Index tests that unlock everything else: specific gravity, phase relations and Atterberg limits
Specific gravity, moisture content, density and void ratio are linked by a few lines of algebra. With grading and plasticity they classify a soil before any strength test is run.
A soil sample is a mix of solids, water and air. Once you know the specific gravity of the solids Gs, the moisture content w and the bulk density, every other volumetric property follows from the phase diagram.
Specific gravity by pycnometer
Weigh the empty bottle (M1), the bottle plus dry soil (M2), the bottle plus soil topped up with de-aired water (M3), and the bottle full of water (M4). Then Gs = (M2 − M1) / [(M4 − M1) − (M3 − M2)], corrected to 20 °C for water density. Entrapped air is the usual source of error, so boil or vacuum the slurry. Most mineral soils fall between 2.6 and 2.8. Organic soils are lower, and iron-rich soils are higher.
Plasticity and grading
Atterberg limits locate a fine-grained soil on the Casagrande chart. The A-line, PI = 0.73(LL − 20), separates clays from silts. A particle size distribution, by sieving and then hydrometer below 75 µm with Stokes' law, gives D10, D30 and D60, and from those Cu and Cc for coarse soils. Together they give a USCS group symbol before any strength test.