The beds of soil is considered here unloaded i.e. no foundation is rest on them. When no directional influences are available, the crack becomes approximately hexagonal shape at the soil surface.
We know black cotton soil has great affinity to water and swell when get moisture. When it loses water, the shrinkage is observed. This type of soil shows extreme consistency in respect of both hardness and plasticity. They become very hard in dry condition and very plastic and sticky when they are wet.
When soil loses water they have tendency to shrink or contract. But this shrinkage is not uniform over depth of layer. While top layer tend to contract, bottom layer/ material at bottom tend to remain in same size. This results a strain.
When the strain is large enough, development and propagation of cracks occurs. When cracks are developed, the strain is relieved.
This cracking mechanism for clay soils also fits good for black cotton soil too. In our previous post we have discussed about phase of development of shrinkage in this soil.
The pattern of cracking are shown as found in soil surface –
According to Hill and Johnstone in 1944, Fox 1964:
In agricultural field where crops in the surface are planted in rows, a parallel orientation of major cracks to these rows is observed. Smaller cracks are found within crop row. The major cracks are also lies in between them. This pattern is attributed to spreading of roots into soil holding soil together.