Smith's Elements of Soil Mechanics. Ian Smith
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Fig. 1.9 (a) Plasticity chart based on BS 5030 (BSI, 2015) (b) Plasticity chart based on BS EN ISO 14688‐2 (BSI, 2018c).
Two versions of the plasticity chart exist (BS 5930 and BS EN ISO 14688‐2) and either may be used: the former in the UK, the latter elsewhere in Europe. The boundaries between the degrees of plasticity are the same from both charts. It is only the symbols used to indicate the plasticity group that differ between the two.
The U‐line in Fig. 1.9b indicates the approximate upper limit for natural soils and has equation,
Example 1.5 Soil classification (i)
1 Classify the soil of Example 1.2 whose particle size distribution curve is shown in Fig. 1.3.
2 If the soil tested in Example 1.3 experienced 100% passing the 63 mm sieve, 96% passing the 2 mm sieve and 85% passing the 63 μm sieve, classify the soil.
Solution:
1 Using Tables 1.2 and 1.3, the soil is classified a medium graded gravelly SAND.
2 The PSD may be plotted to visualise the results (see spreadsheet example_1.3_and_1.5b.xls) or the classification may be made by reviewing the data directly:Particle fractions:Gravel: 100 – 96 = 4%Sand: 96 − 85 = 11%Silt: 85 − 58 * = 27%Clay: 58*%
*In the pipette analysis, 68.7% of the sample was found to be clay. That test was performed only on the fraction passing the 63 μm sieve. To establish the percentage of clay in the whole soil, we must consider the proportion passing the 63 μm sieve as a fraction of the whole soil sample.
Based on the percentages of each fraction, the soil may therefore be classified as a slightly sandy, silty CLAY.
Since it is a clay, it must also be classified in terms of its plasticity. To do this we use the results of Example 1.4 together with the plasticity chart (Fig. 1.9). It is seen that the soil therefore is a slightly sandy, silty CLAY of high plasticity.
Example 1.6 Soil classification (ii)
A set of particle size distribution analyses on three soils, A, B and C, gave the following results:
| Percentage passing | ||||
| Sieve size (mm) | Soil A | Soil B | Soil C | |
| 20 | 90 | – | – | |
| 10 | 56 | – | – | |
| 6.3 | 47 | – | – | |
| 2 | 43 | – | – | |
| 0.6 | 39 | 93 | – | |
| 0.425 | – | 78 | – | |
| 0.300 | 28 | 16 | – | |
| 0.212 | – | 5 | – | |
| 0.150 | – | – | 100 | |
| 0.063 | 5 | 2 | 92 | |
Soil C: Since more than 10% passed the 63 μm sieve, a pipette analysis was performed. The results were:
| Particle size (mm) | Percentage passing |
| 0.04 | 79 |
| 0.02 | 62 |
| 0.006 | 47 |
| 0.002 | 40 |
Soil C was found to have a liquid limit of 48% and a plastic limit of 29%. Plot the particle size distribution curves and classify each soil.
Solution:
The particle size distribution curves for the three soils are shown in Fig. 1.10. The curves can be used to obtain the following particle sizes for soils A and B.
| Soil | D10 (mm) | D30 (mm) |
|