Properties of Black Cotton Soil

Rich proportion of  montmorillonite is found in Black cotton soil from mineralogical analysis. High percentage of montomonillonite renders high degree of expansiveness. These property results cracks in soil without any warning. These cracks may sometimes extent to severe limit like ½” wide and 12” deep. So building to be founded on this soil may suffer severe damage with the change of  atmospheric conditions. 

As plasticity index and linear shrinkage decreased with the increase of lime content, a mixture of both lime and cement is necessary for adequate stabilization of road bases for heavy wheel loads on the black cotton soils. Previously derived results from African and Indian black cotton soils are also matched with these results.

SI No.	Description of properties	Value
1	Shear strength	soaked CBR of only 1.5%,
2.	residual strength parameter Φr	12°
3.	permeability	10−10 cm/sec
4.	Liquid limit (Indian subcontinent)	40% ~ 100%.
5.	Free Swell index	Sometimes > 50%
An increase in lime content of 0 to 9% results in plasticity index to decreases from 47% to 5% and linear shrinkage to decreases from 11 to 3.6%  respectively.

Structure:

we have already noticed that black cotton soils of different region show considerable variation of properties. So while explaining any property, we will try to mention region where the tests were conducted.

In Hyderabad, the formation of this soil is similar to lentil seed; their subsurface structure takes shape like double-convex lens.A study on African soil (Sudan) shows such aggregated structure is a consequence of variation of pressure exerted under swelling shrinkage process due to seasonal change.

Infiltration rate:

At the beginning of wet season, black cotton soil shows high initial infiltration,but this rate decreases drastically when water content is increased. Infiltration rate decrease during first on hour from 34 mm/h to 45 mm/h. The decreasing rates are:

Elapse time	Decreasing rate(mm/hr)
After 1 hr	34 to 45
>1hr and ≤2 hr	4
>144h	0.2
Krantz et al. (1978)

 
Jewitt et al. tested some African soils and found infiltration rate ranging from 0.2 mm/h ~ 0.5 mm/day when cracks in soil are sealed and profile becomes thoroughly wet. In such extremely low infiltration rate, surface runoff is occurred.

Bulk density:

Black cotton soil shows high bulk density in dry condition and low value at swollen stage. Depending on moisture condition bulk density of these soils may be (1~2) gm/cm³ (Jewitt et al.) 

Black cotton soil of American origin have bulk density ranging from (1.81~2.08) gm/cm³. Huston clay has bulk density (1.59~2.1) gm/cm³. 

Indian black cotton soil (Hyderabad) was found to swell up to 60% when get saturated from dry state.

Again some researchers recommended to correct bulk density of black cotton soil for particular moisture content [25% w/w]. It facilitates to reduce errors observed due to initial moisture content in these soils. Following table helps us to realize this fact.

Adsorption:

We know water molecule behaves as electrical dipoles, of which positive charge is situated near atoms two hydrogen and have negative charge near oxygen atom. This enables water molecules to interact/attract neighbor charged particles. The mechanism of adjoining water molecules with clay crystals is termed as adsorption. 

Montmorillonite is the key compound of black cotton soil which can absorb large amount of water molecules. The adsorbed water takes place their position between crystal sheets of montmorillonite which yields large volume change due to absence and presence of water.

Shrinkage

Black cotton soil is usually known as expansive soil; but the mass that expand under wetting will shrink after drying. The shrinkage process often associated with cracking. When structural issue is concerned shrinkage is more destructive than swelling.

Swelling process affects to lighter building than heavier one. As vertical expansion may be mitigated by mass of structure. But shrinkage is harmful to all type of structures and careful design and sometimes limitation of loading may be needed to be imposed.

Depending on moisture-volume relation, three stages shrinkage are defined (Ritchie & Yule, 1980); these are:

a. Structural shrinkage; in this stage, water is lost due to air drying from large pores in soil i.e. no volume change is occurred.

b. Normal shrinkage, this shrinkage facilitates us to determine COLE (coefficient of linear expansion). Water is lost due to matric potentials of (-0.3~ -15) bars.

c. Residual shrinkage, matric shrinkage potential also responsible for this shrinkage but range is -15 bars to -20 bars. Water is lost from crystal of montmorillonite. No significant change in volume is occurred as air fills inter-crystalline spaces.

That is, first and last stage of moisture loss doesn’t contribute to significant volume change. Other structural volume changes in these soils are slickensides, gilgais etc. 

P^H: 

Black cotton soils have consistent chemical properties and which are not much influenced by their formation. The chemical tests on these soils of Sudan, Ethiopia, India and many other regions of Australia and Africa show that their P^H varies within ranges of 7.5 ~ 8.5 (variation may be region to region or within profiles with increase in depth). 

P^H is attributed to soil profile due to presence of CaCo3; calcium and magnesium bases dominate in these soils.

In some tropical areas exchangeable sodium, due to irrigation or under natural gravity flow (area located at valleys), may be accumulated over top surface of soil. When underlying soil is black cotton soil containing high percentage of montmorillonite PH may reach to 9.5 which may destroy soil structure.