water content, as a percent of the dry soil weight. |
| Dry density | Wet density with the water taken out. This is the number compared to the Proctor. |
| Maximum dry density (MDD) and optimum moisture content (OMC) | The reference values from the laboratory Proctor test. |
| Percent compaction | Field dry density divided by MDD, as a percent. |
| Pass or fail, with the specified minimum | Not always printed. Some labs only report the numbers and leave the comparison to you. |
How the numbers connect
Dry density comes from wet density and moisture content:
dry density = wet density / (1 + moisture content / 100)
Percent compaction compares the field result with the laboratory reference:
percent compaction = field dry density / MDD x 100
Example with made-up numbers: a wet density of 130.0 pcf at 9.0% moisture gives a dry density of about 119.3 pcf. If the Proctor MDD is 123.0 pcf, the percent compaction is about 97%. Against a 95% minimum that passes. Against a 98% minimum it fails.
Moisture is usually judged separately, as a window around the optimum, for example OMC plus or minus 2 percentage points. The window and the minimum compaction both come from your specification, not from the standard.
Checks to run before you accept a result
- Is it the right Proctor? The percent compaction is only as good as the reference curve. A different soil, borrow source or layer can have a different MDD. If the wrong curve was used, a good field result can look like a failure, or the reverse.
- Do the numbers agree with each other? Recompute dry density and percent compaction from the wet density and moisture. A transcription error shows up immediately.
- Does it meet the contract? Check the minimum compaction and the moisture window for that material and use (for example subgrade versus base). They are not the same everywhere on a project.
- Is the location clear? A failed test with no usable station or elevation cannot be paired with a retest.
- Is the test frequency covered? Many agencies require a minimum number of tests per area, volume or length. The requirement depends on the agency and the spec, so use your own contract.
- Is there a passing retest for every failure? See our guide on tracking failed compaction tests and retests.
Nuclear gauge readings depend on the gauge calibration, the standard count and the surface preparation. This guide does not audit the technician or the lab. It assumes the reported values are correct and focuses on whether they are complete and meet your specification.
Where this goes wrong in practice
Most problems are paperwork, not soil. Reports arrive as multi-page PDFs from different labs with different layouts. Numbers get retyped into a spreadsheet by hand. A retest sits in another email. The tracker looks complete until someone tries to assemble the closeout package. Reading each report against the same short list above, every time, is what prevents that.
Related: Proctor tests, MDD and OMC explained.