Specific Gravity of Soil
The specific gravity of soil is an important weight-volume property that is helpful in classifying soils and in finding other weight-volume properties like void ratio, porosity, and unit weight. In this lab we first weighed an empty 1000 mL graduated cylinder. Then we filled it to 500 mL with distilled water and weighed it again. This allowed us to find the density or unit weight of the water. Secondly we filled another graduated cylinder with a mixture of soil and water. This mixture was heated on a stove to remove air from the mixture. Then it was filled to the 500 mL mark with additional distilled water and weighed. Finally an oven-safe container was weighed empty and then the soil/water mixture was poured into the container and dried in the oven for a 24-hour period. This enabled us to find the weight of the dry soil solids. Once this procedure was completed and the data collected, the specific gravity could be obtained via the following computations.
The specific gravity is defined as the ratio of the unit weight of the soil solids to the unit weight of water.
The unit weight of the soil solids is given by:
Where Ws is the weight of the soil solids and Vs is the volume of the soil solids. Likewise the unit weight of water is given by:
Consequently we can substitute these expressions into the first equation and obtain:
To obtain the weight of the soil we subtracted the weight of a dish container from the weight of the dish container with the oven-dried soil.
The volume of water we used was known at 500 mL.
The weight of the water corresponding to this volume was simply obtained by subtracting the weight of the flask or graduated cylinder from the weight of the flask with the volume of water.
Determining the volume of the soil was more difficult. We used the mixture that consists of the flask, soil, and water. Here the volume of the soil will be the total volume minus the volume of the water. (The volume of air is negligible)
In order to evaluate this we make the assumption that the unit weight of the water is constant.
We know all the values in this last expression except Ww2. This is the weight of the water in the water-soil-flask mixture. It can be found by:
This is all the information we needed. Simply substitute the solution from this last equation into the equation for the second volume of water, the solution from that into the equation for volume of soil, and all the known and determined values into the substituted equation for specific gravity to obtain the specific gravity. Here are the values and calculations we obtained in our experiment.
This was a reasonable finding since the specific gravity of soils typically falls in the range of 2.6 to 2.9.
Posted on February 22, 2011, in Civil Engineering, Engineering and tagged geotechnical engineering, laboratory, soil, soil mechanics, specific gravity, unit weight. Bookmark the permalink. 10 Comments.