Characteristics of Soil Particles
PURPOSE
The characteristics of soil particles are important in predicting the sub grade performance of the given soil. As the purpose of the sub grade is to given adequate support to the pavement thus the sub grade should posses’ sufficient stability under the adverse climate and loading conditions.
CHARACTERISTICS OF SOIL PARTICLES
What is Soil
Consists largely of mineral matter formed by the disintegration or decomposition of rocks, caused by the action of water, ice, frost or temperature changes or by plant and animal life. Some soils near surface may contain humus organic acid resulting from the decay of vegetation.
In a road there is wide variation in soil type along the road (compared to buildings). Broad classification rather then bearing at a specific point is taken for finding soil properties.
GRAIN SIZE
The grains which soil is composed have been classified by AASHTO as follows.
|
S. No. |
Classes |
Particle size (Diameter) mm |
Sieve Size Passing |
Sieve Size Retained |
|
1 |
Gravel |
80mm to 2mm |
3 |
No 10 |
|
2 |
Coarse sand |
2mm to 0.4 |
No 10 |
No 40 |
|
3 |
Fine sand |
0.4 to 0.08 |
No 40 |
No. 200 |
|
4 |
Silt |
0.08 to 0.002 |
No. 200 |
- |
|
5 |
Clay |
0.002 to 0.001 |
- |
- |
|
6 |
Colloidal Clay |
smaller than 0.001 |
- |
- |
Grain shape
Shape has importance in terms of strength & toughness for large soil particles.
- Rounded: particles found in stream bed have undergone considerable weal and are probably quite strong.
- Flat & Flaky: particles probably have not been subjected to such treatment and may be weak and friable and not suitable for highway use.
- are useful in granular soil mixtures. Which contain little clay and large soil particles having on important influence on their behavior (used in base course) have, an angular particle shape produced by crushing strong, tough rock increase the resistance of soil mass to formation under load since the individual grains tend to lock together.
Surface Texture
The surface texture of large soil particles greatly influences their performance in granular soil mixtures and Rough surface results in high coefficient of friction among the particles. As a result, the particles cannot slide over each other and provide resistance to deformation under load. (Freshly crushed rock)
Rough surface of freshly crushed rock is more important than the angular shape in developing strength.
Desirable properties of Soil
- Stability (Resistance to permanent deformation)
- Incompressibility
- Permanency of strength
- Minimum change in volume & stability under adverse condition of weather & ground water.
- Good drainage
- Ease of compaction (denser)
