In Situ Density Calculator — Sand Cone Method
The sand cone is the reference method for verifying the dry density achieved in a compacted fill. This calculator receives the weight of the extracted material, the moisture content and the mass of calibrated sand that fills the hole, and delivers the in situ wet unit weight, the dry unit weight and the relative compaction with respect to the MDD of the Proctor test. It applies to BS 1377-2 (+ BS EN ISO 17892-4) and BS 5930 (+ BS EN ISO 14688-1/-2), the mandatory standard for structural fills, bases and sub-bases.
What does it measure and when to use it?
The test quantifies how much a soil has been compacted on site, comparing it with the laboratory MDD. It is used to verify finished layers of fills, embankments, improved subgrades and granular bases. The sand cone remains the reference method in the UK: although the nuclear densometer is faster, any significant discrepancy between nuclear and cone is resolved with the cone. It is mandatory in acceptance of works under BS EN 1997-1 (Eurocode 7).
Applied formulas
Volume of hole: V = M_sand / ρ_sand
where M_sand is the mass of calibrated sand that fills the hole (deducting the funnel portion) and ρ_sand is the calibrated density of that sand.
In situ wet unit weight: γ = M_soil / V
Dry unit weight: γd = γ / (1 + w)
Relative compaction: RC = (γd / MDD) × 100 %
Difference from optimum: Δw = w_field − wopt
Calculation example
| Parameter | Value |
|---|---|
| Mass of extracted soil | 3 920 g |
| Soil moisture content (w) | 6.8 % |
| Mass of sand in hole | 2 720 g |
| Calibrated sand density (ρ_sand) | 1.415 g/cm³ |
| Modified Proctor MDD | 21.3 kN/m³ (2 170 kg/m³) |
| Proctor wopt | 7.5 % |
First the volume of the hole: V = 2 720 / 1.415 = 1 922 cm³ = 0.001922 m³. Convert soil mass to kN: 3 920 g = 3.92 kg → 3.92 × 9.81 = 38.46 N = 0.03846 kN. Wet unit weight γ = 0.03846 / 0.001922 = 20.0 kN/m³. Dry unit weight γd = 20.0 / (1 + 0.068) = 20.0 / 1.068 = 18.73 kN/m³. Relative compaction RC = (18.73 / 21.3) × 100 = 88.0 %. Moisture difference Δw = 6.8 − 7.5 = −0.7 % (dry side of optimum).
Result: γ = 20.0 kN/m³ · γd = 18.7 kN/m³ · RC = 88 % · Δw = −0.7 %. Does not comply with the usual minimum of 100 % for sub-base.
Interpretation of results
An RC of 88 % is well below the 100 % required by the UK Highways Agency Manual of Contract Documents for bases and sub-bases. There are two options: re-compact with moisture closer to optimum (adding water if dry) and increasing roller passes, or verify that the Proctor MDD corresponds to the same material. If the RC is between 95 % and 100 %, it may be tolerated on a case-by-case basis; below 95 %, the layer is rejected.
Reference standards
- BS 1377-2 (+ BS EN ISO 17892-4) — Methods of test for soils for civil engineering purposes — Classification tests and determination of particle size distribution
- BS 5930 (+ BS EN ISO 14688-1/-2) — Code of practice for ground investigations — Soil classification
- UK Highways Agency Manual of Contract Documents — Volume 8 sections 8.502 and 8.602
- BS EN ISO 22476-3 — Geotechnical investigation and testing — Field testing — Standard penetration test (equivalent)
Frequently asked questions
How often should I carry out density tests on site?
For sub-bases and bases under UK Highways Agency specifications, typically one test every 1 500 m² or per layer in each 200 linear metres, whichever is less. In structural fills for buildings, one test every 300-500 m² per layer is required. Project-specific specifications may be more demanding.
What do I do if the sand cone gives RC > 100 %?
This can happen with very well compacted crushed bases, but if it exceeds 103-104 %, it is advisable to check the reference MDD. A Proctor test carried out with material slightly different from that on site underestimates the actual MDD. It is recommended to take a sample from the layer and redo the Proctor test.
Can I always replace the sand cone with a nuclear densometer?
Yes for routine control, but the standard UK contract requires comparing at least 1 in every 10 tests with the sand cone. If there is a discrepancy greater than 2 %, the cone prevails. For final acceptance, the cone is almost always required.
How is the sand calibrated?
Its density is determined by filling a container of known volume following the same procedure as the test. It is repeated 3-5 times and averaged. The sand must be calibrated, dry, uniform (typically Ottawa or similar), and its density is re-verified every 3 months or 50 tests.