Hydrometer Test

University of Texas at Arlington

Civil Engineering Department

Laboratory Test Report

Hydrometer Test for Grain Size Analysis on soil from Burleson, Texas

Written by:

Richard Benda

Joseph Muhirwa

Robert Sargent

July 11, 2012

Table of Contents                                                                                                   Page
Abstract.......................................................................................................................3
Introduction.................................................................................................................4
Equipment and Materials............................................................................................4
Methods and Procedure..............................................................................................5
Data, Results, and Discussion....................................................................................7
Conclusion.................................................................................................................9
References.................................................................................................................10
Appendices
            Appendix A: Hydrometer Analysis Table and Sample Calculations............11


List of Tables and Figures                                                                                    Page
Figure 1 – Hydrometer in standard solution...............................................................4

Figure 2 – Soil solution being mixed………………………………………….……..5

Figure 3 – Hydrometer in soil solution…………………………………………...….6

Table 1 – Calculated grain size and percent finer……………………………………7

Figure 4 – Percent Finer vs. Grain Size……………………………………………...8
Table 2 – Calculated grain size and percent finer relative to entire sample……..….9
Table 3 – Hydrometer Analysis Table………………………………………………12

Abstract
            Grain size distribution, also known as particle-size distribution, is one of the most important and most elementary classification schemes of a soil. Sieve analysis can be performed in order to determine the grain size distribution and classify the soil according to the Unified Soil Classification System (USCS). However, this method only works for soils up to a certain size and it can be significantly less accurate with soils finer than 0.15 mm. To classify soils finer than 0.075 mm, other methods, such as the Hydrometer test, must be used. The procedure used followed ASTM standard D422, Standard Test Method for Particle-Size Analysis of Soils. The soil that was used in this particular laboratory test was from Burleson, Texas. The process of performing the hydrometer test involves preparing a solution of soil and a deflocculating agent and allowing it to sit for 8-12 hours. Next, a standard solution is prepared in one graduated cylinder containing water and the deflocculating agent. The hydrometer is inserted in this solution and various calibrations are performing, including meniscus reading on the hydrometer and the temperature of the water. After this the soil that was allowed to sit prior to this is mixed and placed in another graduated cylinder and the remainder is filled with water. The contents of the cylinder are thoroughly mixed and a timer is started. Readings are taken at various times with the hydrometer, and in between readings the hydrometer is placed in the standard solution. At the end of the time elapsed the readings can be used to determine the grain size distribution of the fine portion of the soil. Determining the grain size distribution of a soil is important because the grain size distribution can affect the physical and chemical properties of a soil. Performing this test is an essential step in knowing more about the basic properties of a soil.

Introduction

Since sieve analysis is limited to grain sizes coarser than the number 200 sieve, a different test must be done for grain sizes finer than the 200 sieve. The hydrometer test is the procedure typically used to get the grain size distribution of those remaining particles. The theory behind this test is that the bigger particles will fall to the bottom faster than the smaller particles (based on Stoke’s Law). The two particle types that are finer than the 200 sieve are clay and silt. Since the silt has a bigger grain size than clay, the silt will fall to the bottom before the clay does. When the hydrometer is place in the fluid it floats like a fishing bobber because of buoyancy. The denser the fluid is the more buoyant the hydrometer. As the larger silt particles settle to the bottom the hydrometer will float lower and the readings from the hydrometer will get smaller. By knowing the time the hydrometer readings were taken, the lowering of the hydrometer was expressed as a function of time allowing a graph of grain size vs. percent finer to be made.      

Equipment and Materials

Below are the Materials that were used to perform the hydrometer test.

