Specific Gravity of Soil Solids

Gs is one of the few genuinely stable numbers in geotechnics, because most soils are made of the same few minerals. Quartz is 2.65, feldspars 2.5–2.6, the clay minerals 2.7–2.8, and micas somewhat higher. So while a soil's bulk density can vary two-fold with packing and water content, the density of the grains themselves barely moves — which is exactly why phase relationships are written in terms of Gs, void ratio and water content rather than measured densities.

Everything else follows from it: dry unit weight γd = Gs γw/(1 + e), saturated unit weight γsat = (Gs + e) γw/(1 + e), and degree of saturation S = w Gs/e. Hydrometer particle-size analysis depends on Gs directly through Stokes' law. The exceptions are worth knowing: organic soils and peats have Gs as low as 1.5–2.0 because of the plant matter, and iron-rich or mine-tailings soils can exceed 3.0. Measure it (ASTM D854) when the soil is unusual; assume 2.65 for ordinary sand and you will rarely be more than 2 % out.