Specific Gravity of soil test - Civil Engineering Blog

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Thursday, August 31, 2017

Specific Gravity of soil test

Specific Gravity of soil test



Aim of the experiment:

The aim of the experiment is to calculate the value of specific gravity of a soil sample passing sieve #4 (4.75 mm).

Engineering Use:

Determination of specific gravity is very important for engineers since it helps them in determining the weight – volume relationships of a soil sample which in turn will help them determine the required soil from the borrow site to take in order to construct at the construction site. For example this value enters the calculation for embankments or sub-grade.
Moreover, as stated downwards in the theory part, the range of specific gravity also helps engineers to determine the type of soil to be used in their engineering purposes.

Theory:

The specific gravity of solid soil is often needed for various calculations. And it is defined as the ratio of the weight of a given volume of the soil to the weight of an equal volume of distilled water. The general ranges of the values of Gs for various soils fall within ( 2.6 – 2.9 ) and  are given in the table below:

Table 1: Shows the soil type varying with the range of the specific gravity:

Specific Gravity of soil


In order to calculate the specific gravity of a soil the following relations should be known:
Gs = unit weight or density of soil solid / unit weight or density of water
Gs = Ws / (Vs * ρ)
Where:
Ws: mass of soil solid (gm)
Vs:  volume of solid (cm3)
ρw : density of water (gm / cm3)
Since Vs =Vw
Gs = Ws / Ww
Ww = (Ws + W1) – W2
Where:
Ww: mass of equal volume of water
Vs: volume of water
W1: mass of flask & 500 ml of water
W2: mass of flask & soil & water filled to the mark

Procedures


  • Two 50 g of oven dry soil samples that passes sieve #4 were used.
  • 2 flasks (A,B) of 500 ml volume were filled with water to the marker then weighted on the balance
  • 2/3 of the water was removed from each flask into a beaker
  • The soil that passed through sieve # 4 was added to the flask
  • A vacuum pump was used to remove the air that was between the soil particles
  • Distilled water was added again to each flask till the marker
  • After the air was removed each flask was weighted with the water and the soil sample  in it
  • The thermometer was used to determine the water’s temperature in the beaker

Equipments and materials:


  • Volumetric flask 500ml (Pycnometer)
  • Vacuum pump
  • Balance sensitive up to 0.01 g
  • Distilled water

Data Collection and Calculations:

Weight of soil sample passing sieve # 4 = 50 gm

Table 2: Shows the collected data with the calculation process to determine Gs for a sample of soil:



Sample calculations:
For the first trial for Beaker A:
Ww= W1 + Ws - W2
 = (655.0 + 50.0) - 686.6
= 18.4 g
Gs = Ws/Ww
 = 50/18.4
 = 2.7174

 Average specific gravity = (Gs 1+Gs 2)/2 = 2.6474

Conclusion and Evaluation:

The average value of specific gravity of the tested soil sample equals to 2.6474.
It is clear from (table 1) in the theory part that the test was done on a sand sample of soil due to the range of the values of specific gravity. It should be noticed that the value of specific gravity changes with temperature, so we tried to maintain the value of the temperature in order to get better and close results. The error between the specific gravity in the first trial and second trial was slightly high but the values are in the accepted range. There might be some errors that caused that variation such as:
1.      Vibration in the experiment causing air bubbles to take place.
2.      When vacuuming the volumetric flask, the vacuuming process may not be efficient in the desired way.
3.      Errors in weighing the equipments and materials used during the experiment.

References:

1.      Soil laboratory manual, Birzeit University.
2.     Braja M. Das, .Principle Of Geotechnical Engineering, seventh edition.

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