• Oven-dried soil passing No. 200 sieve
• 40 g/L  Sodium hexametaphosphate solution (deflocculating agent)
• Distilled water
• Weighing scale
• 250 mL beaker
• Stirring device and dispersion cup
• Two 1000-mL etched graduated cylinders
• Timing device
• Thermometer
• Squeeze battle
• 152-H type hydrometer
• Rubber stopper or  plastic bag

Methods and Procedure

To conduct the Hydrometer test ASTM Standard D422: Standard Test Method for Particle-Size Analysis of Soils was followed.  The material passing No.200 sieve is used to perform the hydrometer test. The following steps were used to carry out the hydrometer test:

1. Mix a 50 g soil that has passed through No. 200 sieve with 125 mL of 4% solution of Sodium hexametaphosphate (deflocculating agent) in a 250 mL beaker. This mixture is allowed to sit for at least 2 to 12 hours.
2. Fill the 1000 mL graduated cylinder with 125 mL of dispersing agent and add distilled water to it up to 1000 mL mark. Next, the solution is mixed well by moving the cylinder around with the top covered by a plastic bag.
3. Use a thermometer to record the temperature of the solution. Next, transfer the hydrometer in the cylinder and record the top of the meniscus which is taken as the zero correction factor (F_z). Also, the meniscus correction (F_m) is observed and recorded by taking the difference between top and bottom menisci.
4. Using a small spatula, mix the solution prepared in step 1and pour it into a mixing cup. A squeeze bottle filled with distilled water should be used to wash all the soil particles out of the beaker. Also, distilled water is added to the mixing cup to make it about two thirds full.
5. After 1 minute of mixing, pour the solution into the second graduated cylinder and make sure all the soil particles are transferred into it. Fill the graduated cylinder with distilled water up to the 1000 mL mark and mix the solution by agitating the cylinder. A plastic bag or a rubber stopper is used to cover the top opening of the cylinder.
6. Place the cylinder right next to the cylinder described in step 2 and record the time immediately. This is taken as the starting point or time zero (t=0).  
7. Insert the hydrometer into the cylinder containing the soil-water suspension and start taking the hydrometer readings at cumulative times. The upper level of the meniscus is used to take readings.
8. Take the hydrometer out of the cylinder after two minutes and place it into the next cylinder described in step 2. Hydrometer readings are to be taken at cumulative times and the hydrometer should be read after staying in the soil-water suspension for about 30 seconds. After taking the reading, the hydrometer is placed back into the cylinder with no soil in it (cylinder described in step 2). Lastly, proceed with calculations to determine the size distribution of fine aggregates of the soil of interest.

Data, Results and Discussion

Using the hydrometer readings taken over time, the grain size and the percent finer was calculated (see Appendix A for data and sample calculations).

Table 1 – Calculated grain size and percent finer

Grain size, D(mm)	Percent Finer (PT)
0.0734	86.4
0.0524	84.4
0.0371	84.4
0.0265	82.4
0.0189	80.4
0.0136	76.4
0.0100	74.4
0.0073	68.5
0.0052	64.5
0.0038	60.5
0.0027	56.5
0.0020	50.5
0.0012	44.6

Figure 4 - Percent Finer vs. Grain Size

From the results of the hydrometer test in Table 1, the gradation of soils passing the number 200 sieve (0.075 mm) was able to be determined. This was determined using Equation 1:

                                                  (Eq. 1)

Results are shown in Table 2 for 90% passing number 200:

Table 2 - Calculated grain size and percent finer relative to entire sample

Grain size, D(mm)	Percent Finer (PT)
0.0734	39.1
0.0524	38.2
0.0371	38.2
0.0265	37.3
0.0189	36.4
0.0136	34.6
0.0100	33.7
0.0073	31.0
0.0052	29.2
0.0038	27.4
0.0027	25.6
0.0020	22.9
0.0012	20.2

From the results in Table 2, one can observe that way over a quarter of the entire sample contains particles finer than 0.075 mm, which is a number 200 sieve. This indicates that a large portion of the soil contains clay and silt particles. Clay and silt particles in soil is an indicator of plasticity, and from prior experiment results on the soil this fact has been already confirmed.

