Figure 16-4.—Grain size distribution chart.You should plot these values on a form like the oneshown in figure 16-4. The graph on this form is alogarithm type of layout; coordinates horizontally aresieve sizes (at the top) and grain sizes in millimeters (atthe bottom). Vertical coordinates are percents passing.The formula for determining D60 means the grain size, in millimeters, indicatedby the gradation curve at the 60-percent passing level.In figure 16-4, follow the 60-percent passing line to thepoint where it intersects the gradation curve forFT-P1-1; then drop down and read the grain size inmillimeters indicated below. You read about 1.25mm.D10 means, similarly, the grain size indicated by thegradation curve at the 10-percent passing level. In figure16-4, this is about 0.11mm.for this sample, then, is 1.25/0.11, or about 11.4.COEFFICIENT OF CURVATUREf o rThei s a s f o l l o w s :is as follows:means the coefficient of curvature of thegradation curve. Sometimes the symbolcoefficient of gradation) is used instead of formula for determiningD30 is the grain size, in millimeters, indicated by thegradation curve at the 30-percent passing level. In figure16-4, it is (for FT-P1-1) about 0.35. Therefore,0.352 divided by (0.11 x 1.25), or about 0.89.FT-P1-1 is obviously a sand, since more than halfof its coarse fraction passes the No. 4 sieve. It is a cleansand, since less than 5 percent of it (see table AV-1,appendix V) passes the No. 200 sieve. However, it is nota well-graded sand (SW), because although itsgreater than 6 (prescribed for SW in appendix V), its is less than 1, the minimum prescribed for SW.isis16-14
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