THE COOPER UNION
Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 3 - Grain Size Distribution Analysis
To determine the grain size distribution of a soil.

 

REFERENCES:

1) "Soil Testing for Engineers" by T. W. Lambe - Chapter IV.

2) "Engineering Properties of Soils and Their Measurements", 4th edition by Joseph E. Bowles; Experiments No. 5 & No. 6.

3) A.S.T.M. Standards, 1978, Part 19; Designation D422-63, D1140-54.

APPARATUS AND REAGENTS:
 

1) Soil Hydrometer (range 0.995 - 1.050).

2) Mechanical mixer.

3) Graduate (1000 ml.).

4) Electronic top loading balance (sensitive to .01 gm.).

5) Triple-beam balance (sensitive to 0.1 gm.).

6) Electric oven.

7) Thermometer (sensitive to 0.10C.).

8) Set of U. S. Series sieves containing the following sizes:

1-1/2", 1", 3/4", 1/2", 3/8",

and Nos. 4, 10, 16, 30, 60, 100 and 200.

9) Miscellaneous apparatus and reagents, watch glasses, spatulas, evaporating dishes, sodium hexametaphosphate, sieve pans, etc.

EXPERIMENTAL PROCEDURE:
  1) Select a representative sample of the soil as close to its natural moisture content as possible, amounting to approximately 500 gms. of equivalent oven dry material and place in a pan.

2) Separate this soil sample into coarse, and fine fractions, using the following procedure:

 

a) Set up a No.4 and No. 10 sieve and a pan.

b) Take approximately a 50 gm. portion of the sample and gently grind it in a mortar using a rubber covered pestle. Make certain that none of the individual grains are crushed.

c) Transfer this material to the No. 4 sieve and using a stiff brush and the fingers, work it through the sieve. Be extremely careful to see that every soil grain retained in this sieve is a discrete particle and not an aggregation of finer particles. This condition is very common in soils with plastic fine fractions where grinding in the mortar and pestle has not been sufficiently thorough. It will probably be necessary to examine many grains individually by attempting to crush them between the fingers, to properly test for this condition.

d) Remove the No. 4 sieve and set aside the material retained on it (+No. 4 material) in an evaporating dish.

e) Follow the procedure of (c) for the material remaining on the No. 10 sieve; observe the caution mentioned in (c) even more diligently, since the soil grains are much smaller in size and the detection of these aggregations of particles will be more difficult.

f) Remove the No. 10 sieve and set aside the -No. 4 to +No. 10 material in another evaporating dish.

g) The sieve pan now contains the material passing the No. 10 sieve (-No. 10 material). Leave it in this pan.

h) Repeat steps (a) to (g) inclusive a sufficient number of times to separate the entire 500 gm. sample into the three fractions +No. 4, -No. 4 to +No. 10, and -No. 10.

i) Place the +No. 4 and -No. 4 to +No. 10 samples in the electric oven at 110oC.

  3) Record the weight of a crystallizing dish and watch glass in Table 1, and the weight of the sieve pan in Table 2.

4) Weigh the fine fraction (-No. 10 material) plus the pan to 0.1 gm. on the triple-beam balance; record in Table 2 and compute the total weight of the wet fine fraction.

5) Take approximately 20 gm. of the fine fraction and place in the weighed crystallizing dish and watch glass of step 3. Place the remainder of the fine fraction in a tightly covered labeled jar.

6) Weigh the crystallizing dish and watch glass plus the soil sample to 0.01 gm. on the electronic top loading balance and record in Table 1.

7) Remove the watch glass and place beneath the crystallizing dish and put both of them, together with the soil sample, in the electric oven at 110oC. If some soil has adhered to the glass, place the glass beside the crystallizing dish in the oven for at least 6 hours.

8) Remove the crystallizing dish and watch glass from the oven and cover the soil sample with the watch glass so that it will not absorb moisture from the atmosphere. Place the container in a dessicator to cool, remove the container from the dessicator and weigh to 0.01 gm. on the electronic top loading balance and record the weight in Table 1.

9) Compute the moisture content of the fine fraction, and record in Table 1.

