The shape of soil grains is a very important property influencing engineering behavior of soil, particularly of coarse grained soils.
Coarse grained soils are soils which particles are of relatively larger size and fine grained soils have relatively smaller particle size.
Shape of coarse grains can be examined with naked eyes. But fine grained soils require microscopic examination.
There is no well defined classification of the shape of soil grains but we may consider the following general classification depending upon the dimensions of grains
- Bulky grains
- Flaky grains
- Elongated grains
1. Bulky Grains
Soil grains in which all the dimensions of particle, that is length, width and thickness, are more or less the same are called bulky grains. Bulky particles are formed by physical disintegration, which is mechanical breakdown, of the bigger rocks.
During their transportation by wind or water, the sharp edges of grains may wear out and grains may become rounded. So depending upon their degree of roundedness bulky grains can also be further divided in to sub-categories.
- Well rounded
Soils containing bulky grains behave like a heap of stone pieces. Such soils can bear heavy loads in the static conditions. But if vibration takes place, due to moving load or any other reasons, large amount of settlement may occur.
Soils containing particles with high angularity tend to resist displacement because of interlocking between the particles and hence possess higher shearing strength compared to those soils which have less angular particles.
Cohesionless soils like gravel and sand contain bulky particles.
2. Flaky grains
These particles are extremely thin as compared to its length and width that is why they are also called plate shaped grains.
These particles are similar to dry leaves. Consider they are in a basket. Now under the load these particles are highly compressible and will deform easily. But such soils are relatively more stable when subjected to vibrations.
These particles are mostly present in cohesive soils like clays
3. Elongated grains
In these type of particles only one dimension is prominent relative to other two. They seem more like a needle hence they are also called needle shaped grains.
These types of particles are found in clay soils.