We know bulk density is an important property which is always required in geotechnical engineering for determining slope stability, bearing capacity and also to verify and making decision about compaction method, selection of equipments etc.
This important property is not constant for black cotton soil, though many soils show variations, but these soils subjected to maximum variation. The cause behind this is changes in moisture content in soil and subsequent shrinkage and swelling of soil.
High bulk density is reported when they are dry and low dry density in swollen stage. Jewitt et al. in 1979 provided that bulk density of these soils may vary within range of (1~2) gm/cm3 (approximately) and the variation obviously associated with moisture content.
We know there have influence of overburden pressure on bulk density, black cotton soil also subjected to same influence i.e. bulk density increase with depth.
A study in soil profiles in Mississippi and Arkansas showed that bulk density varies from (1.81-2.08) and black cotton soil of Texas showed values ranging from 1.59-2.1. We mean Houston clay as black cotton soil of Texas and the study was conducted by Dudal in 1965.
Ritchie and Yule in 1980 concluded that in Texas black cotton soil, gravimetric moisture decreased at swelling limit and with increasing depth bulk density increased. The depression at two gilgai was found to have higher moisture content and it has, at swelling limit, lower bulk density than that of mounds.
Rao et al. in 1978 found that black cotton soil of Hyderabad subject to volume change up to 60% when dry soil get saturated. Another researchers suggested to correct bulk density for a selected moisture content (25 percent w/w) to minimize errors due to initial water content in soils, which is best understood from following table
The black cotton soil or vertisols are dominated by montmorillonite in their crystals; the expansion, surface area and water-attraction or holding capacity have discussed in this blog and some others are still to come.