Soil blending is one of the popular means of soil modification. Fuller’s method appears to be the most popular method while other methods such as the Triangular chart method and Rothfuch’s Graphical method are often commonly used too.
Well-graded soils are often the most desirable type of soils for engineering construction works especially where high shear strength is required. This is because well-graded soils have the capacity to achieve high shear strength under compaction due to the fact that the fine particles of the soil adequately fill the voids created by the interlocking of the coarse particles, thus, giving little or zero room for excess void and water spaces. Besides this property, good quality and mechanical stability of a material are dependent upon a grading suitable for achieving maximum packing and controlling the plastic behaviour of the material. It also depends on the achievement of high-density compaction, and on the provision of drainage or sealing to prevent the ingress of water, which would cause deterioration.
If there is too much fine material in the mix, the strength decreases when wet, and significant deformation can occur. If such a material is used in unpaved roads, the running surface will be slippery and the material will shrink when it dries, with attendant cracking. Too few fines on the other hand cause a lack of binding and difficulty in compaction
To know whether the soil is well-graded or not, one needs to first carry out a sieve analysis on the soil and plot the grading curve. From the grading curve, one would determine the coefficient of curvature (Cc) and coefficient of uniformity (Cu).
Mathematically, Cc = D302/ (D60 x D10)
While Cu = D60/D10
Cc and Cu are dimensionless parameters. If Cu is greater than 6 (Cu ˃ 6) and Cc lies between 1 and 3 (1 ≤ Cc ≤ 3), the soil is said to be WELL GRADED.
In a situation where this well-graded soil is desirable and the present soil does not have the full properties required, it is possible to get the desired soil grading by soil blending. One of the most popular methods of achieving this is the Fuller-Talbot method. The method aims at achieving mechanical stabilization by the addition of sized material to achieve suitable particle size distribution for maximum packing. Three important factors are required in this method:
- The shape of the grading curve
- The maximum size of the particle, and
- The content of fines passing 75-micron metre sieve
Mathematically, the Percentage passing any sieve in the Fuller-Talbot method = 100 √ (aperture size of that sieve – d) / (size of largest particle – D).
Or P = 100 (d/D)n where n varies from 0.3 to 0.5.
Generally, n is taken as 0.5 for spherical-shaped particles and 0.3 to 0.35 for angular-shaped particles. Note that the number, n, is based on the angularity number of the aggregates. Aggregates with an angularity number less than 4 are not suitable for soil-aggregate mixes for road works.
Fuller’s curve is a graph plotted between the percentage of soil passing a certain sieve size, P, and the aperture size, d, of the particular size. Two samples are commonly used, one is predominantly fine-grained soil (FGS) while the other is predominantly coarse-grained soil (CGS). For our example below, sample A is FGS while sample B is CGS.
Example
The table below shows sieve sizes for a given soil, determine the suitable grading appropriate for the soil
| Sieve sizes (mm) | Fuller’s grade (F) | Soil (A) | F-A | Soil (B) | F-B |
| 4.75 | 100 | 100 | 0 | 98.93 | 1.07 |
| 2.00 | 64.85 | 100 | -35.15 | 97.34 | -32.49 |
| 1.18 | 49.84 | 97.15 | -47.31 | 95.68 | -45.84 |
| 0.85 | 42.30 | 93.20 | -50.90 | 94.55 | -52.25 |
| 0.6 | 35.54 | 83.34 | -47.80 | 92.56 | -57.02 |
| 0.425 | 29.91 | 69.70 | -35.79 | 91.1 | -61.19 |
| 0.3 | 25.13 | 47.69 | -22.56 | 89.49 | -64.36 |
| 0.15 | 17.77 | 29.98 | -12.21 | 84.21 | -66.44 |
| 0.075 | 12.57 | 25.60 | -13.03 | 57.10 | -44.53 |
| Total | -264.75 | -423.05 |
For 4.75 sieve, P = 100 (4.75/4.75)0.5 = 100%
For 2.00 sieve, P = 100 (2.00/4.75)0.5 = 64.85
For 1.18 sieve, P = 100 (1.18/4.75)0.5 = 49.84
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Grade F should be the desirable grade of the soil when blending. That is, the target should be to get soil whose particles should fall within the range of particles under Fuller-sized particles (F) in the table above.
The mix ratio = (F-A)/ (F-B) = -264.75/-423.05 = 1/1.6
Thus, suitable grading of sample F should comprise 1 FGS to 1.6 CGS which implies that the two soils should be mixed at the ratio of 38.46 % FGS to 61.54 CGS.
Limitations:
- It usually gives a very low percentage of fines but the use depends on application.
- Often, it has insufficient material passing 75-micron sieve and modifications are needed to increase the binding quality of the mix.
Thanks for reading!
3 Comments
It captured me when you explained that sieve analysis must be done to know if the soil is well-graded. The project that my friend is handling requires lime stabilization. I think he should partner with a contractor that is well-versed in providing soil stabilization solutions.
I like that you said that the percentage of the application is crucial to measure that is why it is important to do the process properly. My uncle mentioned to me last night that his friend is looking for a soil stabilization service for his upcoming project, and he asked if I have any idea what is the best option to do. Thanks to this informative article and I’ll be sure to tell him that it will be much better if they consult a trusted soil stabilisation company as they can answer all their inquiries.
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