Specific Gravity of Soils

Courses > Soil Mechanics > Geotechnical Laboratory and In-Situ Testing Methods > Specific Gravity of Soils

Table of Contents

Introduction

The specific gravity of a given material is defined as the ratio of the weight of a given volume of the material to the weight of an equal volume of distilled water. In soil mechanics,the specific gravity of soil solids (which is often referred to as the specific gravity of soil) is an important parameter for calculation of the weight-volume relationship. Thus specificgravity, Gs, is defined as

Gs =unit weight (or density) of soil solids only /unit weight (or density) or water

Specific Gravity of Soils (2)

where W_s = mass of soil solids (g);V_s = volume of soil solids (cm³); and p_w = density of water (g/cm³)

The general ranges of the values of G, for various soils, are given in Useful Relationships and Typical Values In Geotechnical Engineering. The procedure for determination of specific gravity, Gs described here is applicable for soils composed of particles smaller than 4.75 mm (No.4 U.S. sieve) in size.

Concepts and Formulas

Equipment

1. Volumetric flask (500 ml)
2. Thermometer graduated in O.soC division scale
3. Balance sensitive up to 0.01 g
4. Distilled water
5. Bunsen burner and a stand (and/or vacuum pump or aspirator)
6. Evaporating dishes
7. Spatula
8. Plastic squeeze bottle
9. Drying Oven

Procedure

1. Clean the volumetric flask well and dry it.
2. Carefully fill the flask with de-aired, distilled water up to the 500 ml mark (bottom
of the meniscus should be at the 500 ml mark).
3. Determine the mass of the flask and the water filled to the 500 ml mark (W1)
4. Insert the thermometer into the flask with the water and determine the temperature of the water T = T1 .
5. Put approximately 100 grams of air-dry soil into an evaporating dish.
6. If the soil is cohesive, add water (de-aired and distilled) to the soil and mix it to the form of a smooth paste. Keep it soaked for about one-half to one hour in the evaporating dish. (Note: This step is not necessary for granular, i.e., noncohesive, soils.)
7. Transfer the soil (if granular) or the soil paste (if cohesive) into the volumetric flask.
8. Add distilled water to the volumetric flask containing the soil (or the soil paste) to make it about two-thirds full.
9. Remove the air from the soil-water mixture. This can be done by:

a. Gently boiling the flask containing the soil-water mixture for about 15 to 20 minutes. Accompany the boiling with continuous agitation of the flask. (If too much heat is applied, the soil may boil over.) Or
b. Apply vacuum by a vacuum pump or aspirator until all of the entrapped air is out.

This is an extremely important step. Most of the errors in the results of this test are due to entrapped air which is not removed.

10. Bring the temperature of the soil-water mixture in the volumetric flask down to room temperature, i.e., T1 see Step 4. (This temperature of the water is at room temperature.)
11. Add de-aired, distilled water to the volumetric flask until the bottom of the meniscus touches the 500 mlmark. Also, dry the outside of the flask and the inside of the neck above.the meniscus.
12. Determine the combined mass of the bottle plus soil plus water (W2).
13. Just as a precaution, check the temperature of the soil and water in the flask to see if itis T1.± 1 or not.
14. Pour the soil and water into an evaporating dish. Use a plastic squeeze bottle and wash the inside of the flask. Make sure that no soil is left inside.
15. Put the evaporating dish in an oven to dry to a constant weight.
16. Determine the mass of the dry soil in the evaporating dish (Ws).