Felix Rodriguez - University of Florida
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CEG 4011 Soil Mechanics
Date Performed: 30 August 2004
Date Submitted: 9 September 2004
The intention of this lab is to determine the specific gravity of a sample of soil. The specific gravity is defined by ASTM as “the ratio of the mass of a unit volume of a material at a stated temperature to the mass in air of the same volume of gas-free distilled water at a stated temperature.” Two types of specific gravities are defined as follows:
Mass specific gravity of the soil: [pic]
Specific gravity of the soil: [pic]
Here, ρ and γ refer to the density and the unit weight of the soil, while ρs and γs refer to the density of solids and unit weight of solids, respectively. The mass specific gravity is defined as such using the density of water at 4˚ C. Another definition of specific gravity exists, this one for water:
In order to determine the specific gravities, it is necessary to determine the mass and volume of the soil sample accurately. They will be found by using a procedure called the indirect constant volume method. This procedure exploits the use of Archimedes’ Principle, where the volume of a solid is found by submerging the sample in water and measuring the difference in the water volume. A calibrated volumetric flask, called a pycnometer, will be used as the constant volume reference. The procedure involves the following definitions and relations:
Mf = mass of the empty flask
Ms = mass of the soil solids
Mfw = mass of the flask filled to the calibration mark with water
Mfws = mass of the flask filled to the calibration mark with soil and water
Mw = mass of the water displaced by soil solids
Combining the above definitions allows us to create an expression for the specific gravity of solids:
The soil sample is very coarse, angular, well-graded sands of light gray color.
1. The empty flask was weighed. This quantity is Mf. The flask was then filled with distilled water to about an inch below the calibration mark.
2. The flask was attached to a vacuum pump for de-airing for 10 minutes.
3. Water was then filled to the calibration mark. The outside of the flask was dried, to make sure the weight of the flask with water was not over-estimated. The flask was weighed again, with the water this time. This quantity was saved as Mfw.
4. The temperature of the water was then taken to the nearest 0.1˚ C. A calibration curve was plotted for varying temperatures and masses of the flask with water.
1. Approximately 150g of a cohesionless soil sample was weighed. It was then placed in the volumetric flask. The mass of the solids was weighed, and recorded as Ms.
2. The flask was filled with distilled water to a level just below the base of the neck. The flask was then placed in a vacuum pump and de-aired for another ten minutes.
3. Water was added to the flask until the meniscus was raised to the calibration mark. Water inside the flask above the calibration mark was removed.
4. The flask was weighed to the nearest 0.1g, and recorded as Mfws.
It appears that the specific gravity of the soil sample is near 2.6, which is a reasonable number. This is a typical value for the specific gravity of a soil sample of any kind. The calibration curve is reasonable in curvature, and appears that the information thus provided is legit.
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