Understanding In-Situ Dry Density
Checking that field-compacted soil actually met the compaction target
Why field testing is necessary
A laboratory Proctor test establishes the maximum dry density a specific soil can achieve at its optimum moisture content, under controlled compaction effort. In-situ dry density testing checks whether the soil actually compacted in the field reached an acceptable percentage of that laboratory-determined maximum — a direct quality-control check on the compaction work performed.
Poorly compacted fill settles over time, loses strength when wet and can crack slabs, pavements and utilities built on top of it. A density test is cheap compared with fixing any of those, which is why structural fill is almost always specified with a density requirement.
Bulk density vs. dry density
A field test first measures bulk (wet) density: the total mass of soil and water in a known volume. Dry density removes the water: ρd = ρ ÷ (1 + w), with w as a decimal. Compaction is judged on dry density because moisture varies from test to test, while the mass of solids packed into a volume is what compaction actually changes. The moisture content comes from a sample taken at the same spot — see What Is Soil Moisture Content?.
Common field methods
The sand-cone method (ASTM D1556) excavates a small hole, weighs the removed soil, and fills the hole with a calibrated sand of known density to determine the hole's volume, from which dry density is calculated. The nuclear density gauge (ASTM D6938) measures density non-destructively using radioactive source readings, and is widely used for its speed, though it requires trained, licensed operators due to the radioactive source involved. The drive-cylinder method (ASTM D2937) is another option, pressing a known-volume cylinder into the soil.
| Method | Standard | Best for | Limitations |
|---|---|---|---|
| Sand cone | ASTM D1556 | Soils that hold a stable hole, without appreciable stones over 1½ in | Not for organic, saturated or highly plastic soils; may not suit loose granular soil; seepage or vibration affects the hole |
| Drive cylinder / core cutter | ASTM D2937 | Fine-grained soils that can be sampled intact | Not for organic or friable soils, very hard soils, or soils with many particles over 4.75 mm |
| Nuclear gauge | ASTM D6938 | Fast, repeated testing | Licensing; not for clean gravel or clean crushed rock |
Comparing to the compaction target
The field result is expressed as a percentage of the laboratory maximum dry density — 'percent compaction' or relative compaction. FHWA's geotechnical manual notes that specifications often require a minimum, giving 95% as an example, and its table of AASHTO recommended minimums, based on the standard Proctor test, lists at least 95% for embankments and 95–100% for subgrades depending on the soil class. The project specification sets the actual figure.
The comparison is only valid against the Proctor test named in the specification — standard (ASTM D698, 12,400 ft-lbf/ft³) or modified (ASTM D1557, 56,000 ft-lbf/ft³). The two use different compactive efforts, so their maximum densities for the same soil differ; never mix them.
Worked example: core cutter
A 1,000 cm³ cutter weighs 1,250 g empty and 3,390 g after being driven into the fill and trimmed flush. Bulk density = (3,390 − 1,250) ÷ 1,000 = 2.14 g/cm³. A companion sample gives 12% moisture, so dry density = 2.14 ÷ 1.12 = 1.91 g/cm³. With a laboratory maximum of 2.00 g/cm³, percent compaction = 1.91 ÷ 2.00 × 100 = 95.5%, which passes a 95% requirement.
The In-Situ Dry Density Calculator follows this core-cutter sequence in kg/m³: 2,140 kg/m³ wet, 2,140 ÷ 1.12 = 1,911 kg/m³ dry, and 1,911 ÷ 2,000 = 95.5%. A useful sanity check: the field dry density can't exceed the zero-air-voids density for the measured moisture, which needs the soil's specific gravity — see What Is Soil Specific Gravity?.
When a test fails
- Check moisture first — soil dry of optimum rarely reaches density however many passes the roller makes. Add water and re-compact.
- If the soil is wet of optimum, dry it by discing or aerating before re-compacting.
- Reduce the lift thickness so the compactor's energy reaches the bottom of the layer.
- Confirm the right Proctor curve is being used; fill from a new borrow source may need its own.
- Re-test at the same location after rework, and record both results.
Where it's used
Density testing is routine for building pads, trench backfill, embankments, and road and driveway base. For base layers, Road Base Materials Explained covers placement and compaction; for subgrade strength, What Is a CBR Test? explains how density ties into design values.
Common mistakes
- Comparing to a Proctor result from a different soil.
- Testing only the surface of a thick lift, where compaction is always best.
- Using the moisture content from the cutter sample itself after it has dried.
- Driving a cutter into gravelly fill, which disturbs the sample and misreads volume.
FAQ
- What is a good percent compaction?
- It is whatever the project specification requires. FHWA gives 95% of the laboratory maximum dry density as a typical example, and AASHTO's recommended minimums (based on standard Proctor) are at least 95% for embankments and 95–100% for subgrades, depending on the soil.
- How do you calculate dry density from wet density?
- Divide the wet (bulk) density by (1 + moisture content as a decimal). For example, 2.14 g/cm³ at 12% moisture gives 2.14 ÷ 1.12 = 1.91 g/cm³.
- Which field density test is most accurate?
- Each has its place. The sand cone suits soils that hold a stable hole; the nuclear gauge is fastest; the drive cylinder suits fine-grained soils that can be sampled intact. Accuracy depends more on careful technique than on the method chosen.
- Can percent compaction be over 100%?
- Yes. Field equipment can sometimes exceed the laboratory compaction effort, or the soil may differ from the Proctor sample. Results well over 100% are worth checking against the correct Proctor curve.
Related Calculators
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More From the Blog
Sources
ASTM D1556 / D6938 / D2937
Standard Test Methods for Density of Soil in Place (Sand-Cone Method / Nuclear Methods / Drive-Cylinder Method).
ASTM D698 / D1557
Laboratory compaction (standard and modified Proctor).
FHWA NHI-05-037, Geotechnical Aspects of Pavements, Chapter 5
Tables 5-12, 5-13 and 5-16 (field density methods, compaction, AASHTO recommended minimum compaction) and Equation 5.1 (dry unit weight).
FHWA NHI-05-037, Geotechnical Aspects of Pavements, Chapter 5 — reference