What Is Soil Specific Gravity?

What it measures

Specific gravity of soil solids compares the density of the actual solid soil particles (not the soil mass as a whole, which includes air and water in its voids) to the density of water. Common values for most mineral soils cluster in a fairly narrow range, though organic soils and some unusual mineral compositions can fall outside it.

It is written Gs and has no units. A Gs of 2.65 means the mineral grains themselves are 2.65 times as dense as water — about 2,650 kg/m³ or 165 lb/ft³ of solid mineral. The soil in the ground weighs much less than that per unit volume because part of its volume is pore space.

Why specific gravity of soil is determined

The purpose of the test is to link the mass of the soil solids to the volume they occupy. Almost every weight-volume calculation in soil mechanics needs that link, so Gs is usually measured as an input to other results rather than reported on its own.

• Volumetric properties: unit weight, void ratio, porosity and degree of saturation all use Gs.
• Hydrometer analysis: the particle-size distribution of fine-grained soils, measured by hydrometer, is calculated using Gs.
• Compaction checks: the zero-air-voids line on a compaction curve is calculated from Gs (see the formula below), and no compacted density at that moisture can lie above it.

Typical values

Because most soils are made of a small number of common minerals, Gs varies less than almost any other soil property. That's why it's sometimes assumed on small jobs — but an assumed value should be stated as such, and organic or unusual soils should always be tested.

The typical values below cover common mineral soils. They don't include organic soils or soils rich in unusual minerals, which should be tested rather than assumed.

How it's tested

A common laboratory method (ASTM D854, or AASHTO T 100) uses a pycnometer — a specialized flask of known volume — to measure the volume of water displaced by a known mass of oven-dried soil solids, from which specific gravity is calculated.

The key step is removing all trapped air from the soil-water mixture, by vacuum, gentle boiling or both. Any air left in the slurry makes the soil appear to displace more water than it really does, which lowers Gs. The flask's water mass is also temperature-dependent, so readings are corrected to a standard temperature, normally 20 °C.

Worked example

Using four weighings: pycnometer empty W1 = 155.0 g; with dry soil W2 = 205.0 g; with soil and water W3 = 686.2 g; with water only W4 = 655.0 g. The soil mass is W2 − W1 = 50.0 g. The water it displaces is (W4 − W1) − (W3 − W2) = 500.0 − 481.2 = 18.8 g. So Gs = 50.0 ÷ 18.8 = 2.66.

The Specific Gravity of Soil Calculator uses exactly this formula. It doesn't apply a temperature correction, so apply one to W4 first if your test temperature differs from the calibration temperature.

Why other calculations depend on it

Specific gravity is a key input for calculating a soil's void ratio, porosity, and degree of saturation — properties that in turn feed into settlement, permeability and strength analyses, such as the unconfined compressive strength test for clays. It's rarely the final answer engineers need on its own, but it's frequently a required input to get there.

• Void ratio, e = Gs × ρw ÷ ρd − 1
• Porosity, n = e ÷ (1 + e)
• Degree of saturation, S = w × Gs ÷ e
• Zero-air-voids density, ρzav = Gs × ρw ÷ (1 + w × Gs)

Soil specific gravity vs. aggregate specific gravity

Aggregate testing uses the same idea but reports several values, because aggregate particles have internal pores that can absorb water. ASTM C127 (coarse) and C128 (fine) give bulk (oven-dry), bulk saturated-surface-dry and apparent specific gravity, plus absorption — and mix designers use the SSD value to proportion concrete. Soil Gs, by contrast, is the specific gravity of the solids alone. The aggregate specific gravity guide and the Aggregate Specific Gravity Calculator cover the aggregate version.

Common mistakes

• Incomplete de-airing of the slurry, which understates Gs.
• Ignoring the temperature difference between calibration and test.
• Confusing Gs with the bulk or dry density of the soil mass.
• Assuming 2.65 for an organic or unusual soil.