What is the purpose of a soil mechanics laboratory?

A soil mechanics laboratory tests soil samples to determine physical and mechanical properties such as strength, compressibility, and permeability. These data are essential for designing foundations, retaining walls, and earthworks, ensuring structures are safe and stable under expected loads.

Which ASTM standards are commonly used in soil testing?

Common ASTM standards include ASTM D1586 for SPT, ASTM D422 for grain size, ASTM D4318 for Atterberg limits, ASTM D698 for Proctor compaction, and ASTM D2850 for unconsolidated-undrained triaxial tests. These ensure consistency and reliability across laboratories.

How does seismic zone affect foundation design?

Seismic zones, defined by ASCE 7, indicate ground motion intensity during earthquakes. Higher zones require stronger foundations, deeper embedment, and sometimes soil improvement to mitigate liquefaction risk. Our laboratory provides dynamic soil properties for seismic analysis.

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 provides the in-place wet unit weight, the dry unit weight, and the relative compaction with respect to the MDD of the Proctor. It applies to ASTM D1556 and ASTM D5080-20, mandatory standard for structural fills, bases, and subbases.

What does it measure and when to use it?

The test quantifies how much a soil has been compacted in the field, comparing it to the laboratory MDD. It is used to verify finished layers of fills, embankments, improved subgrades, and granular bases. The sand cone remains the referee method in Latin America: although the nuclear densometer is faster, any significant discrepancy between nuclear and cone is resolved with the cone. It is mandatory in ASTM D698 project acceptances.

Applied Formulas

Volume of the 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-place 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

Input data — interurban highway subbase
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 hole volume: 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 meet the usual minimum of 100 % for subbase.

Interpretation of results

An RC of 88 % is well below the 100 % required by the ASTM D698 for bases and subbases. There are two options: recompact 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 were between 95 % and 100 %, it may be tolerated with case-by-case analysis; below 95 %, the layer is rejected.

Reference standards

ASTM D1556 — Standard Test Method for Density of Soil in Place by Sand-Cone Method
ASTM D5080-20 — Soil Mechanics Laboratory — Determination of density in the field — Sand cone method
ASTM D698
AASHTO T 191 — Sand-cone method (equivalent)

Frequently asked questions

How often should I perform density tests in the field?

For ASTM D698 subbases and bases, typically one test every 1 500 m² or per layer every 200 linear meters, whichever is less. For structural fills in 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 review the reference MDD. A Proctor performed with material slightly different from the field material underestimates the actual MDD. It is recommended to take a sample from the layer and redo the Proctor.

Can I always replace the sand cone with a nuclear densometer?

Yes for routine control, but the standard ASTM D698 contract requires comparing at least 1 out of 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 test procedure. It is repeated 3-5 times and averaged. The sand must be calibrated, dry, uniform (typically Ottawa or similar), and its density is rechecked every 3 months or 50 tests.