Smith's Elements of Soil Mechanics. Ian Smith
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e.g. 2 mm sieve:
The particle size distribution curve is shown in Fig. 1.3. Using the vertical axis, we can easily see that the soil has approximate proportions of 30% gravel and 70% sand.
It is also seen that the grading curve has a regular slope and therefore contains roughly equal percentages of particle sizes. The soil is a medium graded, gravelly SAND.
1.5.4 Sedimentation analysis
The fraction of soil smaller than 0.06 mm cannot be separated by sieves, so a sedimentation analysis is used to establish the proportions of silt and clay fractions. The procedure is only considered necessary if more than 10% of the soil passes the 63 μm sieve. In the test, a sample of the dry soil passing the 63 μm sieve is placed into suspension with water in a sedimentation cylinder and allowed to settle over a period of time. Measurements of the percentage of particles remaining in suspension at set time intervals are established by either using a pipette or a hydrometer. Details of the test procedures are given in BE EN ISO 17892‐4:2016 and Head (2006). The pipette procedure is described below.
Fig. 1.4 Pipette analysis arrangement.
The dry soil of known mass is placed into a sedimentation cylinder of volume 500 ml and distilled water, containing a small amount of dispersing agent, is added. A dispersing procedure (e.g. end over end shaking) is adopted to separate all the particles in the suspension. The test begins by placing the upright cylinder into a water bath at 25 °C and a stopwatch is started. Three separate sampling dips are made at a depth of 100 mm using the pipette (Fig. 1.4) at pre‐determined times. Different particle sizes will be captured in each sample, since the larger particles fall to the bottom of the cylinder before the smaller ones.
The pipette has a standard volume of 10 ml. This equals the volume of the solution sampled. That sample is flushed from the pipette into a small glass weighing bottle. By weighing the bottle before sampling and again after oven drying, the mass of the sampled soil is determined. By scaling the mass retained in the pipette sample (10 ml), less the mass of the dispersant, to that of the cylinder (500 ml), the mass of soil in suspension is determined. That, together with the determination of the particle size at the time of sampling, allows us to establish the percentage of particles passing that size.
The particle size, d (mm), sampled at depth, h (mm), at time, t (min) is established by:
(1.4)
where
η is the dynamic viscosity of water (mPa s) (=0.89 at 25 °C)
ρs = particle density of the soil (Mg/m3) – see Section 1.7.3.
If samples are taken at 4, 46 and 414 minutes for a soil with particle density of 2.65 Mg/m3, the particle sizes in each sample will be 0.02, 0.006 and 0.002 mm respectively.
Example 1.3 Pipette analysis
The results of a standard pipette analysis, carried out on a sample of soil passing the 63 μm sieve of dry mass 27.25 g and particle density = 2.65 Mg/m3, were:
| Sample 1 | Sample 2 | Sample 3 | |
| Time (min) | 4 | 46 | 414 |
| Mass of empty sample bottle (g) | 5.1926 | 5.3710 | 5.2983 |
| Mass of sample bottle plus dry soil (g) | 5.6927 | 5.8052 | 5.6898 |
During a control test on the dispersant/water only solution (500 ml), it was established that the mass of dry dispersant in a sample of 10 ml was 0.0173 g.
Determine the percentages of fine silt and clay in the soil.
Solution:
Standard test, therefore:Volume of cylinder = 500 ml;Volume of pipette = 10 ml;Water bath temperature = 25 °C;Sampling depth = 100 mm
| Sample 1 | Sample 2 | Sample 3 | |
| Particle size, d (mm) | 0.02 | 0.006 | 0.002 |
| Mass of soil in sample (g) | 0.5001 | 0.4342 | 0.3915 |
| Equivalent mass in cylinder (g) |
|
8.0 | 5.5 |
| Percentage passing d (%) |
|
76.5 | 68.7 |
From the results it is seen that the percentage of