Compaction and CBR
Proctor curve, maximum dry density and optimum water, field acceptance, and the CBR of road subgrades.
Turns the raw readings of a Proctor test into the compaction curve, with the maximum dry density, the optimum water content, the zero-air-voids and saturation lines and the degree of saturation at optimum, then checks a field density test against the specification. The CBR part draws the load–penetration curve, corrects a concave start, gives the CBR at 2.54 and 5.08 mm, rates the material for road layers and estimates its resilient modulus.
Overview
Every embankment, road, airfield and earth dam is built in compacted layers accepted against a laboratory reference. These are the two tests a soil laboratory runs most often, and their sheets are usually drawn by hand; the tool gives the laboratory and the site engineer the same figures in a minute.
The compaction curve is fitted by least squares through all points rather than drawn by eye, and the tool warns when the peak is not bracketed by the test points.
What you enter · What you get
What you enter
- Test type, specific gravity, mould volume and mass
- Mass of mould with compacted soil and water content of each point
- Field dry density and water content, required compaction
- CBR penetration–load readings and swell
What you get
- Maximum dry density and optimum water content
- Relative compaction and acceptance of the field test
- CBR at 2.54 and 5.08 mm, rating and resilient modulus
- Compaction and CBR curves; data as CSV
How it works in the field
Method and equations
Compaction
Bulk density ρ = (M − Mmould)/V and dry density ρd = ρ/(1 + w); a polynomial of third order (second order for 3–4 points) is fitted by least squares and its maximum within the tested range gives ρd,max and w opt (ASTM D698 / D1557).
CBR
Loads at 2.54 and 5.08 mm are divided by the standard loads 13.24 and 19.96 kN. When the curve starts concave, the steepest tangent is extended to zero load and the origin moved to that point (ASTM D1883, BS 1377-4).
Resilient modulus
Heukelom & Klomp (1962) correlation Mr ≈ 10.34 CBR (MPa), applied up to CBR 20 where it was calibrated.
Accuracy and error limits
- Repeatability of the Proctor test is about ±0.02–0.03 g/cm³ in maximum dry density and ±0.5–1 % in optimum water; the fitted peak adds about ±0.01 g/cm³ when the points bracket it well. Field density tests carry ±1–2 % in relative compaction.
- CBR results scatter by ±10–20 % between specimens of the same soil; the Mr correlation is approximate (±50 %) and should not replace repeated-load triaxial tests for important pavements.
- To reduce the error: use at least five points, two on each side of the optimum, measure Gs rather than assume it, test CBR in triplicate on soaked specimens, and correct for oversize particles when more than 5 % is retained on the 19 mm sieve.
Who uses it
Standards and references
- ASTM D698-12(2021). Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort.
- ASTM D1557-12(2021). Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort.
- ASTM D1883-21. Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils.
- BS 1377-4:1990. Methods of test for soils for civil engineering purposes — Compaction-related tests.
- Heukelom, W. & Klomp, A.J.G. (1962). Dynamic testing as a means of controlling pavements during and after construction. 1st International Conference on the Structural Design of Asphalt Pavements, Ann Arbor, 667–679.
At a glance
- Category
- Engineering & geotechnics
- Runs on
- Phone, tablet and desktop browser
- Export
- Compaction and CBR curves as PNG, JPG and PDF; data as CSV; full PDF report
- Access
- Free
Run this survey on your next site
All 64 tools, report export and the AI assistant come with one subscription.