10) Perform a sieve analysis on the coarse fraction (+No. 10 material) in the following manner:

 

a) Remove the +No. 4 and -No. 4 to +No. 10 material from the oven. Because of the fact that almost all of the surface area of each of these relatively large particles is exposed to heat, the drying will be quite rapid.

b) Set up a nest of sieves that include the following sizes: 1-1/2", 1", 3/4", 1/2", 3/8", and No. 4 sieves and a pan.

c) Deposit the +No. 4 material obtained in steps 2 (a-e) on the 1-1/2" sieve and work the material through the sieve.  

d) Place the material retained on the 1-1/2" sieve in a weighed evaporating dish and weigh the evaporating dish and the 1-1/2" material on the triple-beam balance to 0.1 gm.; record this weight in Table 3 as the weight retained on the 1-1/2" sieve.

e) Set this 1-1/2" material aside in a tightly covered labeled jar.

f) Repeat step (c) for the material retained on each of the 1", 3/4", 1/2", 3/8" and No. 4 sieves. Deposit the material retained on each of these sieves in the jar of (e).

g) If any material should pass the No. 4 sieve at this time, it merely indicates that the operations of step 2 (c) was not carried out too carefully. Add such material to the -No.4 to +No. 10 material which has been removed from the oven.

h) If any material apparently finer than the No. 10 sieve should appear in the -No. 4 to +No. 10 material at this time, this -4 to +No. 10 material should be put through a No. 10 sieve again and any -No. 10 material thus obtained should be added to that of step 4 and a correction made in Table 2.

i) Weigh this total -No. 4 to +No. 10 on the triple-beam balance and record in Table 3 as the weight retained on the No. 10 sieve.

j) Mechanical sieve shaker may also be used to separate different grain sizes.  

  11) Empty the contents of the jar of steps 10e, f and h into the evaporating dish, weigh and record in Table 2.

12) As a check, compare the sum of the weights retained on each of the 1-1/2", 1", 3/4", 1/2", 3/8" #4 and #10 sieves with the value of step 11.

13) Calibrate the hydrometer and the 1000 ml. graduate using the following procedures:

 

a) Determine the cross-sectional area of the 1000 ml. graduate by measuring the distance in cm. between any two graduations on the graduate, e.g., the 100 ml. and 900 ml. graduations; cross-sectional area, A, is equal to the volume included between these two graduations divided by the measured distance between the two graduations. Record the value of this area in Table 6.

Note: Should use same cylinder in Step 18.

 

b) Determine the volume of the hydrometer bulb. Add 500 ml. of water (tap water will be adequate for this determination), submerge the hydrometer bulb, and observe the volume (volume of the hydrometer bulb is the difference between the final volume after submergence of the bulb, and the original 500 ml. volume). Record the value of this volume in Table 6.

c) Measure and record the distance, H, from the neck of the bulb to any four calibration marks well distributed along the hydrometer stem. Record the values of H and the corresponding hydrometer reading in Table 6.

d) Measure the distance from the neck of the bulb to its tip and record this as h, the height of the bulb in Table 6. For a symmetrical bulb h/2 locates the center of volume of the bulb.

e) Compute the distances HR and HR' from the center of volume of the bulb to each of the four calibration marks of part (c) making use of Formulas 1a and 1b.

f) Determine the value of the meniscus correction, cm, by filling the 1000 ml. graduate with distilled water, inserting the hydrometer and observing the difference in readings taken at the surface of the water and the upper rim of the meniscus; this difference is the meniscus correction. Record the value in Table 6.

g) Determine the value of the density correction, cd, by filling the 1000 ml. graduate with 100 ml. of the deflocculating agent and 900 ml. of distilled water, inserting the hydrometer and taking a reading at the surface of the liquid, and noting the difference between this reading and the one taken at the surface of the distilled water in (f); this difference is the density correction. Record the value in Table 6.

  14) Select a representative sample of the fine fraction from Step 4 amounting to approximately 75 gm.; place in a weighed evaporating dish, and weigh dish and soil to 0.01 gm. on the electronic top loading balance.

15) Record the weight of the dish and the weight of the dish and soil in Table 5.

16) Add sufficient distilled water to the evaporating dish to cover the soil sample with approximately 1/4 inch of water; stir until the soil is thoroughly wetted.

