the specific gravity is necessary for certain tests,such as hydrometer analysis. It is also necessaryfor computations involving volume and weightrelationships. The specific gravity of a soil masscan be expressed in one of three different formsas follows:SPECIFIC GRAVITY OF SOLIDS (G,) is theratio of the weight in air of a given volume of soilparticles to the weight of an equal volume ofdistilled water, both at a stated temperature. Thespecific gravity of solids is only applied to thatfraction of a soil that passes a No. 4 sieve.APPARENT SPECIFIC GRAVITY (Ga) isthe ratio of the weight in air of a given volumeof the impermeable portion of soil particles to theweight in air of an equal volume of distilled water,both at a stated temperature. The impermeableportion of a porous material, such as most largesoil grains, includes the solid material plus im-permeable pores or voids within the particles.BULK SPECIFIC GRAVITY (G~) is the ratioof the weight in air of a given volume ofpermeable material (including permeable and im-permeable voids) to the weight of an equal volumeof distilled water at a stated temperature.Sample SelectionFor specific gravity tests, the soil samples maybe either disturbed or undisturbed. Care must betaken, however, to ensure that representativesamples are obtained. When the sample containsboth large and small particles, the sample shouldbe separated on a No. 4 sieve. Then the specificgravity of the fine fraction is determinedseparately from the coarse fraction. A compositespecific gravity for the entire soil sample is thencalculated in the manner to be described later.For samples smaller than the No. 4 sieve, itis easier to begin the test with an ovendried sam-ple. However, some soils, particularly those withhigh organic content, should be tested at theirnatural water content; the ovendried weight deter-mined at the end of the test.Specific Gravity of SolidsAs discussed earlier, the specific gravity ofsolids is applied to soil that passes a No. 4 sieve.However, when the specific gravity is to be usedin conjunction with hydrometer analysis, it isdetermined only on the fraction that passes a No.200 sieve. In either case, the specific gravity maybe determined for soil at natural water contentor ovendried.APPARATUS.—A 500-milliliter (ml)volumetric flask is required for this test. For thediscussion in this TRAMAN, it is assumed thatthe flask has been calibrated. This means that theweight of the flask and water has been calibratedover a range of temperatures that would likely beencountered in the laboratory. As a matter of in-terest, calibration procedures are located in ASTMD 854. Some other apparatus used to perform testare as follows:Balance, 2,000-g capacityBalance, 200-g capacityCans, moisture contentDishes, evaporatingFunnelMortar and pestlePump, vacuum (optional)Stirrer, soil dispersion (optional)Thermometer, general laboratoryPROCEDURE.— AS mentioned previously,you can perform the specific gravity test on soilsat natural water content. When possible, however,you should first oven-dry the sample, as this makesit easier to perform the test. The procedure forperforming the specific gravity test is as follows:1. Record all identifying information regard-ing the sample on a data sheet similar to figure15-33. Also, record identifying information forthe flask and dish (or moisture can) that will beused for the test.2. Air-or oven-dry the sample and breakupall lumps with a mortar and pestle. About 50 gof clay and about 100 g of coarser samples arethe usual quantities.3. Weigh and record the tare weight of amoisture can. Then fill the can with the dry sam-ple; oven-dry and determine the weight to thenearest 0.01 g. This weight minus the tare weightis the weight of the dry soil (W.) entered in block6g of figure 15-33. This weight is critical to theaccuracy of the test. YOU MUST TAKE GREATCARE NOT TO LOSE ANY OF THE MATE-RIAL DURING THE REMAINDER OF THETEST.4. Transfer the material to the volumetricflask, using a funnel. Use a battery filler, orsyringe, to CAREFULLY wash ALL materialfrom the can and funnel into the flask.5. Fill the flask two-thirds full of clean water(for exact analysis use distilled or demineralizedwater). Allow the material to soak from 4 to 6hr, except for clean, sandy soil, which does notrequire soaking.15-26
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