Particle size in mm

Particle size in mm

Institute of Technology) system. The boundaries defined by this system can be seen on the particle size distribution sheet in Fig. 1.2 The results of the sieve analysis are plotted with the particle sizes horizontal and the summation percentages vertical. As soil particles vary in size from molecular to boulder it is necessary to use a log scale for the horizontal plot so that the full range can be shown on the one sheet.

The smallest aperture generally used in soils work is that of the 0.063 mm size sieve. Below this size (i.e. silt sizes) the distribution curve must be obtained by sedimentation (pipette or hydrometer). Unless a centrifuge is used, it is not possible to determine the range of clay sizes in a soil, and all that can be done is to obtain the total percentage of clay sizes present. A full description of these tests is given in BS 1377: Part 2.

Examples of particle size distribution (or grading) curves for different soil types are shown in Fig. 1.8. From these grading curves it is possible to determine for each soil the total percentage of a particular size and the percentage of particle sizes larger or smaller than any particular particle size.

The effective size of a distribution, Du)

An important particle size within a soil distribution is the effective size which is the largest size of the smallest 10 per cent. It is given the symbol D10. Other particle sizes, such as D60 and D85 are defined in the same manner.

Grading of a distribution

For a granular soil the shape of its grading curve indicates the distribution of the soil particles within it.

If the shape of the curve is not too steep and is more or less constant over the full range of the soil's particle sizes then the particle size distribution extends evenly over the range of the particle sizes within the soil and there is no deficiency or excess of any particular particle size. Such a soil is said to be well graded.

If the soil has any other form of distribution curve then it is said to be poorly graded. According to their distribution curves there are two types of poorly graded soil:

• If the major part of the curve is steep then the soil has a particle size distribution extending over a limited range with most particles tending to be about the same size. The soil is said to be closely graded or, more commonly, uniformly graded.

• If a soil has large percentages of its bigger and smaller particles and only a small percentage of the intermediate sizes then its grading curve will exhibit a significantly flat section or plateau. Such a soil is said to be gap graded.

The uniformity coefficient C„

The grading of a soil is best determined by direct observation of its particle size distribution curve. This can be difficult for those studying the subject for the first time but some guidance can be obtained by the use of a grading parameter, known as the uniformity coefficient.

If Cu < 4.0 then the soil is uniformly graded.

If Cu >4.0 then the soil is either well graded or gap and a glance at the grading curve should be sufficient for the reader to decide which is the correct description.

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