Abstract
Black cotton soil covers large parts of central and southern India and gives foundation engineers a recurring
headache: it swells when it gets wet and shrinks badly when it dries out, and a structure sitting on it can heave or crack long
before any load-bearing failure ever happens. This paper reports what happened when such a soil, dug from a site known for
this behaviour, was mixed with steel slag - a waste product from steel plants - together with potassium chloride, a simple salt that
is known to interfere with the swelling chemistry of montmorillonite clay. Before any treatment, the soil was put through the
standard battery of index and strength tests: hydrometer analysis, Atterberg limits, free swell, specific gravity, compaction and
CBR. It turned out to be a CH soil with 110% differential free swell, a liquid limit above 83%, and an almost unusable soaked
CBR of 1.395%. Steel slag was then fixed at 15% of the dry mix, and potassium chloride was varied from 0 to 2% in half-percent
steps. Every one of these five mixes was re-tested for free swell, liquid limit, compaction, soaked CBR and triaxial compressive
strength, the last of these at both 7 and 14 days of curing. The numbers moved in a consistent direction: free swell fell from 90%
(soil + slag alone) down to roughly 40% at the highest KCl dose, the liquid limit dropped to the low seventies, and density, CBR
and triaxial strength all climbed steadily up to 1.5% KCl before easing off slightly at 2%. Soaked CBR rose from 1.395% in the
raw soil to close to 8.7% at the best mix, and triaxial deviator stress improved further with longer curing. Taken together, the
results point to 15% steel slag with 1.5% potassium chloride as a practical, cheap and waste-reusing way to turn this problematic
clay into a workable foundation bed.