17) Add 100 ml. of the deflocculating agent to this soil-water mixture, stir thoroughly, cover the evaporating dish to retard evaporation, and allow to soak for at least 18 hours. If it will be necessary to allow a longer soaking period (in the case of students, class schedules may prevent the strict adherence to the 18 hour soaking period) transfer the soil-water mixture to a jar with a screwtype cover. This will, of course, prevent evaporation and contamination from dust. Note that in this transferral not a single drop of this soil-water mixture must be lost since the soil sample itself represents a definite weighed quantity from step 14. A wash bottle or syringe filled with distilled water are very convenient devices for making this transferral.

18) At the end of the soaking period transfer the soil-water mixture containing the deflocculating agent to a 1000 ml. graduate and add distilled water to bring the graduate total volume up to 250 ml.

19) The soil sample will now be given further dispersion with the compressed air mechanical mixer. Open the compressed air regulating valve on this apparatus until a pressure of one p.s.i. is registered on the gauge.

20) Insert the mechanical mixer tube in the graduate, seat it properly at the top of the graduate, and increase the pressure to 25 p.s.i.

21) Mix the soil sample for the following periods of time:-

Plastic Index

Mixing Period, minutes

0-5

5

6-20

10

over 20

15

  22) At the end of the mixing period, reduce the pressure to 1 p.s.i.

23) Carefully unseat the mechanical mixer tube and slowly lift it from the graduate while simultaneously washing, with wash bottle or syringe filled with distilled water, any soil grains adhering to the mixer tube or injection head into the graduate.

24) Add distilled water to the graduate to bring the suspension level to the 1000 ml. mark.

25) Thoroughly mix the contents of the graduate for one minute; create a soil suspension of uniform density using the following procedure:

 

a) insert the stirring rod, consisting of a two foot-long metal rod having a 1-1/2" diameter rubber disk at one end, into the graduate and vigorously pump the rod up and down throughout the entire depth of the suspension, for 40 seconds.

b) At the end of 40 seconds, slowly pump the rod up and down in the upper-half of the suspension, for 15 seconds.

c) For the last 5 seconds slowly move the stirring rod up through the upper-half of the suspension, and withdraw it from the graduate.

  26) At the instant that the stirring rod is removed from the suspension start the timer, insert the hydrometer to a depth slightly below its floating depth, and allow it to float freely to rest. Take hydrometer readings at elapsed times, measured from the beginning of sedimentation, of 1/2, 1 and 2 minutes; the hydrometer is to remain continuously in the suspension for these two minutes. Make a record of these readings.

27) Remove the hydrometer from the suspension after the 2 minute reading has been taken, wipe and dry it, and place it in its container.

28) Repeat steps 25 and 26 twice more. It is the average of the three 1/2 minute, 1 minute and 2 minute hydrometer readings which are recorded in Table 7 as R'H values.

29) Additional hydrometer readings will be taken at 4, 8, 15 and 30 minutes; and 1, 2, 4, and 24 hours of elapsed time (all elapsed times being measured from the beginning of sedimentation of the third run of step 28). Times, other than those just stated, may be substituted for the sake of convenience.

30) When it is desired to take one of the hydrometer readings stated in Step 29, carefully insert the hydrometer in the soil suspension to a depth slightly below its floating depth 15 to 20 seconds before the due-time of the reading, thus allowing it to float freely to rest (as in step 26) in time for the reading to be taken. Record these readings as R'H values in Table 7. Insert and withdraw the hydrometer very slowly and carefully to cause as little disturbance in the suspension as possible.

31) After the two-minute reading and all subsequent readings, insert the thermometer slowly and carefully in the soil suspension, take the temperature, and record the values in Table 7.

32) After taking the last hydrometer reading, transfer the soil suspension to a No. 200 sieve, and wash with tap water until the wash water is clear. Transfer the material retained in the sieve to an evaporating dish and dry in an electric oven at 110oC.

