Hydrometer method.

Hydrometer method.

Apparatus:

a) Density hydrometer confirming to IS: 3104-1965 – (Range 0.995 – 1.030).

b) Two glass-measuring cylinders of 1000ml capacity with ground glass or rubber stoppers about 7cm diameter and 33cm high marked at 1000ml volume.

c) Thermometer to cover the range 0 to 50⁰C, accurate to 0.50⁰C.

d) Water bath or constant temperature room

e) Stirring apparatus

f) 75 micron sieve.

g) Balance accurate to 0.01g

h) Stop watch

i) Wash bottles containing distilled water

j) Glass rod, about 15 to 20 cm long and 4 to 5 mm in diameter

k) Reagents: Hydrogen peroxide, Hydrochloric acid N solution and Sodium hexametaphosphate.

l) Conical flask of 1000ml capacity

m) Funnel, filter paper, measuring cylinder of 100ml capacity and blue litmus papers.

Procedure:

(A) Calibration of Hydrometer:

1. Determination of volume of the hydrometer bulb (V_h): Pour about 800ml of distilled water in the 1000ml-measuring cylinder and note the reading at the water level. Immerse the hydrometer in water and note the water reading. The difference between the two readings is recorded as the volume of the hydrometer bulb plus the volume of that part of the stem, which is submerged. For practical purposes, the error due to the inclusion of this stem volume may be neglected. Alternatively, weigh the hydrometer to the nearest 0.2g. This weight in grams is recorded as the volume of the hydrometer in ml. This includes the volume of the bulb plus the volume of the stem. For practical purposes the error due to the inclusion of the stem may be neglected.

2. In order to find the area of cross-section (A) of the measuring cylinder in which the hydrometer is to be used, measure the distance, in cm, between two graduations of the cylinder. The cross-section area (A) is then equal to the volume included between the two graduations divided by the distance between them.

3. Measure the distance (h) from the neck to the bottom of the bulb, and record it as the height of the bulb.

4. With the help of an accurate scale, measure the height (H) between the necks of the hydrometer to each of the other major calibration marks (R_h).

5. Calculate the effective depth (H_e) corresponding to each of the major calibration marks (or hydrometer readings, R_h) by the following expression:

                                                 1              V_h

                   He     =         H +        (h -           ) 

                                                 2                A

6. Draw a calibration curve between H_e and R_h, which may be used for finding the effective depth (H_e) corresponding to hydrometer readings (R_h) obtained during the test.

7. Meniscus correction (C_m) : Insert the hydrometer in the measuring cylinder containing about 700ml of water. Take the readings of the hydrometer at the top and bottom of the meniscus. The difference between two readings is taken as the meniscus correction (C_m), which is a constant for hydrometer. During the actual sedimentation test, the readings should be taken at the bottom of the meniscus but since the soil suspension is opaque, readings are taken at the top of meniscus. The meniscus correction is always positive.

(B) Pre- treatment of soil: 

1. Weigh accurately (to 0.01g) 50 to 100 g of oven dried soil sample (W_d) passing the 0.075mm IS sieve. If the percentage of soluble salts is more than one percent, the soil should be washed with water before further treatment, taking care to see that the soil particles are not lost.

2. Add 150ml of hydrogen peroxide to the soil sample placed in a wide mouth conical flask and stir it gently for few minutes with a glass rod. Cover the flask with glass and leave it to stand overnight.

3. Next morning, the mixture in the conical flask is gently heated in an evaporating dish, stirring the contents periodically. Reduce the volume to about 50ml by boiling. With very organic soils additional peroxide may be required to complete the oxidation.

4. If the soil contains insoluble calcium compounds, add about 50ml of hydrochloric acid to the cooled mixture of soil obtained in step 3. The solution is stirred with a glass rod for a few minutes and allowed to stand for one hour or for longer periods, if necessary. The solution will have an acid reaction to litmus.

5. Filter the mixture and wash it with warm water until the filtrate shows no acid reaction to litmus. Transfer the damp soil on the filter paper and funnel to the evaporating dish using a jet of distilled water. Place the dish and its contents to the oven. Take the weight (W_b) of the oven-dried soil remaining after pre-treatment and find the loss of weight due to pre-treatment.

