Contents

1. THE DETERMINATION OF THE GRAIN SIZE DISTRIBUTION IN SOILS BY MEANS OF THE HYDROMETER...............................................................................2

i) Objective:..........................................................................................................2

ii) Description:.......................................................................................................2

2. Flow Chart...........................................................................................................3

3. Apparatus............................................................................................................4

4. Method.................................................................................................................5

5. Results.................................................................................................................6

5.1. Test results.......................................................................................................6

a) Coarse sand:.................................................................................................6b) Fine sand:.....................................................................................................6c) Silt:..............................................................................................................6d) Clay:............................................................................................................7e) Percentage silt + clay in total sample:..............................................................7f) Percentage passing the 0.075mm sieve in total sample:.........................................7

5.2. Calculations:.....................................................................................................7

6. Interpretation of Results......................................................................................8

7. Recommendations...............................................................................................8

8. Conclusion...........................................................................................................8

9. Appendices..........................................................................................................9

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1. THE DETERMINATION OF THE GRAIN SIZE DISTRIBUTION IN SOILS BY

MEANS OF THE HYDROMETER

i) Objective:

To determine the soil distribution of the various particle sizes using a hydrometer, and

analyzing this information on completion.

ii) Description:

This method covers the quantitative determination of the distribution of particle sizes in soils

by means of a sedimentation process (downward movement of small suspended particles by

gravity), based on Stokes’ Law, by using a specially calibrated hydrometer.

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2. Flow Chart

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Aim: To determine the soil distribution of the various particle sizes using a hydrometer

Apparatus:

Balance sensitive to 0.1g.

A canning jar, wide mouth, about 1000ml capacity.

A bouyoucos cylinder.

A bouyoucos hydrometer.

A dispersing apparatus with paddle.

A stop watch.

Thermometer sensitive to 1°C.

Sodium silicate.

Test Procedure:

Weigh 100g of fine soil, add 400ml of distilled water and 40g sodium hexametaphosphate, stir

well and let it stand for 2hrs.

Mix contents with dispersing apparatus for 15min. and pour

suspension in cylinder.

Fill cylinder with 1205ml distilled water and place hydrometer inside

the cylinder.

Take reading after 18sec, shake, and then take reading after 40sec. After 1hr take reading again with hydrometer and temperature with thermometer.

Remove hydrometer and place cylinder in temperature bath of

20°C.

Shake cylinder until suspension is obtained. Return cylinder to water

bath and start stop watch.

3. Apparatus

- Balance sensitive to 0.1g.

- A canning jar, wide mouth, about 1000ml capacity.

- A bouyoucos cylinder.

- A bouyoucos hydrometer.

- A dispersing apparatus with paddle.

- A stop watch.

- Thermometer sensitive to 1°C.

- Sodium silicate.

- Pen and paper.

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4. Method

- Weigh out 100g of the soil fines.

- Place soil fines into canning jar.

- Add 400ml of distilled water and 40g hexametaphosphate to canning jar.

- Mix contents thoroughly with a dispersing apparatus for approximately 15 minutes

and pour this suspension into the Bouyoucos cylinder.

- Fill the cylinder with distilled water to the 1205ml mark with the hydrometer inside

the cylinder.

- Remove the hydrometer and place the cylinder in a constant temperature bath at 20°C

- When the content is at 20°C, shake the cylinder end over end until a homogenous

suspension is obtained.

- Return the cylinder to the water bath and start the stopwatch.

- After 1 Hour, the hydrometer is inserted and a reading is recorded to the nearest 0.5.

Simultaneously the temperature of the content is also recorded using a thermometer.

- A 40 second reading is then done following the same procedure done in the previous

step along with the temperature.

- Lastly, an 18 second reading is taken in the same way along with the temperature.

- The readings are then recorded as follows:

1 hour reading - C (Clay)

40 second reading - F (Silt and Clay)

18 second reading - E (% passing the 0.075mm)

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5. Results

5.1. Test results

Time (sec) Actual Hydrometer Reading

Temperature (°C) Effective Depth, L (cm)

18 seconds 60 21.5 °C 6.5 cm40 seconds 60 21.5 °C 6.5 cm

1 hour 10 21.5°C 14.7 cm

The soil mortar is divided into four fractions (expressed as percentages of the soil mortar)

a) Coarse sand:

P1 =

Sm = % soil mortar in total sampleSf = % soil fines in total sample

P1 =

b) Fine sand:

P2 =

F = 40 second hydrometer reading

P2 =

c) Silt:

P3 =

C = 1 hour hydrometer reading

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P3 =

d) Clay:

P4 =

P4 =

e) Percentage silt + clay in total sample:

P5 =

P5 =

f) Percentage passing the 0.075mm sieve in total sample:

P6 =

E = 18 second hydrometer reading

P6 =

5.2. Calculations:

D =

18s

40s

60min

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6. Interpretation of Results

The results indicate that the soil sample is not very well graded.

7. Recommendations

My recommendations are that the hydrometer should be replaced as it is clear from carrying

out the practical that the hydrometer is defective.

8. Conclusion

The hydrometer test measures the density of these grains and their terminal velocity as they

fall through the solution. These values allow the diameter of the soil grains to be calculated,

thus providing the percentage of particles in the sample that exceed a given size.

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9. Appendices

Figure 1: Filling the test sample

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