33) Remove the material from the oven and perform a sieve analysis on it in the following manner:

 

a) Set up a nest of sieves consisting of Nos. 16, 30, 60, 100 and 200. Additional sieves may be added if desired.

b) Transfer the dried material from the evaporating dish to the No. 16 sieve.

c) Gently sweep the material over the surface of the screen with a fine-haired brush. Continue brushing until not more than one per cent of the material retained on the sieve passes that sieve in one minute of brushing.

d) Transfer the residue on the No. 16 sieve to an evaporating dish, weigh the dish and soil to 0.01 gm. on the electronic top loading balance, and record the weight in Table 4.

e) Repeat (c) for the material on the No. 30 sieve, or whatever sieve finer than the No. 16 is being used and now contains the material passing the No. 16 sieve.

f) Transfer this residue to the evaporating dish containing the residue of the No. 16 sieve, determine the cumulative weight, and record in Table 4.

g) Repeat the sieving procedure of (c) on the material on each succeeding finer-opening sieve, transferring the residue of each sieve to the evaporating dish containing the previously weighed residues of the coarser sieves, and obtaining a new cumulative weight. Record these cumulative weights in Table 4.

COMPUTATIONS:
 

1) Correct total weight of the fine fraction sample used in the hydrometer analysis, (Step 14) to a dry weight using the moisture content determined in Step 9. Record this weight in Table 5.

2) The computations for the sieve analysis of the material coarser than the No. 10 sieve will be made in the following manner:

 

a) Compute the dry weight of the whole soil sample, coarse fraction plus fine fraction, by adding the dry weight of the coarse fraction (Steps 11 and 12) to the dry weight of the entire fine fraction. This dry weight of the fine fraction is computed by correcting the total weight of the fine fraction (Step 4) for the moisture content (Step 9).

b) Compute the cumulative weight finer than each corresponding coarse sieve using the individual weights retained on each sieve from Step 10. Record in Table 3.

c) Compute the percent finer than each corresponding coarse sieve by dividing the cumulative weight finer than each sieve, (b), by the dry weight of the whole sample, (a). Record the values in Table 3.

 

3) The computations for the sieve analysis of the material finer than the No. 10 sieve will be made in the following manner:

 

a) Compute the cumulative weight finer than each corresponding fine sieve by subtracting each cumulative weight retained, (step 33g) from the dry weight of the hydrometer sample (computations-1). Record in Table 4.

b) Compute the percent finer than each corresponding fine sieve by dividing the cumulative weight finer than each sieve size by the dry weight of the hydrometer sample, and multiplying this quotient by the per cent of the whole soil sample that is finer than the No. 10 sieve.

 

4) Compute the R"H values by adding the meniscus correction (13f), to the observed hydrometer readings, R'H, and record in Table 7.

5) Compute the RH values by subtracting the density correction (13g) and applying the temperature correction, (adding or subtracting it depending upon the temperature of the suspension at the time of reading and the calibration temperature of the hydrometer).

6) The computations for the hydrometer analysis will be made in the following manner:

 

a) Compute the values of the equivalent grain size, D, in mm., corresponding to each hydrometer reading, using Stokes Law (Formula 3); obtain necessary values of HR from the plot of Graphs - 1.

b) Compute the percent of the whole soil sample that is finer than each of these corresponding equivalent grain sizes of (a) by:

 

1) Computing the percent of the hydrometer sample that is finer than each of the corresponding equivalent grain sizes, using Formula 2. To use Formula 2 values of RH, the specific gravity of the hydrometer sample; and dry weight of the hydrometer sample are needed. RH values have been computed in Computations - 5; the specific gravity of the hydrometer sample (material passing the No. 10 sieve) has been determined in Experiment No. 2; and the dry weight of the hydrometer sample has been computed in Computations - 1.

2) Multiplying each of the percents finer of the hydrometer sample, computed in the previous step, by the percent of the whole soil sample that is finer than the No. 10 sieve.

GRAPHS:
 

1) Plot the calibration curves for the hydrometer and graduate used in the hydrometer analysis. Both curves will be plotted on cartesian coordinates with the hydrometer reading as ordinate and the corresponding distance to the center of volume of the hydrometer bulb as abscissa. Curve 1 will be the plot of Formula la and curve 2 will be the plot of Formula lb; values for both plots have been computed in Experimental Procedure - 13e, and values are recorded in Table 6.

2) Plot the grain size distribution curve for the whole soil sample analysed in this experiment. The data for the plot have been computed in Computations - 2, 3, and 6 and are recorded in Tables 3, 4, and 7. Plot the equivalent grain size in mm. on a logarithmic abscissa vs. the corresponding percent finer on an arithmetic ordinate, using a second-quadrant plot.