2. Weigh and record the total weight after cooling.

3. Place the sample in a clean pan and add clean water until it is completely covered. Allow it to soak until it is completely disintegrated from 2 to 12 hr. Stir to break up lumps and hasten the action.

4. Wash the material thoroughly on a No. 200 sieve under running water and discard the material that passes.

5. Oven-dry and reweigh. Record the difference between this weight and the original weight as washing loss.

6. Continue as for sieve analysis, dry. 

Figure 15-32 shows a data sheet for sieve analysis with prewashing. The ovendry weight of the original sample was 75.0 g; the ovendry weight after prewashing was 55.0 g; therefore, the washing loss was 75.0 -55.0 or 20.0 g. The sum of the weights retained (53.0 g, the total of col-umn b) plus the 2.0 g that, in spite of prewashing, was still left in the sample to pass the No. 200 sieve, equals 55.0 g. This was the original weight after prewashing. Therefore, no error was made.

As you learned in the preceding discussion, the determination of grain size distribution by sieve analysis is limited to those materials larger than the No. 200 (0.074-mm) sieve. For uses such as soil classification, this is sufficient since grain size distribution is not used to classify fine-grained soils. For determination of frost susceptibility, however, the distribution of particles smaller than the No. 200 sieve is necessary. A soil is considered frost susceptible if it contains 3 percent or more by weight of particles smaller than 0.020 mm in diameter. Frost susceptibility should always be considered in areas subject to substantially freezing temperatures, since repeated freezing, and subsequent thawing, of water in the soil can seriously affect the ability of the soil to support a structure. Hydrometer analysis is the test used to determine the grain size distribution of the soils passing the No. 200 sieve.

Hydrometer analysis is based on Stokes law, which relates the terminal velocity of a free-falling sphere in a liquid to its diameter. The relation is expressed by the following equation.

It is assumed that Stokes law can be applied to a mass of dispersed soil particles of various shapes and sizes. Larger particles settle more rapidly than the smaller ones. The hydrometer analysis is an application of Stokes law that permits the calculation of the grain size distribution in silts and clays, where the soil particles are given the sizes of equivalent spherical particles.

The density of a soil-water suspension depends upon the concentration and specific gravity of the soil particles. If the suspension is allowed to stand, the particles will gradually settle out of the suspension, and the density will be decreased. The hydrometer is the instrument used to measure the density of the suspension at a known depth below the surface. The density measurement, together with knowledge of specific gravity of the soil particles, determines the percentage of dispersed soil particles in suspension at the time and depth of measurement. Stokes law is used to calculate the maximum equivalent particle diameter for the material in suspension at this depth and for the elapsed time of settlement. A series of density measurements at known depth of suspension and at known times of settlement gives the percentages of particles finer than the diameters given by Stokes law. Thus the series of readings will reflect the amount of different sizes of particles in the fine-grained soils. The particle diameter (D) is

Figure 15-32.-Data sheet for sieve analysis with prewashing.

The procedures used to perform the hydrometer analysis are not discussed in this TRAMAN but are contained in ASTM D 422.