Essay On Specific Gravity

2. 0 DETERMINATION OF SPECIFIC GRAVITY

2.1 Introduction

2.1.1 OBJECTIVE

To determine the specific gravity of a given soil by Density Bottle or Pycnometer Method

2.1.2 Learning Outcome

Explain the Concept of Specific Gravity and its Uses

Describe Lab Method for Determining Specific Gravity of Soils

Perform the Specific Gravity Test on a given Soil Sample

Determine Specific Gravity of the Soil

Produce a report of the exercise

2.1.3 NEED AND SCOPE

A knowledge of specific gravity is often used together with soil moisture content and unit weight in the calculation of soil properties like void ratio, porosity, degree of saturation, soil identification, soil classification, etc. It is particularly important when compaction and consolidation properties

It tells us how many times the soil grain is heavier than water. The specific gravity of solids of a soil sample depends upon the mineral constituents of the soil sample. However, the average specific gravity of solids of sand sample is 2.65 whereas for clay it is about 2.70. For most soils, Gs varies in a very narrow range of 2.6 to 2.8, Holtz and Kovacs (1981).

The specific gravity of soil mass can be determined by experiment in the laboratory. This is done by immersing a known mass of dry soil, ms in a fluid contained in a density bottle or glass jar. The bottle is completely filled with the fluid. The mass of the bottle plus the fluid and the soil is determined, say m3. If the weight of the bottle filled with water only is m4, then the mass of a volume of fluid equivalent to that of the soil is m4 - m3+ ms and if the fluid is water, then

Specific gravity, Gs = m_s/(m_4-m_3+m_s ) 2.3 If the fluid used for the test is not water but a fluid with a specific gravity, GL then

Gs = (m_s G_L)/(m_4-m_3+m_s ) 2.4

2.2 APPARATUS REQUIRED Calibrated Pycnometer or volumetric flask, 250

Cohesive Soil (Clay) Take about 50 gm of the air or oven-dried soil sample and put in a 250-ml beaker. Cover soil with 125 ml of sodium hexametaphosphate solution. Stir until thoroughly wetted and allow mixture to soak for at least 16 hours After soaking period, carefully pour mixture into the dispersion cup. With an electric mixer, mix soil-water slurry for a period of 1 minute Pour slurry into a clean calibrated pycnometer or volumetric flask of known weight. Weigh the flask and slurry, Wfs. Carefully fill flask/bottle about three-quarter full with deaired distilled water. Remove any air entrapped by applying partial vacuum to the slurry. In some cases, you may need to boil the contents for about 10 minutes and reapply the partial vacuum. Now add distilled water to fill the flask to the bottom meniscus at the calibration mark. Dry the outside of the bottle and weigh the bottle and its contents, Wbws. Mix contents in the bottle until temperature is uniform. Take temperature at different depths, cover bottle and turn it upside down and back several times to mix the contents. Record the uniform temperature,