(C) Dispersion of soil:

1. To the oven-dried soil, add 100ml of sodium hexametaphosphate solution and warm the mixture gently for about 10 minutes. Transfer the mixture to the cup of the mechanical mixer using a jet of distilled water, and stir it well for about 15 minutes. The sodium hexametaphosphate solution is prepared by dissolving 33 g of sodium hexametaphosphate and 7 grams of sodium carbonate in distilled water to make one liter of solution. This solution is unsuitable and should be freshly prepared approximately once in a month.

2. Transfer the soil suspension to the 75 micron IS sieve placed on a receiver and washes the soil on this sieve using jet of distilled water from a wash bottle. The amount of distilled water used during this operation may be about 500ml.

3. Transfer the soil suspension passing the 75-micron IS sieve to the 1000ml-measuring cylinder, and adds more water to make the volume to exactly 1000ml in the cylinder.

4. Collect he material retained on 75-micron sieve and put it in the oven for drying. Determination the dry weight of soil retained on 75-micron sieve.

(D) Sedimentation test with hydrometer:

1. Insert a rubber bung or any other suitable cover on the top of the 1000ml-measuring cylinder containing the soil suspension and shake it vigorously end over end. Stop shaking and allow it to stand. Immediately, start the stopwatch, and remove the top cover from the cylinder.

2. Immerse the hydrometer gently to a depth slightly below its floating position and then allow it to float freely. Take the hydrometer readings after periods of 0.5, 1, 2 and 4 minutes. Take out the hydrometer, rinse it with distilled water and allow it to stand in a jar containing distilled water at the same temperature as that of the test cylinder.

3. The hydrometer is re-inserted in the suspension and readings are taken after periods of 8, 15 and 30 minutes; 1, 2 and 4 hours after shaking. The hydrometer should be removed, rinsed and placed in the distilled water after each reading. After the end of 4 hours, readings should be taken once or twice within 24 hours.

4. Composite correction (C): In order to determine the composite correction, put 100ml of dispersing agent solution in another 1000ml measuring cylinder and make it to 1000ml by adding distilled water. The cylinder should be maintained at the same temperature as that of the test cylinder containing soil specimen. Insert the hydrometer in this comparison cylinder containing distilled water and the dispersing agent and take the reading corresponding to the top of the meniscus. The negative of the hydrometer reading so obtained gives the composite correction (C). The composite correction is found before the start of the test, and also at every time intervals of 30 minutes, 1 hour, 2 hours and 4 hours after the beginning of the test, and afterwards, just after each hydrometer reading is taken in test cylinder.

5. The temperature of the suspension should be observed and recorded once during the first 15 minutes and then after every subsequent reading.

Calculations:

(1) The loss in weight in pre-treatment of the soil in percentage is calculated from the following expression:

                                                       

                              W_b                                 Where,  P    = loss in weight in percentage

P       =       1 -   -------   X 100                     W_d = weight of dry soil sample taken from the soil passing 2mm Sieve

                              W_d                                                      W_b = weight of the soil after pre-treatment

(2) The diameter of the particle in suspension at any sampling time t is calculated from:

D       =   10^-5 M ( H_e / t ) ^0.5              Where, M = poise constant factor

                                                                  H_e = effective depth of the hydrometer

                                                                    t  = elapsed time, minutes

(3)  The percentage finer N^/ based on the weight W_d is calculated from:

                 100 G                     Where, N^/ = percentage finer, based on the weight of dry soil sample W_d

N^/      =                    X R                     W_d = weight of dry soil sample taken from the soil sample passing 2mm sieve.

               W_d ( G – 1 )                          G = specific gravity of the soil passing 75 micron sieve.

                                                            R = corrected hydrometer reading

                                                             R = R_h^/ + C

                                                             R_h = R_h^/ + C_m

                                               Where, R_h^/ =  observed hydrometer reading

                                                            R_h = hydrometer reading, corrected for meniscus correction 

(4) The percentage finer (N) based on the total weight of dry soil sample (W) is obtained from the relation:

N       =  N^/  X (W^/ / W)           Where,   W^/ = cumulative weight passing 2mm sieve.

Values of Factor ‘M’