MECHANICAL ANALYSIS

Engineering Aid 3 - Beginning Structural engineering guide book
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Figure 15-29.-Varying sieve sizes. 3. Place three scoops (24 g) of reagent in the cap. Then, with the pressure vessel in a horizon- tal position, insert the cap into the pressure vessel and tighten the clamp to seal the cap to the unit. 4. Raise the moisture tester to a vertical position so that the reagent falls into the vessel. 5. Hold the moisture tester horizontally; vigorously shake the device with a rotating motion for 10 sec to put the steel balls into orbit around the inside circumference; then rest for 20 sec. Repeat the shake-rest cycle for a total of 3 min. Do not allow the steel balls to fall against either the cap or orifice leading to the dial; this may cause damage. 6. Holding the tester horizontally at eye level, read and record the dial reading as the percent of moisture by wet mass. 7. When the sample is dumped, examine it for lumps. If the soil sample is not completely broken down, increase the time limit (shaking unit) by 1 min on the next test. 8. To determine the percentage of moisture by dry mass (ovendry moisture percentage), read the direct reading obtained in No. 6 above into a calibration curve that is also supplied with the test set. MECHANICAL ANALYSIS Mechanical analysis is the determination of grain sizes and the percentage distribution of each size. A complete mechanical analysis is ac- complished in two parts: sieve analysis and hydrometer analysis. Sieve Analysis A sieve analysis is applicable to soils that are larger than the No. 200 sieve or that contain small amounts of material passing the No. 200 sieve. You can conduct the sieve analysis either on the entire sample or on the sample after the fines are removed by prewashing. The apparatus and pro- cedures used to conduct a sieve analysis are described below. APPARATUS.— Typical sieve analysis ap- paratus includes a gram weighing balance and a number of sieves with apertures of varying sizes used to determine grain sizes (fig. 15-29). Sieves may be of the ordinary circular SIFTER type (usually about 8 in. in diameter) or the ROCKER type, which consists of a rocker frame in which screens with apertures of various sizes can be placed. The sieve used for analysis is the so-called stan- dard sieve. A standard sieve has a square aper- ture. Screen sizes are designated as follows: A sieve with fewer than four apertures to the linear inch is designated by the size of an aperture; for example, a 1/4-in., 1/2-in., 3/4-in., or 1-in. sieve. A sieve with four or more apertures to the linear inch is designated by a number that represents the number of apertures to the linear inch. A No, 4 sieve, for example, has four aper- tures to the linear inch, a No. 6 has six apertures, and so on. The finest sieve used is a No. 200, with 200 apertures to the linear inch and an aperture size slightly smaller than one two-hundredth of an inch square. To conduct a sieve analysis, you need an electric or hand-operated sieve shaker. 15-21

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