# What Is The Specific Gravity Of Soils And Why Does It Matter?

20th Feb 2019

There are many different ways that engineers can learn about the properties of soil at a construction site. Testing the specific gravity of soils is one of the most important. Learn more about this informative test, how to perform it and what makes it so crucial to your project's success.

## What Is Specific Gravity Of Soils?

The "specific gravity" of soils and soil solids refers to the mass of solids in the soil compared to the mass of water at the same volume. The name "specific" is a bit misleading since soil solids are made of so many different particles, each with their own separate gravity measurements. As such, the specific gravity of soil solids is actually an average of the specific gravity of all included soil particles. In general, the specific gravity tends to fall between 2.65 and 2.80, with coarser soils generally having lower specific gravities than finer soils.

## Why Does This Test Matter?

Knowing the specific gravity of soils helps engineers understand how porous the soil is or how many voids it contains. It also indicates how saturated the soil is with water. Engineers use these measurements to perform important calculations that predict whether the soil at a site will be stable enough to support a structure and allow proper drainage.

## How Do You Perform A Specific Gravity Of Soils Test?

There are a couple of ways you can perform this test. The two most common involve a specific gravity bench or a specific gravity flask, also known as a volumetric or Le Chatelier flask.

### Using A Bench

A specific gravity bench allows you to weigh suspended samples in water. Using a bench is effective for determining the specific gravity of hardened concrete, aggregates, refractory brick, bituminous mixtures and other similar materials. To perform the specific gravity of soils test using a bench, follow these steps:

- Oven-dry a soil sample and weigh it.
- Submerge the sample in water and weigh it again.
- Calculate the specific gravity of soil solids by dividing your first measurement (weight in air) by your second measurement (loss of weight in water).

The calculation described will look like this:

- Specific Gravity = Weight In Air/Loss Of Weight In Water

### Using A Flask

A Le Chatelier flask is the second most commonly used piece of equipment for determining the specific gravity of soil solids. However, this method is more effective for powdered materials, such as limes, slag and hydraulic cement. To perform the test using a flask, follow these steps:

- Oven-dry the soil sample and crush it into a powder (or at least a finer texture).
- Measure out a sample of the dried, crushed soil to a precise mass (2 grams, for example).
- Pour the sample into a volumetric (Le Chatelier) flask.
- Fill the same flask with water to just below the top of the bulb. Take note of how much water you've added.
- Roll the flask at an inclined angle to release any air bubbles in the water without spilling it.
- Fill the flask with enough water to reach the 500 mL mark. Again, take note of how much water you've added.

The specific gravity of soil solids calculation is slightly more complex for this method than the bench method. To begin, you must find the density of soil solids (Ps) by dividing the mass of the oven-dried soil sample by 500 minus the volume of water added to reach 500 mL. The calculation will appear as such:

- Ps = Ms/(500-Vw)

Now that you have the density of soil solids, you can calculate the specific gravity of soil solids (SG). To find it, divide the density of soil solids by the density of water (Pw), which is 1,000 kg/m3. The calculation should then be:

- SG = Ps/Pw

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