7. Grain Size Analysis

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.Grain-Size Analysisof a SoilI( In clu din g b oth M e ch an ic a l

a nd 'H yd romQ te r A n aly ses ) ..

Grain-size analysis, which is among the oldest of soil tests, is w idely used inthe engineering classification of soils, G raln-slze analysis is also utilized inpart of th e speciticatloun of soil (o r airfields, roads, earth dam s, and othersoU em bankm ent construction. Additionally, the (rost susceptib ility of soilscan be fairly accurately predicted from the results of this analysis, T hestandard grain-size analysis test determines the relative proportions of dif-feren t grain sizes as they are distributEld am ot:lg certain size ranges.

Grain-size analysis for soils con taining relatively larg e p articles is a cc om -plished by using s ie ve s .. A sieve is sim ilar to a cook's flo ur s itte r, It is anapparatus having equal size and sh ap e o pe nin gs through whic h gra in s sm allerthan the size of the opening will pass while larger grains are to be retained .Obviously, such a sieve can b~ used to separate so il grains, in a soil sampleinto two groups: one containing grains sm aller than the size of the sieveopening and the other, larger grains. By passing th e soil.sample downwardthrough a series of sieves each of decreasing size openings, the soil grains canbe separated into a number o C groups, each of which contains grains within aparticular size range. These various s ie ve siz es are usually specified and arestandards.

. Soils having very small grain sizes can generally not hove their grain-sizec-:analysls performed using sieves because of the very small size of sieve open-iM that would be required and the difficu lty of getting such sm allpartlclest~ pass through. Grain-size nnnlysis fo r these soils is therefore done by

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APPARATUS ANDSUPPLIES,

Grstn-Size Analysis of ,1 Soilanother method-the hydrometer analysis. The hydrometer method is busedon Stokes' law, which says that the larger the grain size, the greater its settlingvelocity w i l l be.H a soil sam ple contains both large and sm all partic les, which isof'tenth e elise, th e g rain -size analysis may be performed using n comblnation of thetwo methods described above.

SievesA full set of sieves includes the following [1):

3 in, (75 mm)2 in . (5 0 mm)l{-lo. (38.1 m m) - . - "lin. (25.0 II1tn)tin. (19.0 mm),in. (9.5 mm)No.4 (4.75 mm),No. 10 (2.00 mm)No. 20 (0.850 mm)r No. 40 (0.425 mm)No. 60 (0.250 mm)No. 1'40 (0.140 mm)No. 200 (0.075 mm)

A set of sieves that gives uniform spacing of points for the grain-sizedistribution curve to be prepared may b e used. It consists of thefollowing { 1]:,

3 Ill. (7 5 nun)1in. (38.1 mm)-e {in. (19.0 mm)- - - c - in.(9.5 rum)

-~ No.4 (4.75 m rn)v No.8 (2.36 rum), No. 16 (1.18 mrn)v. - - - 4 J No. 30 (0.600 mm)

No. 50 (0.300 mm)No. 1 00 (O,150 mm)No. 200 (0.075 nirn)

Balance (with ;IC"'!:iI'Y to 0.01 g)

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A PP Brs ru & ,n d S up plie s

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M echanlcally operated stirring device (such as 1 1 mul l, m ixe r)(see Fig. 7.1)Hydrometer (conform ing to requirements for hydrometer 1 51H or152H in ASTM Specification E 1 00)

in

No. 1 8 a W G n(ll! - O .O t1 !'!"J_p ~c:'\t

0.001Metric equivalents

0.049 0.203

1.24 . 5.16 12.7 19 .0m[lI) Del~il of stirting pl!ddles

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7. Grain Size Analysis