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HYDROMETER ANALYSIS You learned in the EA3 TRAMAN that a soil is considered susceptible to frost when it contains 3 percent or more by weight of particles smaller than 0.020 mm in diameter. To determine whether or not a soil contains an excessive amount of that size particle, you must perform a particle-size analysis of the materials passing the No. 200 (0.074-mm) sieve. You do this by hydrometer analysis. This TRAMAN discussion identifies the items used for a hydrometer analysis and will briefly summarize the procedures used. For a full discussion of the procedures, you should refer to NAVFAC MO-330 or to ASTM D 422. You also should review the topic of hydrometer analysis on page 15-24 of the EA3 TRAMAN before continuing with the following discussion.Apparatus Figure 13-13 shows some of the items that you will need to perform a hydrometer analysis. Another item you will need is an ASTM hydrometer (fig. 13-14). ASTM hydrometers are graduated by the manufacturer to read in either specific gravity or in grams per liter and are calibrated at a standard temperature of 68F (20C). Other needed items are a dispersion cup and stirrer (fig. 13-15), a thermometer accurate to 1F (0.5C), and a clock, or watch, with a second hand. Sample Preparation and Test Procedure Samples for hydrometer analysis are taken from representative material finer than the No. 10 sieve. The approximate size of the sample varies according to the type of soil being tested. For clays and silty soils, you
Figure 13-14.ASTM hydrometer.
Figure 13-15.Dispersion cup and stirrer. will need 30 to 50 grams of air-dry material. When the sample is mostly sand, you will need approximately 75 to 100 grams. First, place the sample in an evaporating dish. Then pour a solution of water (distilled or demineralized) and a dispersing agent, such as sodium hexametaphosphate, over the sample to make a soil-water slurry. Only enough water to submerge the sample completely and enough dispersing agent to keep the soil particles from flocculating (adhering to each other in clusters) should be used. After it has soaked for approximately 16 hours, the soil-water slurry is transferred to the dispersion cup (fig. 13-15) and is mixed for 1 to 10 minutes depending on the plasticity of the soil.Next, you transfer the soil-water suspension to a 1,000-milliliter (mL) sedimentation cylinder and add distilled or demineralized water to fill the cylinder to the
Figure 13-16.Data sheet for hydrometer analysis. 1,000-mL graduation. Bring the suspension to the temperature expected to prevail during the test. Vigorously agitate the solution for 1 minute before taking and recording (fig. 13-16) your first hydrometer reading. Then, without further agitating the solution, take and record additional readings after elapsed times of 4, 15, and 30 minutes and 1, 2, 4, and 24 hours. Remove the hydrometer from the suspension after each reading and place it in a graduate of clean water. |
 
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