Maximum particle size Minimum dry weight (sieve opening) of test specimen

Samples that contain cohesive soil, which forms hard lumps, must be prewashed. This procedure is described later. Other samples are analyzed DRY by the following procedure:

2. Break up lumps. For coarse material, use a rolling pin on a clean, hard, smooth surface. For fine material, use a mortar and pestle (usually a part of the laboratory apparatus). Take care not to crush individual grains. The object is to separate aggregations of clustering grains.

4. Select and weigh the sieves and pan to be used in the test. The sieve selection varies according to the type of soil being tested. The following is a selection commonly used:

Stack (nest) the sieves one on top of the other such that the largest sieve is on top. The coarsest sieve actually recorded is the next above the first one that retains any material. The weight recorded as retained on this sieve is 0 g; the weight recorded as passing it is the total weight of the sample.

5. Place the sieve pan under the stack of sieves; place the total sample in the top sieve and shake. The shaking interval depends on the amount of fine material. Five minutes is usually enough for most coarse-grained soils, and 15 min is enough for most fine-grained soils.

6. Remove the sieves from the shaker. Start-ing with the first to retain any material, carefully weigh each sieve with the retained material. Sub-tract the weight of the sieve from the combined weight of the sieve and material to determine the weight of the material retained on each sieve. Finally, determine the weight of the material that reached the pan; that is, that passed the No. 200, or finest, sieve.

Enter the results on a data sheet like the one shown in figure 15-31. In this analysis, all the material (359. 1 g) passed the 3/8-in. sieve; none was retained on this one. The No. 4 retained 51.0 g. This means that 308.1 g (359.1 51.0) passed this sieve. You can see how the weight pass-ing was determined from the weight retained in each subsequent case. In column d, the percent passing is computed for each sieve by multiply-ing the weight passing by 100 and dividing the result by the total weight of the sample.

Figure 15-31.-Data sheet for dry sieve analysis.

The total weight of fractions plus the weight of the material that reached the pan comes to 359.0 g. The weight of the sample originally was 359.1 g; there is an error here of 0.1 g. At the lower right, you can see how the percentage of error is computed. The maximum permissible percentage of error is normally () 1 percent. If the percentage exceeds the maximum, the test must be rerun. For an error smaller than the maximum permissible, correction is made by adding the value of the error to the largest amount listed as retained. The value of the error in this case is 0.1 g. The largest amount retained is 83.3 g for the No. 20 sieve. This amount would be changed to 83.4 g.