Conclusion
            By performing ASTM standard D422 the grain size distribution for the fine portion of a soil from Burleson, Texas was able to be determined. Over 39% of the soil sample used contained particles which were finer than a number 200 sieve (0.075 mm), which means that this soil contained a fairly large amount of clay and silt particles. Though it was known that this soil was of a somewhat high plasticity before this test was performed, the large amount of clay and silt particles in the sample confirmed this since fine particles (particularly clay) are often a good indicator of plasticity. The results from this laboratory test will be most important in classifying the soil according to the Unified Soil Classification System (USCS). This test was fairly simple to perform and had very little potential for error. Any error that may have resulted was probably due to omission of one reading. This resulted from lack of access to the lab in the evening. Determining the grain size distribution of a soil is a very important aspect of working with a soil sample and this test was just part of that step.

References

ASTM. (n.d.). Standard Test Method for Particle-Size Analysis of Soils, ASTM D422, West Conshohocken, PA.

Das, Braja M. (2009). Principles of Geotechnical Engineering, 25th ed., Cengage Learning, Stanford, CT.

Das, Braja M. (2002). Soil Mechanics Laboratory Manual, 7th ed., Oxford University Press, New York, N.Y..

Appendix A

Hydrometer Analysis Table and Sample Calculations

Table 3: Hydrometer Analysis Table

Time (min)	Hydrometer reading, R	Rcp	Percent finer, a*Rcpx100/Ms	RcL	L (cm)	A	D (mm)
0.25	48	43.4	86.366	49	8.23	0.0128	0.07344
0.5	47	42.4	84.376	48	8.39	0.0128	0.05243
1	47	42.4	84.376	48	8.39	0.0128	0.03708
2	46	41.4	82.386	47	8.56	0.0128	0.02648
4	45	40.4	80.396	46	8.72	0.0128	0.0189
8	43	38.4	76.416	44	9.05	0.0128	0.01361
15	42	37.4	74.426	43	9.22	0.0128	0.01004
30	39	34.4	68.456	40	9.71	0.0128	0.00728
60	37	32.4	64.476	38	10.04	0.0128	0.00524
120	35	30.4	60.496	36	10.37	0.0128	0.00376
240	33	28.4	56.516	34	10.7	0.0128	0.0027
480	30	25.4	50.546	31	11.19	0.0128	0.00195
1440	27	22.4	44.576	28	11.69	0.0128	0.00115

R_cp (Equation 2), is the hydrometer correction factor (where R is the actual hydrometer reading) is determined by factoring in the temperature correction (Equation 3) and the zero correction (the reading of the hydrometer in the standard solution).

                                                                                                        (Eqn. 2)

                                          (for T between 15 and 28^oC)             (Eqn. 3)

To use the hydrometer data to determine the percent finer, a correction for specific gravity has to be done as well (since the hydrometer is calibrated for G_s = 2.65) (Equation 4).

                                                                                                                (Eqn. 4)

From this and the prior data (including the soil mass, M_s), the percent finer can be calculated (Equation 5).

                                                                                        (Eqn. 5)

To continue and determine the grain size, D, a few more parameters must be determined. ­R_cL, the determination for effective length of the hydrometer, can be determined by adding R and the meniscus correction F_m (Equation 6).

                                                                                                                  (Eqn. 6)

From a chart, the effective length L can be determined from corresponding values of R_cL. A, an additional parameter varying based on temperature and specific gravity, can also be determined from a chart. With these parameters, D can be determined (Equation 7).

Sample calculation for hydrometer reading at t=0.25

The initial solution temperature, T, was found to be 25^oC. From equation 3:

With this value and the meniscus correction (which was found to be 6 upon observation), ­R_cp can be determined using equation 2:

From prior lab tests, the specific gravity G_s is known to be 2.67, which requires a correction with equation 4:

The percentage finer can then be calculated from equation 5:

From observation, the meniscus correction (F_m) is 1, which is added to R to get R_cL (equation 6):

From the chart, L is determined to be 8.23 cm. For a temperature of 25^oC and a specific gravity of 2.67, A is determined to be 0.0128. Using equation 7, D can be determined:

This is done for each reading. Grain size is plotted against percent finer to determine grain size distribution.