The total weight of fractions plus the weightof the material that reached the pan comes to359.0 g. The weight of the sample originally was359.1 g; there is an error here of 0.1 g. At thelower right, you can see how the percentage oferror is computed. The maximum permissiblepercentage of error is normally (±) 1 percent. Ifthe percentage exceeds the maximum, the testmust be rerun. For an error smaller than the max-imum permissible, correction is made by addingthe value of the error to the largest amount listedas retained. The value of the error in this case is0.1 g. The largest amount retained is 83.3 g forthe No. 20 sieve. This amount would be changedto 83.4 g.SIEVE ANALYSIS WITH PREWASH-ING.— When inspection indicates that a samplecontains an excessively high portion of superfinematerial (material that passes the No. 200 sieve),analysis with prewashing is done as follows:1. Oven-dry the sample.2. Weigh and record the total weight aftercooling.3. Place the sample in a clean pan and addclean water until it is completely covered. Allowit to soak until it is completely disintegrated—from 2 to 12 hr. Stir to break up lumps and hastenthe action.4. Wash the material thoroughly on a No. 200sieve under running water and discard the materialthat passes.5. Oven-dry and reweigh. Record the dif-ference between this weight and the originalweight as washing loss.6. Continue as for sieve analysis, dry.Figure 15-32 shows a data sheet for sieveanalysis with prewashing. The ovendry weight ofthe original sample was 75.0 g; the ovendry weightafter prewashing was 55.0 g; therefore, thewashing loss was 75.0 -55.0 or 20.0 g. The sumof 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. 200sieve, equals 55.0 g. This was the original weightafter prewashing. Therefore, no error was made.Hydrometer AnalysisAs you learned in the preceding discussion, thedetermination of grain size distribution by sieveanalysis is limited to those materials larger thanthe No. 200 (0.074-mm) sieve. For uses such as soilclassification, this is sufficient since grain sizedistribution is not used to classify fine-grainedsoils. For determination of frost susceptibility,however, the distribution of particles smaller thanthe No. 200 sieve is necessary. A soil is consideredfrost susceptible if it contains 3 percent or moreby weight of particles smaller than 0.020 mm indiameter. Frost susceptibility should always beconsidered in areas subject to substantiallyfreezing temperatures, since repeated freezing,and subsequent thawing, of water in the soil canseriously affect the ability of the soil to supporta structure. Hydrometer analysis is the test usedto determine the grain size distribution of the soilspassing the No. 200 sieve.Hydrometer analysis is based on Stokes’ law,which relates the terminal velocity of a free-fallingsphere in a liquid to its diameter. The relation isexpressed by the following equation.Where:It is assumed that Stokes’ law can be appliedto a mass of dispersed soil particles of variousshapes and sizes. Larger particles settle morerapidly than the smaller ones. The hydrometeranalysis is an application of Stokes’ law that per-mits the calculation of the grain size distributionin silts and clays, where the soil particles are giventhe sizes of equivalent spherical particles.The density of a soil-water suspension dependsupon the concentration and specific gravity of thesoil particles. If the suspension is allowed to stand,the particles will gradually settle out of the suspen-sion, and the density will be decreased. Thehydrometer is the instrument used to measure thedensity of the suspension at a known depth belowthe surface. The density measurement, togetherwith knowledge of specific gravity of the soilparticles, determines the percentage of dispersedsoil particles in suspension at the time and depthof measurement. Stokes’ law is used to calculatethe maximum equivalent particle diameter for thematerial in suspension at this depth and for theelapsed time of settlement. A series of densitymeasurements at known depth of suspension andat known times of settlement gives the percentagesof particles finer than the diameters given byStokes’ law. Thus the series of readings will reflectthe amount of different sizes of particles in thefine-grained soils. The particle diameter (D) is15-24

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