Improved technique in grading of coarse and fine sands during mechanical analysis of soils

Extract

In the accompanying article attention is drawn to a source of error which arises in grading coarse and fine sand subsequent to dispersion. The method of mechanical analysis in which the error arises, and to which the writer refers, is the revised official method adopted several years ago by the Agricultural Education Association(1). The usual procedure is to wash the dispersed soil on to a No. 70 I.M.M. sieve with a jet of hot water, and to rub it gently with a rubber pestle or the finger until no more will pass through. Robinson(2) states that this rubbing is generally unnecessary. The writer has found, however, that even prolonged washing and rubbing with a rubber pestle are ineffective in removing all the fine sand. In the wet mixture the sands are held together closely by surface tension. In order to overcome this, it would seem reasonable first to dry the sieve and its contents at 105° C. and give a further light sieving. This has been done with a large variety of soils, and it has been found that an extremely high proportion of fine sand is withheld when working with certain soils. The amount of fine sand sieved out of the dried mixture increases or decreases in proportion to the amount of total sands present. This is illustrated in the accompanying graph (Fig. 1), where it is to be noticed that, in soils increasing in total sands from 50 per cent (or thereabouts) upwards, the error begins to assume serious proportions.